2006. november 3., péntek

Trapping Messages Sent to an Application


I wrote code for the OnMessage event handler of Application object to trap all Windows messages sent to my application, but it doesn't seem to fire on all messages. Is there a way to trap all messages sent to my application?


There sure is. And the answer to this "problem" is amazingly simple. But before I go into trapping messages at the application level, I should probably discuss some mechanics.

TApplication's "Hidden" Window
It's not a commonly known fact that the default Application object creates a hidden window when your application is started. But you can seen evidence of this by creating a new application saving it, then running it (make sure you don't rename anything - just keep the main form as "Form1" and the project as "Project1). When you run the application, you'll notice that the caption bar for your main form says, "Form1" while the icon displayed on the task bar says "Project1." That icon represents the application's hidden window, and it affects your program in many ways, especially when you're trying to handle messages sent to your application.
Delphi surfaces the OnMessage event for the Application object. The OnMessage event handler is "supposed" to allow you trap every message sent to your application. But there's a problem with this: OnMessage will only fire when there's something in the Application object's message queue. These messages are typically window management messages such as WM_PAINT or messages sent to the application from Windows through PostMessage, Broadcast or SystemMessage . However, messages sent directly to a window using SendMessage bypass the Application object's message queue, so OnMessage doesn't fire for those types of situations.
Some of you more familiar with handling windows messages might think that a solution to the problem above might be to override the WndProc method for the Application object. Unfortunately, that's not possible because TApplication's WndProc method is not only private, it's also declared as a static method which means it's not overrideable. So it's not only invisible, you can't create a TApplication subclass to override WndProc (not that you'd want either). But that doesn't mean that you can't get to the WndProc method using alternative means.

"Hooking" All Messages
Even though WndProc is all but closed to direct subclassing, TApplication does include a method called HookMainWindow that allows you to insert your own message handler at the top of WndProc to intercept messages sent to your application before they're handled by the Application object. This is convenient for all developers, and solves the problem of trapping any message sent to your application.
HookMainWindow is declared under TApplication as follows:

procedure HookMainWindow(Hook: TWindowHook);

Notice that HookMainWindow takes one parameter, Hook of type TWindowHook. TWindowHook is a method pointer type that's defined like so:

  TWindowHook = function(var Message: TMessage): Boolean of object;

Since TWindowHook is a method pointer, you can define your own method as the hook function as long as it follows the nomenclature defined for TWindowHook. Notice that the return value of the function is of type Boolean. This is the equivalent of the "Handled" parameter of OnMessage. If your function handles a particular message, you'd return true. This will be passed back to the Application's WndProc and message processing for that message will be terminated. Otherwise, you'd return False. Here's an example method:

function TForm1.AppHookFunc(var Message: TMessage): Boolean;
  Result := False; //I just do this by default
  if Message.Msg = WM_ < SomethingOrOther > then
      Result := True;

Okay, now that we've set up everything, we need to make the application hook the messages. This can be done in the main form's OnCreate method:

function TForm1.FormCreate(Sender: TObject);

I should mention that you need to clear the hook using, you guessed it, UnHookMainWindow, after you're done using it, and this can be done in the OnDestroy for the main form:

function TForm1.FormDestroy(Sender: TObject);

Okay, disgustingly simple. But I feel the best things in life are those that give maximum satisfaction for the least amount of cost (please don't read ANYTHING into that <G>). So, now you've got the tools to create your own message "hooker" (sorry, had to do that at least once). Until next time...

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