How does Delphi's KeyPreview work?

Question

A colleague and myself were debating over which way was less of a burden on the system resources. (Note: this is not the question I want an answer to. Rather the title and t

Answer 1

The bottom line here is that you can't generate input quick enough to make the computer even notice. The computer would not be troubled if you produced input messages at rates hundreds or even thousands greater than you typically do.

You won't be able to measure the difference between handling something in OnMessage and using KeyPreview.

So the decision as to which to use comes down to which is most convenient. If you need handling to happen at an application wide level, and you don't have a common base class for all your forms, then you use OnMessage. If you want different behaviour for different forms then you need to use KeyPreview.

Personally I strongly recommend refactoring so that all forms in your projects derive from a common base (a subclass of TForm). This allows you much more flexibility. Done this way you can, for example, use the KeyPreview mechanism to apply intervention points for all forms in your applications.

As for how KeyPreview is implemented, the input messages get redirected in KeyDown, KeyPress etc. in TControl. To learn more read the source code.