When to Use - How can GEF and the Eclipse Platform be used
Editing and Edit Policies - Adding editing support to your Graphical View
Editpart Lifecycle - Interesting events to know about
Interactions - the GEF interactions and the players involved
Draw2d focuses on efficient painting and layout of figures. The GEF plug-in adds editing on top of Draw2d. The purpose of this framework is to:
Facilitate the display of any model graphically using draw2d figures
Support interactions from mouse, keyboard, or the Workbench
Provide common components related to the above
The diagram below shows a high-level view of GEF. GEF can be loosely defined as the region in the middle. The framework provides the link between an application's model and view. It also provides input handlers, such as tools and actions, that turn events into requests. Requests and Commands are used to encapsulate interactions and their effects on the model.

In MVC (model-view-controller) design, the controller is often the only connection between the view and the model. The controller is responsible for maintaining the view, and for interpreting UI events and turning them into operations on the model. These roles as they apply to GEF are described below:
Model
The model is any data that gets persisted. Any model can be used with GEF. The model must have some sort of notification mechanism. Although not technically the model, commands are closely related. A command is how the model is modified in a way that can be undone and redone by the user. In general, commands should work only on the model itself.
View (Figures/TreeItems)
The view is anything visible to the user. Both Figures and TreeItems can be used as view elements.
Controller (EditPart)
There is usually one controller per visualized model object. The controller is called an
EditPart. Editparts are the link between the model and the view. They are also responsible for editing. Editparts contain helpers called EditPolicies, which handle the much of the editing task.
Viewers
An
EditPartVieweris where editparts display their view. There are two types of viewers provided in GEF. A graphical viewer hosts figures while a tree viewer displays native treeitems. GEF viewers are similar to JFace viewers in that they manage an SWT Control. Viewers are also a selection provider, and the unit of selection is the EditPart.
GEF can be used anywhere that you can use an SWT Control inside a Workbench. It could be an editor, a view, a wizard page, etc. Most commonly it is used inside an EditorPart and sometimes in that editor's outline page.
GEF requires the Eclipse Rich Client Platform (RCP) and the "views" plug-in (org.eclipse.ui.views), 
which provides property sheet support.
Editparts associate their view and model, but they also form their own structure. An editpart maintains children. Usually this corresponds to a similar containment found in the model. For example, the model may consist of a diagram containing nodes. There would then be a corresponding diagram editpart which contains multiple node editpart children.
The parent-child relationship of editparts carries over into their figures. The parent's figure will contain the children's figures. Or, in some cases the parent part's view is a compound view composed of several figures, one of which is designated as the content pane. The figure of each child is added to the content pane. The end result is that you have three separate hierarchical data structures which are approximately parallel to each other.

Connections are the exception to this simple tree structure. Connections represent a link or association between two objects. A Draw2d connection figure is used as the view. Connections may be stored in any way in the model, but the editparts are managed by the source and target editpart at each end. A connection editpart's figure containment is also handled specially. The connection figure is added to a special layer in the diagram so that it paints above the primary layer containing non-connection parts.
There are two types of editpart implementations provided with GEF. Graphical editparts use figures as their view. Graphical editparts also add the ability to support connections, which themselves are graphical editparts. The other type is a tree editpart. Tree editparts use SWT treeitems as their view.
The responsibilities of each editpart include:
Note that maintaining the view and other editparts implies that the editpart will be notified of changes in the model. Usually, the editpart hooks a listener directly to the model object(s) with which it is associated. When it receives notification, it updates its view or structure depending on the change.
As the name implies, editparts must support editing the model. But first we will focus on the initial steps of building an application, which is to display the initial model.
Once you have a model and some figures with which to view it, the next step is to put the pieces together. This means creating the editparts that are going to work with each model and figure combination. GEF's implementations are abstract and must be extended for your application. But first, we need to set up the foundation.
GEF includes the class ScrollingGraphicalViewer. This is a viewer 
implementation which uses a Draw2d FigureCanvas. Most applications 
use this viewer unless, for some reason, scrollbars are not needed. The next step 
is to decide which root editpart to use. Each editpart viewer requires a 
special editpart called the root. This editpart does not correspond to anything 
in the model. Its function is to setup the viewer and provide a uniform context 
for all of the application's "real" editparts. There are two implementations to 
consider using:
ScalableRootEditPart - provides the standard set of layers and 
	supports zoom should the application decide to expose this to the user.
ScalableFreeformRootEditPart - similar to above, but all of the 
	layers conform to the freeform interface, which allows the diagram to 
	additionally extend into negative coordinates (to the left and up). This is 
	the most flexible and commonly used root editpart.
Now we have a viewer and its root editpart, so next we'll actually 
set some contents into the viewer. Contents refers to the base model 
object that seeds the viewer with the graphical diagram being displayed. The 
viewer's EditPartFactory is then responsible for taking the 
contents and 
constructing the appropriate editpart, which is then set on the root editpart. 
Its figure gets added to the root's. At that point the contents editpart will 
construct its children editparts, reusing the viewer's factory, which in turn 
then create their children and/or connections, etc., until all of the editparts 
and their views have been created.

So the first implementation you will need to write is the 
contents editpart. You'll also need to implement a factory which recognizes the 
contents model object and constructs this editpart. This part's figure provides the 
backdrop for the rest of the diagram. Usually it doesn't even need to paint, but 
you still need to choose the layout manager and the figure type based on the 
root editpart. To control which figure gets created, override createFigure(). When using a freeform root, the contents figure must implement 
the freeform API. The FreeformLayer figure will suffice. The layout 
manager is often the XYLayout or the FreeformLayout, again depending on whether 
the root is freeform or not.
During initialization, the contents editpart will construct its 
children based on the list returned when calling getModelChildren() 
on itself. Editparts should override this method to return the list of objects 
in the model for which child editparts should be created. By default, editparts 
go back to the viewer's factory to create each child.
The children of the diagram, sometimes called nodes, usually 
display some information to the user. Their figures may be one of the default 
provided figures, a custom figure, or a composition of multiple figures. During 
the population of the viewer, each editpart will have its refreshVisuals() 
method called. This method is responsible for reflecting the model's properties 
in the view. Editparts must override this method based on the model and figure 
they work with. In complex cases this method can be factored out into several 
smaller helper methods. Later, when we start listening to model changes, this 
method or its helpers may be invoked again.
If a child of the diagram is also a parent with its own children, 
it needs to override getModelChildren() in the same manner that the 
diagram editpart did.
Connections are special editparts that connect any two editparts in 
a diagram. An editpart is called a node if it can be the source or target of a 
connection. The connections are created and managed in a shared way by both of 
its source and 
target nodes. Each node in the diagram must override getModelSourceConnections() and getModelTargetConnections() to 
return the model objects representing the connections. GEF then checks to see if 
the connection editpart has already been created (by the other node at the other 
end), and if not 
it asks the factory to create the connection editpart. The source node is 
responsible for activating and adding the connection figure to the diagram.
A connection editpart's figure must be a Draw2d Connection. The connection editpart attempts to set its figure's anchors by 
casting the source and target nodes to a NodeEditPart. The source 
and target must return the appropriate anchors or default anchors with bogus 
fixed locations will be used instead.
In many ways connections are just like other editparts. They typically have properties in their model that must be visualized. They may even themselves have children or be nodes to other connections.
So far we have focused on just displaying a model graphically. This 
requires that you extend AbstractGraphicalEditPart and override 
behavior based on each part's model. Here is a summary of the methods discussed:
createFigure() - this method creates the editpart's view, or 
	figure. This method does not reflect the model's state in the figure. 
	That is done in refreshVisuals()
refreshVisuals() - this method reflects model attributes in the 
	view. Complex editparts may further decompose this method into several 
	helper methods.
getModelChildren() - this method is called to determine if 
	there are model elements for which children editparts should be created.
getModelSource/TargetConnections() - similar to children, 
	but model elements returned here indicate connections for which the editpart 
	is the source or target.
Once you have some editparts displayed it's time to start editing. Editing is usually the most complex task an editpart performs. Editing includes not only making changes to the model, but also showing graphical feedback during interactions with the view. To abstract away the source of interaction, GEF uses a request. Tools or other UI interpreters will create requests and then call the various API on EditPart based on the interaction. A subset of the EditPart API is shown below.
|  | Methods on EditPartwhich take a Request:
 | 
| 1 | The first step of editing is to decide which editparts 
		are involved. Usually, it is some combination of the viewer's current selection and an 
editpart calculated using the current mouse location. The selection can be 
trimmed by asking if each selected part understands a request. The part under 
the mouse, called the target, is found with the viewer's help and the 
 | 
| 2 | During interactions, especially mouse interactions and 
		dragging, editparts are asked to show feedback based on their role in 
		the interaction. An editpart is considered the source if it is 
		the part being acted on. A target editpart is the part underneath 
		the mouse. For example, when dragging a node around a diagram, the node 
		is the source, and the diagram is the target. The node is asked to show 
		source feedback, which might be a rectangle or some other modified 
		representation of the node. The diagram is asked to show target 
		feedback. When reattaching the end of a connection, a node might be 
		showing target feedback instead. Some interactions only operate on a 
		source. | 
| 3 | The command is what eventually changes the model. 
		Editparts are asked for a command for a given request. Commands also 
		help determine if the interaction is possible. If there is no command, 
		or it is not executable, the UI will indicate that the interaction is 
		not allowed. If an editpart contributes NULL as its command, it does not 
		prevent the interaction from occurring, unless of course no commands are 
		provided by any of the editparts. To indicate something is not 
		allowed, the editpart must return a command that is not executable. | 
| 4 | Finally, there is a generic API telling an editpart to just "do something". This is generally something that does not immediately result in a model change. For example, opening a dialog or activating the "direct-edit" mode. | 
 Editparts 
don't handle editing directly. Instead, they use EditPolicies. Each editpolicy 
is then able to focus on a single editing task or group of related tasks. This also allows editing 
behavior to be selectively reused across different editpart implementations. Also, behavior 
can change dynamically, such as when the layouts or routing methods change.
Editparts 
don't handle editing directly. Instead, they use EditPolicies. Each editpolicy 
is then able to focus on a single editing task or group of related tasks. This also allows editing 
behavior to be selectively reused across different editpart implementations. Also, behavior 
can change dynamically, such as when the layouts or routing methods change.
When any of the above editing 
methods are called (except performRequest()), the editpart delegates to its 
policies to satisfy the request. Depending on the method, the editpart may stop 
at the first policy that handles a request, or it may allow each policy a chance to 
contribute. Refer to each method's javadoc for more details.
During the editpart's creation, createEditPolicies() is called and the editpart 
should install the appropriate policies.
EditPolicies are installed using roles, which are nothing more than 
identifiers that serve as keys. GEF 
provides several identifiers for commonly used roles. Roles become important 
when an editpart needs to have its policies removed or swapped out. The rest of 
the time it is just a good habit. GEF also provides several policies 
for use with these roles. Many of these policies must be extended to fill in the missing 
pieces that work with the application's model. EditPolicies are 
discussed in more detail in the section on interactions.
Commands are passed around throughout editing. They are used to encapsulate and combine changes to the application's model.
An application has a single command stack. Commands must be executed using the command stack rather than directly calling execute.
With respect to lifecycle, editpart implementations typically only have to worry about extending activation and deactivation, which is when the editpart should add and remove its model listeners. Still, an understanding of the entire lifecycle is important.
The first thing that happens is creation. Most editparts will be created by the viewer's factory, which is invoked by either the viewer or the managing editpart which is typically the parent. Immediately after being created, the following methods are called:
setModel() - If the constructor does not take the model as an 
argument, it should be set immediately. Other events that follow may be based 
on the part's model.
setParent(...) - The very first step is to tell the part its parent. 
This gives the part a path back to its viewer, which it might need for things 
like accessing the editpart registry.
createFigure() - Although this is a protected method, the timing of when it 
gets called is somewhat interesting. Both the parent and the model are known at the time the figure needs to be 
created.
addNotify() - Signals the completion of the child being added to the parent. 
At this point, the child will do the following:
activate() - Indicates that the editpart should become active for 
editing, meaning that the model might change. The parent only activates its 
child if it is also active. The root is only active if the viewer has created 
its Control. An editpart should do the 
following on activation:
At this point the editpart is in its normal editing state. It gets selected, shows feedback, and returns commands, etc., until it is no longer needed, meaning it is either removed from the diagram or the viewer is being disposed.
deactivate() - The opposite of activate. Once again, subclasses 
should extend this method to remove the listeners added during activation.
 The remaining steps only occur when the editpart gets 
removed, meaning its model was removed from the diagram. If the viewer is being disposed, then deactivation is the only thing 
that is guaranteed to occur. For this reason, activate and deactivate are the 
commonly extended methods while the remaining methods can usually be ignored.
The remaining steps only occur when the editpart gets 
removed, meaning its model was removed from the diagram. If the viewer is being disposed, then deactivation is the only thing 
that is guaranteed to occur. For this reason, activate and deactivate are the 
commonly extended methods while the remaining methods can usually be ignored.
removeNotify() - Signals that the editpart is about to incur removal. The 
following must happen while the editpart still has access to its surroundings:
setParent(null) - The last step of removal. The parent and 
viewer are no longer reachable at this point.
 Editparts do not come 
back from the grave. When a change in the model is undone, a new editpart 
is created. For this reason, commands should not reference editparts, and 
editparts should not contain any important state that must be restored on undo.
Editparts do not come 
back from the grave. When a change in the model is undone, a new editpart 
is created. For this reason, commands should not reference editparts, and 
editparts should not contain any important state that must be restored on undo.
A tool handles most events from a viewer. The 
EditDomain keeps track of the currently active tool. Applications 
may use the palette (PaletteViewer) to display multiple tools, allowing the user 
to change between modes such as selection, creation, etc.
Tools are implemented like state machines. SWT events provide the input to the state machine. Based on the event and current state, a tool will perform certain actions. These actions could include:
A tool is activated by setting it on the EditDomain. There is only one active tool for all viewers in the domain. If a palette is being used, selecting a tool in the palette will activate that tool.
The Selection Tool is the primary tool used in GEF and is often the default 
for an application. The selection tool is unique in its ability to delegate. It obtains a helper 
called a DragTracker from an editpart or handle below the mouse when a drag 
occurs. A drag is defined as a mouse button being pressed, its being released, 
and any events that occur in between. Events are forwarded to the delegate so that the drag can be handled 
differently based on where and how the drag originated. For example, clicking on a 
handle may result in resizing a shape, or moving the end of a connection. 
Clicking on an editpart typically drags that part to a new location or parent.

Ironically, the Selection Tool doesn't select editparts. All mouse clicks are handled as drags. When the Selection Tool receives a mouse down event over a selectable editpart, it asks for a drag tracker. The editpart returns a tracker derived from SelectEditPartTracker. The tracker also receives the mouse down event, as well as any other events, until the mouse button is released. When the tracker interprets a selection gesture, it modifies the viewer's selection. Trackers even handle events like double-click.
For more on the selection tool and trackers, see the section on Selection Interaction.
GEF's PaletteViewer provides an SWT Control allowing the user to select 
which Tool is active. It can also be a drag source for dragging objects from the 
palette directly into the diagram. Use of the palette is optional, but common.
The palette can be placed anywhere, including inside the editor. 
GEF provides a Workbench View for hosting the Palette. Also, a custom Composite, 
the FlyoutPaletteComposite, will manage the placement of the 
PaletteViewer beside a primary Control. This composite is 
used as the editor's primary control in the Logic Example, shown below:

The PaletteViewer displays a Palette model, which starts with the 
PaletteRoot. The root organizes the contents using either drawers, 
which open and close, or groups, which do not. Each grouping element then 
contains Palette Entries. An Entry defines either a tool or template for the 
User. Templates are described below in Creation.
The Palette provides several display modes, such as icon-only. You can also provide a customizer to allow the user to modify or create palette content.
This section discusses the various types of interactions that are included in the framework, and which parts of the framework are involved in supporting the interaction. An interaction can be anything that affects the model or the UI state. Many interactions are graphical but some are not. An interaction may include:
This section discusses the participants involved in each interaction and what they do. This can include:
RequestConstants class.| Tools | Requests | Edit Policies and Roles | Actions | 
| SelectionTool MarqueeTool SelectEditPartTracker *GraphicalViewerKeyHandler | SelectionRequest DirectEditRequest REQ_SELECTION_HOVER REQ_OPEN REQ_DIRECT_EDIT | SelectionEditPolicy DirectEditPolicy SELECTION_FEEDBACK_ROLE | SelectAllAction | 
No interaction is more basic or universal than selecting items in a viewer. Most of the interactions discussed here operate on what is currently selected. Yet, selection is a complex topic and there are several steps involved. The Selection Tool was briefly discussed in the above section on tools.
Let's first define selection. Selection is a List of EditParts maintained by an EditPartViewer. Changes to the selection are made by invoking methods on the viewer, and not by modifying the list directly. The selection is never empty. If the selection is cleared, the viewer's contents editpart becomes the selection. The last editpart in the list is considered the primary selection.
Closely related to selection is focus. Focus is a single editpart maintained by the EditPartViewer. Focus is used when manipulating selection via keyboard. By moving focus, the user can navigate from one editpart to another without changing the current selection. The user can add/remove the focused editpart from the selection. If focus is not explicitly set, it is the same as the primary selected part.
|  Selection Handles | The editpart is responsible for showing its selected and focused 
state to the user. The viewer tells editparts when they are selected, focused, 
or have primary selection. Typically, selection is shown by one or more 
EditPolicies adding selection handles. The handles shown here on the LED and 
		circuit parts were added by ResizableEditPolicy. The black handles indicate primary selection.Because selection handles are related to how a part can be dragged or sized, which in turn is related to the containing figure's layout manager, it is usually the parent part's editpolicy that installs a policy on the children for displaying the appropriate handles. For example, an XYLayoutEditPolicy would install a ResizableEditPolicy on each child of its host editpart. Connections may ( | 
Selection Targeting and Feedback
For selection to occur, the Selection Tool must first target an 
editpart using a SelectionRequest. In rare cases, an editpart is 
not selectable and targeting "falls through", hitting the editpart below. During 
this continuous mouse targeting, the Selection Tool invokes showFeedback() on the 
current target editpart by passing it a SelectionRequest of type REQ_SELECTION. It is recommended 
that most applications ignore this request since showing and hiding feedback as 
the mouse moves across a diagram can be very distracting to the user. For this 
reason, there is an additional feedback request sent with the type 
REQ_SELECTION_HOVER whenever the user pauses the mouse over an editpart. Often 
an editpart will display a popup shell similar to a tooltip displaying 
additional information about the part. The SELECTION_FEEDBACK_ROLE identifier 
can be used when installing policies which show such feedback.
A benefit to using these feedback requests is that the Selection Tool is smart about asking parts to erase feedback. For example, if the user starts dragging, you would not want a popup message to stay around. Also, selection feedback will not occur when other tools are active.
Selection via a DragTracker
Once the user actually presses down on the mouse, the selection 
target will be asked for a DragTracker. To allow selection, return 
a SelectEditPartTracker or its subclass DragEditPartsTracker, 
depending on whether dragging is permitted. These trackers will modify selection 
at the appropriate time, taking into consideration the SHIFT and CTRL modifier 
keys.
The contents editpart should never be selected by a tracker. 
The reason being that it should never be part of a multiple selection. 
Therefore, it should return either a DeselectAllTracker, 
or the MarqueeDragTracker. Remember, the selection is never empty 
so the contents part will be the selection when all other parts are not 
selected.
Other Selection Requests
Editparts may be asked to perform two additional Requests that are related to selection. These requests are related to selection in that they are interactions associated with clicking the primary mouse button. The first is a double-click, which is called an open (REQ_OPEN). This interaction can be used for editparts that can be opened, expanded, or may display a dialog. The other interaction is called a direct edit (REQ_DIRECT_EDIT). An example of direct editing is when the user wishes to modify the text of a label. The user must first select the part, and then click on it again after it is selected. After a brief delay (used to rule out a double-click), the request is sent.
Selection Actions
GEF provides a SelectAllAction. Given a viewer, this action 
selects all of the contents part's children when invoked.
Selection using the Keyboard
Keyboard selection is supported in graphical viewers by installing a 
GraphicalViewerKeyHandler. The key handler only receives key events 
sent to it by the current tool. The Selection Tool forwards key forwards the 
events necessary for selection.
 Drag trackers are not needed 
inside GEF's TreeViewer. The native tree handles selection already, and dragging 
of treeitems is processed internally using native DND.
Drag trackers are not needed 
inside GEF's TreeViewer. The native tree handles selection already, and dragging 
of treeitems is processed internally using native DND.
| Tools | Requests | Edit Policies and Roles | Actions | 
| REQ_DELETE | COMPONENT_ROLE CONNECTION_ROLE RootComponentEditPolicy | DeleteAction | 

The only universal interaction that all GEF applications should support is delete. The workbench places a global delete action on the edit menu. All applications should register a handler such as the included DeleteAction.
The DeleteAction sends a GroupRequest of type REQ_DELETE to the viewer's 
current selection. All editparts should have an editpolicy which either supports 
or prevents delete from occurring.
Every editpart is either a component or a connection. A component is a basic editpart that is the child of a parent. A connection is slightly different because it is owned by its source and target.
The COMPONENT_ROLE key is used when installing an editpolicy on 
a component editpart. Applications can extend the 
provided 
ComponentEditPolicy to fill in the commands for deletion. The 
RootComponentEditPolicy should be used on the contents editpart. 
This policy prevents the diagram itself from being deleted. Here, "root" refers 
to the model root and is not related to the viewer's root editpart.
The CONNECTION_ROLE key is used when installing a policy on a 
connection editpart. Applications can extend the provided ConnectionEditPolicy 
to fill in the command for deletion.
These editpolicies should handle the tasks most closely associated with the model. In the logic example, this role is responsible for the LED's increment and decrement behavior, which adds or subtracts one from the LED's value.
 Implementing the command that 
performs delete can be difficult, especially when connections are involved. The 
command must consider whether the object being deleted has connections, or if 
children of the object being deleted have connections, and delete the 
connections as well. But, you don't want to delete the same connection twice in 
cases where both source and target nodes are being deleted as part of multiple 
selection. The logic example's delete command addresses all of these concerns.
Implementing the command that 
performs delete can be difficult, especially when connections are involved. The 
command must consider whether the object being deleted has connections, or if 
children of the object being deleted have connections, and delete the 
connections as well. But, you don't want to delete the same connection twice in 
cases where both source and target nodes are being deleted as part of multiple 
selection. The logic example's delete command addresses all of these concerns.
| Tools | Requests | Edit Policies and Roles | Actions | 
| CreationTool | REQ_CREATE Create | CONTAINER_ROLE LAYOUT_ROLE TREE_CONTAINER_ROLE ContainerEditPolicy LayoutEditPolicy | CopyTemplateAction PasteTemplateAction | 
| TemplateTransferDropTargetListener TemplateTransferDragSourceListener | |||

A CreateRequest is used to ask an editpart to create a new child. The request 
is identified by the ID REQ_CREATE. Creation can occur through three different 
methods: clicking, dragging, or pasting. The request provides the location, 
object, and object type being created. The object and its type are provided by a 
CreationFactory. The request hides the factory and provides access 
to the created object directly, caching it in case multiple editpolicies all 
need access to the created object. In some cases, the request will contain a 
size attribute.
Producing CreateRequests
The creation tool provides a "loaded cursor" mode which will attempt 
to create an object at the mouse location when clicking. If the mouse is pressed 
and dragged, the tool will track the size of the rectangle defined by the user. 
The creation tool can be placed on the palette using a CreationToolEntry. 
When the mouse is released, the tool will either repeat the process or switch 
back to the default tool.
Creation can also be performed using native Drag-and-Drop. The drag 
source can be anything, but it is typically the PaletteViewer. A palette entry 
taking a template is added to the palette. The TemplateTransfer is used 
to transfer the template, which is just an Object, from the drag source to the 
drop target. A TemplateTransferDragSourceListener must be added to the 
PaletteViewer. Similarly, the viewer must have a 
TemplateTransferDropTargetListener. Since a template is model-specific, the 
application must extend the drop target listener to convert the template into a 
CreationFactory for the request.
A special palette entry called the CombinedTemplateCreationEntry 
supports both the creation tool and drag-and-drop style of creation.
Consuming CreateRequests
The target editpart is responsible for showing feedback and returning the command for creation. GEF provides two types of policies for handling creation. One type of policy is specific for the view in which creation occurs, either graphical or tree-based. This edit policy corresponds to either the LAYOUT_ROLE or the TREE_CONTAINER_ROLE.
The other type of edit policy is specific to only the model, should applications wish to separate out the portion that is shared between graphical and non-graphical creation. In most cases, any kind of shared logic is in the command implementations, making this type of policy unnecessary.
A LayoutEditPolicy handles the 
process of creation based on the container's layout manager. For example, if the 
XYLayout is being used, then the resulting Command will need to associate an (x, 
y, w ,h) constraint with the created child. Layouts that don't use constraints 
would require that the index of the drop location be determined. GEF provides 
abstract policies for the basic layout types.
The TreeContainerEditPolicy is used for supporting creation in a 
tree-based viewer. The policy is responsible for determining the index of 
creation and showing feedback.
The PasteTemplateAction can be used to create objects 
without the mouse. This is important for accessibility. The CopyTemplateAction 
is added to the Palette. When the user invokes this action, an internal 
mechanism copies the transfer so that it can be pasted in a viewer. When paste 
occurs, the PasteTemplateAction retrieves the template object, constructs a 
CreateRequest, and sends it to the selected editpart. Mouse location is not 
available in this interaction. Paste is only enabled when the selection is 
exactly one editpart.
 When a creation command is 
redone, it must restore the original child that was created the first time it 
was executed. If it creates a new object, then subsequent commands will fail on 
redo when they try to modify the originally created child.
When a creation command is 
redone, it must restore the original child that was created the first time it 
was executed. If it creates a new object, then subsequent commands will fail on 
redo when they try to modify the originally created child.
| Tools | Requests | Edit Policies and Roles | Actions | |||
| DragEditPartsTracker ResizeTracker | ChangeBoundsRequest AlignmentRequest 
 | LayoutEditPolicy ResizableEditPolicy ContainerEditPolicy | AlignmentAction MatchSizeAction | 
|  move interaction |  resize interaction | 
The DragEditPartsTracker extends basic selection behavior to allow 
the selected parts to be dragged within their graphical viewer. Dragging the 
selected parts can result in three potential interactions: move, reparent, and 
clone. All three use the ChangeBoundsRequest, which extends GroupRequest to 
include a size delta, move delta, and mouse location.
While dragging the selection, if the tracker targets the part's 
original parent, the request is typed as REQ_MOVE. If the target changes, the 
interaction becomes a reparent. For a reparent, a request of type REQ_ORPHAN 
is sent to the old parent, while the new target is sent a request of type REQ_ADD. When the CTRL key is pressed (ALT on 
the Mac), the operation is always a REQ_CLONE, which is only sent to the target 
part.
All of these requests are related in that they require the target to process a rectangle and a mouse location. The LayoutEditPolicy is responsible for handling each of these request types. For layouts which use constraints, each part's original bounds is taken and modified by the size and move deltas to determine a new bounds, for which a corresponding constraint is found. For index-based layouts, the mouse location is used to establish the new index.
A ContainerEditPolicy can optionally be used to contribute 
additional commands (not related to the layout) during ADD, ORPHAN, and CLONE 
requests.
Resizing
Resizing falls under the same category as changing bounds. Note 
that when resizing either the top or left sides, the location of the part is 
also changed. Resizing only makes sense for layouts with constraints, such as 
XYLayout. The ResizableEditPolicy adds up to eight resize handles 
to its host. When the Selection Tool is clicked on one of these resize handles, 
a ResizeTracker performs a resize on the selected parts 
understanding "resize". SHIFT and 
CTRL key modifiers can be used to constrain the resize operation.
The types of handles available on an editpart depend on the layout manager in which its figure is placed. For example, parts inside a table might have handles for adjusting insets, padding, column span, or other attributes. Some layouts don't need any handles, but four corner handles should be added just to indicate selection. Dragging these handles would be the same as dragging the part itself.
Because of the relationship between handles and layouts, it is 
recommended that the PRIMARY_DRAG_ROLE editpolicy be installed by the parent's 
LayoutEditPolicy, which defines abstract methods for this 
purpose. If a container changes layout managers during editing, typically the 
layout policy gets swapped with one for the new layout manager. The new 
policy then replaces the stale PRIMARY_DRAG_ROLE policies on each child.
The MatchSizeAction matches the size of the selected 
parts to that of the primary selected part's size. This action is implemented in 
a way similar to manually resizing the individual parts, and it uses the same 
request and type.
The AlignmentAction 
uses an AlignmentRequest, which extends ChangeBoundsRequest. When using 
a ChangeBoundsRequest, the part's current placement in the Control (in 
absolute coordinates) is passed to the request, which then returns a modified 
version. Using this pattern, alignment is able to adjust each part's rectangle by 
different amounts. In most cases, alignment can be treated no differently than a 
move. This action aligns all selected parts with one of the edges of the primary 
selected part.
| Tools | Requests | Edit Policies and Roles | Actions | 
| ConnectionCreationTool ConnectionDragCreationTool | CreateConnectionRequest REQ_CONNECTION_START REQ_CONNECTION_END | GraphicalNodeEditPolicy NODE_ROLE | 

The ConnectionCreationTool is used to create a new connection between nodes. This interaction requires the user to activate the tool (typically 
using the palette), and then click on two editparts supporting 
connections. The creation can be aborted by pressing the ESC key. The 
ConnectionDragCreationTool is similar, but the interaction is a single mouse 
drag. This tool can be returned as the drag tracker from a handle or even an 
editpart in some cases.
The process is separated into two parts. The first part is defining the 
source 
of the connection. The source is a node, but it may also include a specific 
"port" on that node. The tool uses a CreateConnectionRequest 
identified by REQ_CONNECTION_START to determine the target editpart and ask it 
for a command. However, this is only the first half of creating the connection, 
so the command is not complete yet. The tool will not attempt to execute this 
command or even ask if it is executable. This command is only used to pass 
information to the target editpart.
The second part is to define the target node for the connection. The tool uses the same request but retypes it as REQ_CONNECTION_END. The command returned by the source node is now stored on the request and passed during the second part of the interaction. The target is asked for the final command that performs the entire creation of the connection. Any command can be returned at this point, including the command provided on the request, updated with the target node information. At this point, enablement will be determined by asking the command if it can be executed. Creation ends by executing the command.
During the first and second steps, the editpart being targeted as the source or target node is asked to show target feedback. The editpart might visually highlight various attachment points or simply indicate that it is the target.
The source node editpart is also asked to show source feedback during creation. 
The provided GraphicalNodeEditPolicy can be used to display creation feedback. 
This policy will create a connection feedback figure, and set its anchors using 
the NodeEditPart interface. This mix-in interface for 
GraphicalEditPart is used both during creation feedback, and when the 
connection's editpart is created, to provide anchor points.
The "source" and "target" nodes should not be confused with "source" and "target" feedback. For feedback, "source" simply means show the feedback for the connection, while "target" means highlight the mouse target.
| Tools | Requests | Edit Policies and Roles | Actions |  | 
| ConnectionEndpointTracker | ReconnectRequest REQ_RECONNECT_SOURCE REQ_RECONNECT_TARGET | ConnectionEndpointEditPolicy ENDPOINT_ROLE GraphicalNodeEditPolicy NODE_ROLE | 

By dragging the endpoints of an existing connection, its source or target can be changed. This includes changing "ports" on the same node editpart. This interaction is called reconnecting.
A connection adds handles at its endpoints by installing a 
ConnectionEndpointEditPolicy with the ENDPOINT_ROLE. Each of these handles returns a 
tracker for reconnecting the corresponding end of the connection. This policy is 
also responsible for showing the connections feedback during the interaction. 
This policy does not return commands and therefore is not abstract. The 
reconnect command comes from the new target node.
As the source or target endpoint is dragged, the tracker sends 
source feedback requests to the connection, and target feedback requests to the 
current target if there is one. The tracker uses a ReconnectRequest 
typed as either a source or target reconnect.
The target node's GraphicalNodeEditPolicy is responsible for showing target 
feedback and returning the actual command to perform the reconnect. As with 
creation, the target editpart should implement the NodeEditPart 
interface, which allows the ConnectionEndpointEditPolicy to snap the feedback to 
the nodes anchor(s).
| Tools | Requests | Edit Policies and Roles | Actions | 
| ConnectionBendpointTracker | BendpointRequest REQ_MOVE_BENDPOINT REQ_CREATE_BENDPOINT | BendpointEditPolicy CONNECTION_BENDPOINTS_ROLE | 
| 
 Each handle provides a  | 
| 
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