Canvas

From Real Software Documentation

Canvas controls are very useful for implementing your own graphical controls because you can use the Graphics class drawing commands or the Object2D classes to draw inside the Canvas region. It can also be used to display existing graphics, like the ImageWell.

Events

Activate KeyDown Close KeyUp ConstructContextualMenu LostFocus ContextualMenuAction MouseDown Deactivate MouseDrag DoubleClick MouseEnter DragEnter MouseExit DragExit MouseMove DragOver MouseUp DropObject MouseWheel EnableMenuItems Open GotFocus Paint

Properties

AcceptFocus HelpTag PanelIndex AcceptTabs Index Parent Active Left Scope AutoDeactivate LockBottom TabIndex Backdrop LockLeft TabStop DoubleBuffer LockRight Top Enabled LockTop TrueWindow EraseBackground MouseCursor UseFocusRing MouseX Visible Handle MouseY Width Height Name Window

Methods

AcceptFileDrop Invalidate AcceptPictureDrop Refresh AcceptRawDataDrop RefreshRect AcceptTextDrop Scroll Close SetFocus DrawInto

Notes

Coordinates passed to Canvas events are local to the Canvas object.

To use the Scroll method to scroll the picture in a Canvas control, you need to store the last scroll value for the axis you are scrolling so you can use this to calculate the amount to scroll. This can be done by adding properties to the window that contains the Canvas control or by creating a new class based on the Canvas control that contains properties to hold the last X scroll amount and last Y scroll amount.

If the ScrollControls parameter is True, any controls on top of the Canvas control will also be scrolled. This allows the implementation of a scrolling pane of controls.

The following example scrolls a picture that was added to the project. The properties XScroll and YScroll have been added to the window to hold the amounts the picture has been scrolled. The picture is scrolled 8 pixels at a time. In the Keydown event of the window, the following code calls the Scroll method whenever the Up, Down, Left, or Right arrow keys are pressed.

The Scroll method calls the Paint event of the canvas that redraws the picture with the new values of XScroll and YScroll. The Paint event has the following line of code:

Examples

This example draws a 3D rectangle with a raised look.

This example assigns a picture that has been added to the Project Editor to the Backdrop property in the Open event:

You can use the methods of the Graphics class to modify the picture in any way you like. For example, the following code in the object's Paint event handler adds a caption to the picture:

If you instead assigned the graphic to the BackDrop property using the Picture constructor, as shown below, you could manipulate the graphic at the pixel level using the RGBSurface property of the Picture object.

See also the examples for the Control class, which give an example of dragging from a Canvas control and the ImageWell class example, which show drag and drop to and from the ImageWell.Image property.

Simulating a Focus Ring on Windows and Linux

Unfortunately, a Focus Ring does not appear on Windows or Linux when a Canvas control gets the focus, but the GotFocus and LostFocus events fire normally. You can easily use them to simulate a focus ring.

In the GotFocus event, use:

In the LostFocus event, use:

The simulated focus ring looks like this:

See Also

Graphics, Picture classes.Link title