Subclassing and Hooking with Visual Basic

Description

You use the WH_FOREGROUNDIDLE hook mainly to allow an application to do background processing when it is about to go into an idle state. This type of processing is normally called background processing because it occurs after other more important processing has finished. An application achieves an idle state when it has no more messages in the message queue to process.

To be more specific, the system calls this hook when the foreground thread is about to enter an idle state. Every application has one foreground thread; any other thread is considered a background thread. Because Visual Basic (VB) applications run in a single thread, this thread is considered the foreground thread. If a VB application creates multiple threads, the system will call the hook only when the main or foreground thread is about to go into an idle state.

When the application is about to enter an idle state, the system will check to see if any WH_FOREGROUNDIDLE hooks were installed. If so, the system calls the first filter function installed in the chain for this hook.

You can use this hook as a thread-specific hook, or you can place it in a dynamic link library (DLL) and use it as a system-wide hook. A thread-specific hook intercepts all messages within the thread in which it is installed. When you install a hook as a system-wide hook, you must place it in a DLL. This DLL is injected into every process so that the hook can operate on all messages in the system. Chapter 3 discusses this in detail. ...