Display Feedbacks

The set of objects that implement the IDisplayFeedback interface gives you fine-grained control over customizing the visual feedback when using the mouse to form shapes on the screen display. You can direct the precise visual feedback for tasks, such as adding, moving, or reshaping features or graphic elements. The objects can also be used without creating any features or elements for a task, such as measuring the distance between two points. Typically, you would use the display feedback objects within code that handles the mouse events of a tool based on the ITool interface, such as OnMouseDown and OnMouseMove. Which mouse events to program depends on the task at hand. For example, when adding a new envelope, you would program the display feedback objects in the OnMouseDown, OnMouseMove, and OnMouseUp events. Or, when digitizing a new polygon, you would program the OnMouseDown, OnMouseMove, and OnDblClick events. When you are collecting points with the mouse to pass to the display feedbacks, you can use the ToMapPoint method on IDisplayTransformation to convert the current mouse location from device coordinates to map coordinates. Although the feedback objects (excluding the GroupFeedback object) all have common functionality, their behavior does vary. These variations can be divided as follows:

1. Feedbacks that return a new geometry. The interfaces for these objects have a Stop method that returns the new geometry. These objects are NewEnvelopeFeedback, NewBezierCurveFeedback, NewDimensionFeedback, NewLineFeedback, NewPolygonFeedback, MoveEnvelopeFeedback, MoveLineFeedback, MovePointFeedback, MovePolygonFeedback, BezierMovePointFeedback, LineMovePointFeedback, PolygonMovePointFeedback, ReshapeFeedback, ResizeEnvelopeFeedback, and StretchLineFeedback.
2. Feedbacks that are for display purposes only. The developer is required to calculate the new geometry. For example, you can use the start and end mouse locations and calculate the delta x and delta y shifts, and then you can update or create the geometry from this. These feedback objects are MoveGeometryFeedback, MoveImageFeedback, NewMultiPointFeedback, and VertexFeedback.

 

The objects are used within applications to allow graphic elements to be digitized and modified within the map (data view) and layout (layout view) and are also used by the ArcMap feature editing tools. Some of the feedback objects have a Constraint property that determines how the feedback behaves. These constraints can specify, for example, that a ResizeEnvelopeFeedback maintains the aspect ratio of the input Envelope. The details of these constraints are given with the individual feedbacks. The display feedback objects also provide some of the base functionality for the rubberband objects described earlier. You should use the rubberband objects first if they suit your requirements; select the display feedback objects if you want greater control over the user interface when modifying graphics or features. This greater control comes at the cost of more code.

The IDisplayFeedback interface is used to define the common operations on all of the display feedback operations. These include moving, symbolizing, and refreshing the display feedbacks as well as setting a display feedback object's Display property (for example, setting it to IActiveView::ScreenDisplay). The IDisplayFeedback interface is useful only in combination with one of the display feedback objects and its derived interfaces, for example, the NewPolygonFeedback object and its INewPolygonFeedback interface. Nearly all of the display feedback interfaces employ interface inheritance from IDisplayFeedback; hence, there is no need to use Caching to access its methods and properties. Typically, the Display and Symbol properties would be set when a display feedback object is initialized, while the MoveTo method would be called in a mouse move event. Setting the Symbol property is optional. If not set, a default symbol is used. The Refresh method is used to redraw the feedback after the window has been refreshed (for example, when it is activated again), and it should be called in response to the Tool's Refresh event. The hDC parameter, which is required by the Refresh method, is actually passed into the subroutine for you. In the following example, a check is first made to see if m_pNewPolyFeedback, which is a member variable NewPolygonFeedback object, has been instantiated yet, that is, if the user is currently using the feedback. If it has been instantiated, then the Refresh method is called.

[C#]

private void Refresh(int hDC) 
    { 
     if (!(m_pNewPolyFeedback == null))
       { 
         m_pNewPolyFeedback.Refresh(hDC); 
       } 
    }

The following code example shows how to use the IDisplayFeedback interface with the INewEnvelopeFeedback interface to create a display feedback that will allow the user to add a new polygon. Note that this code simply demonstrates the visual feedback; further code is required if you wish to add that drawn shape as a map element or feature. The new envelope feedback object is declared as a member variable as follows:
[C#]

private INewEnvelopeFeedback pNewEnvFeed;

Other objects are locally declared—pEnv as IEnvelope, pScreenDisp as IScreenDisplay, pLineSym as ISimpleLineSymbol, and pStartPoint and pMovePoint as IPoint. The following code would be placed in the OnMouseDown event to set up the Display and Symbol properties and to call INewEnvelopeFeedback::Start with the current mouse location in map units.
[C#]

pNewEnvFeed = new NewEnvelopeFeedbackClass(); 
    pNewEnvFeed.Display = pScreenDisp; 
    pNewEnvFeed.Symbol = pLineSym as ISymbol; 
    pNewEnvFeed.Start(pStartPoint);

The following line of code would be placed in the OnMouseMove event to move the display feedback to the current mouse location in map units, using the MoveTo method from IDisplayFeedback.

 

[C#]

pNewEnvFeed.MoveTo (pMovePoint)

The following line of code would be placed in the OnMouseUp event to return the result using the Stop method from INewEnvelopeFeedback.
[C#]

pEnv = pNewEnvFeed.Stop;