Layers Overview
This page discusses the following topics:
Layer
Defined
Layer
Attributes
Information
Returned by a Layer
Opening
and Closing Layers
The
Buffer Layer
The control is made up of one or more drawing layers. A layer is what shapes are drawn on, and has several
attributes. Only one layer is the current drawing layer at any time. When the control is first initialized
at run-time, two layers are opened, the Buffer Layer and Layer 1. More will be said about the buffer layer
later on.
Layers are the same as the control view. Layers are transparently stacked on top of one another, with
shapes in the bottommost layer drawn first, and the topmost layer (highest in number) drawn last.
The idea behind multiple layers is that shapes with similar meaning and attributes can be kept on
the same layer. Then that layer can be manipulated without affecting shapes on other layers.
For instance, say we had one layer with rectangles, another with circles, and a third with triangles.
Looking at each layer separately:
Layer 1 
Layer 2 
Layer 3 
When viewed on the control, it would look like this, with the bottommost layer drawn first:
Name and Number
A layer has a string (LayerName)
and number (LayerNumber)
associated with it. LayerNumber reflects the current drawing layer, and can be used in calls to
methods and when referencing properties. LayerName can be used as an identifier, as well as to
return the LayerNumber it represents with LayerFindByName.
Colors
A layer has a color
state, with pen and brush settings. When shapes are added to a layer, they take on the layer's color
state if there is no cad level color state. The initial setting is a solid black pen with a transparent
brush.
Below we see two layers with their own color states. The first has a green fill with diagonal lines,
the second has a blue fill with horizontal lines. Notice that the blue shapes are 'on top of' the green
shapes, because the drawing order is from the smallest layer number to the largest.
Action
A layer has a LayerAction
property that defines how shapes on that layer interact with the control. LayerAction sets whether
or not shapes can be found with the mouse, moved, or displayed at all.
Search Method
Shapes can be selected by the mouse depending on the search method. By default, most shapes
are set so that they can only be selected by clicking their border. This can be changed so that clicking
the inside of the shape will select it. The property for doing this is called LayerSearchMethod.
Note that changing this property only affects shapes drawn after the change.
Information Returned by a Layer
The number of shapes on a layer is returned by LayerShapeCount.
The numbers of the shapes on a layer can be returned in an array via LayerShapeNumbers.
A layer is opened with the LayerOpen
method. When a layer is opened, it becomes the current drawing layer.
A layer is closed with the LayerClose
method. Closing a layer completely deletes all shapes on the layer. Closing a layer also affects any existing
layer numbers. If existing layer numbers are higher than the one deleted, then they are changed to be
one less than before. For instance, if we had five layers numbered 0 through 4, and layer 2 was deleted,
layers 3 and 4 would be renumbered to 2 and 3, respectively. Use LayerFindByName
when necessary to find a layer's number with respect to its name.
The buffer layer is provided as a means for hiding shapes in the control. It always has a LayerNumber
of 0, with a LayerName
of "Buffer Layer". The buffer layer cannot be deleted. Its LayerAction
property is set so that shapes on it are not shown.
Some uses for this layer are:
Adding shapes programmatically. Since the points for shapes have to be set one at a time, you may want to hide the shape until its drawing is completed.
Hiding shapes. Call ShapeToLayer to move shapes from one layer to another.
Copying shapes. When using a common shape, you may want to keep it in the buffer layer, without displaying it. ShapeToLayer can be used to copy shapes from one layer to another.
Simulating 'undo'. As shapes are deleted, you could make a copy to the selection buffer. Then an 'undo' would be possible by restoring the shape from the buffer layer.
Setting the color state for selected shapes. See Shape Selection Overview for more information.
See Also
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Last modified on: Tuesday, February 04, 2003