Overview of Graphics
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Delphi encapsulates the Windows GDI at several levels. The most important to you as a component writer is the way components display their images on the screen. When calling GDI functions directly, you need to have a handle to a device context, into which you have selected various drawing tools such as pens, brushes, and fonts. After rendering your graphic images, you must restore the device context to its original state before disposing of it.
Instead of forcing you to deal with graphics at a detailed level, Delphi provides a simple yet complete interface: your component's Canvas property. The canvas ensures that it has a valid device context, and releases the context when you are not using it. Similarly, the canvas has its own properties representing the current pen, brush, and font.
The canvas manages all these resources for you, so you need not concern yourself with creating, selecting, and releasing things like pen handles. You just tell the canvas what kind of pen it should use, and it takes care of the rest.
One of the benefits of letting Delphi manage graphic resources is that it can cache resources for later use, which can speed up repetitive operations. For example, if you have a program that repeatedly creates, uses, and disposes of a particular kind of pen tool, you need to repeat those steps each time you use it. Because Delphi caches graphic resources, chances are good that a tool you use repeatedly is still in the cache, so instead of having to recreate a tool, Delphi uses an existing one.
An example of this is an application that has dozens of forms open, with hundreds of controls. Each of these controls might have one or more TFont properties. Though this could result in hundreds or thousands of instances of TFont objects, most applications wind up using only two or three font handles, thanks to a font cache.