Ruby iterators and C callback functions

Michael Geary | Mon, 2004-08-09 00:00

Mike Sax wonders what’s the fuss about iterators. Aren’t they just a fancy use of function pointers? Indeed, Mike has hit the nail on the head. Consider the window iterator that’s been built into Windows since 1.0:

BOOL EnumWindows( WNDENUMPROC lpEnumFunc, LPARAM lParam );

This function iterates through all of the top-level windows (children of the desktop window) and calls lpEnumFunc for each one, passing it the HWND of each window and the lParam that you passed to EnumWindows.

So lParam is how you get to provide some state that the enumeration function can make use of. Suppose you wanted to write a function that counted the number of visible top-level windows. Your C code might look like this:

struct MyEnumWindowState
{
    int nVisible;
};

BOOL CALLBACK MyEnumWindowsProc( HWND hwnd, LPARAM lParam )
{
    MyEnumWindowState* pState = (MyEnumWindowState*)lParam;
    if( IsWindowVisible(hwnd) )
        pState->nVisible++;
}

int CountVisibleWindows()
{
    MyEnumWindowState state;
    state.nVisible = 0;

    EnumWindows( MyEnumWindowsProc, (LPARAM)&state );
    return state.nVisible;
}

This works, but it is rather tedious. So Windows 2.0 added the GetWindow function, which lets you simply ask for a window’s child or next sibling. That simplifies the overall structure of the code, especially if you use the GetFirstChild and GetNextSibling macros defined in windowsx.h:

int CountVisibleWindows()
{
    int nVisible = 0;
    HWND hwnd = GetDesktopWindow();

    for( hwnd = GetFirstChild(hwnd);
         hwnd;
         hwnd = GetNextSibling(hwnd) )
    {
        if( IsWindowVisible(hwnd) )
            nVisible++;
    }

    return nVisible;
}

That’s it, just one function, no callback function or struct definition needed. We don’t need the struct because the code inside the loop can directly reference the nVisible variable defined in the function.

But the simplification came at a price: We had to write the loop ourselves, asking explicitly for the first child of the desktop window and then the next sibling of each child window.

Also, it doesn’t work.

What if another application creates or destroys a top-level window, or just changes a window’s Z-order, while you’re in the loop? You’ll either miss a window, count one twice, or crash with an invalid window handle.

To handle these cases, you need a bit more complexity. If you had a way to temporarily lock all window creation and destruction, you could quickly create a list of all the windows and then release the lock, then enumerate from that list, perhaps also doing a last-minute check when you enumerate each window to skip any that get destroyed during enumeration. Or, you might set a Windows hook to notify you of any windows created, destroyed, or moved in the Z-order, so you could deal with them appropriately.

Whatever you did, it would be enough code that you wouldn’t want to duplicate it each time you wanted to write a window loop. The GetFirstChild/GetNextSibling style of loop doesn’t really facilitate that kind of code isolation. The EnumWindows style enumerator completely separates the code that does the iterating (EnumWindows itself) from the code that receives the iteration (your callback function). But, it makes it harder to share state between the callback function and the code that called EnumWindows.

If you had a way to use a callback function, but have it more easily share state with the calling function, you’d have a winner. In C# and JavaScript, you can do this by using an anonymous callback function nested inside the surrounding code. Because of lexical scoping, the callback function can access variables in the parent function as easily as it can access its own.

Both those language have enough extra syntactic cruft that when you look at a simple example using nested anonymous functions, it’s easy to be unimpressed. The payoff shows up in more complicated, real-life coding situations.

Code blocks in Ruby simplify this technique down to its essence, making it useful even for simple cases. Assuming a good Rubyesque Windows interface library, our function might be something like:

def countVisibleWindows()
    nVisible = 0

    Win.enumWindows do |window|
        nVisible += 1 if window.visible?
    end

    nVisible
end

In this code, the enumWindows function takes a code block argument and calls that code block for each window, passing it the window as an argument. Because the code block is nested inside the countVisibleWindows function, it can access the nVisible variable directly.

This solves both our problems: The logic for iterating through the windows is separated out into the enumWindows function, and the callback function (code block) can access state variables cleanly and easily.

(In Ruby, a code block is a like a callback function, but it’s not quite a full-fledged function. A code block does not introduce a new scope for variables—it shares the scope of the enclosing function.)

Unfortunately, Ruby does not seem to have a Windows interface library that works like this. Ruby’s standard Win32 module provides a general way to call Windows DLL functions, but it doesn’t have a clean implementation of enumWindows that uses a code block.

However, MoonWolf has written a Ruby port of Perl’s Win32::GuiTest module that includes this kind of enumWindows function. It’s implemented in two parts: a low level function written in C that enumerates HWND values, and a higher level function written in Ruby that constructs Ruby window objects and enumerates them. The window object in Win32::GuiTest is a fairly thin wrapper that encapsulates an HWND and other window information.

The high-level enumWindows looks like this:

def enumWindows
    ret = []

    GuiTest::_enumWindows { |hwnd|
        win = createWindow( hwnd )
        ret << win
        yield win if block_given?
    }

    ret
end

This code calls the low-level _enumWindows function, which passes an HWND to the code block enclosed in curly braces. This code block creates the window object, appends to the ret array, and also yields the window object to a code block that was provided by the caller of enumWindows.

If I were implementing this, I think I would change it a bit. Typically a function like this either yields results to a code block, or it returns a value, but not both. And I would change the confusingly named createWindow function (which has no relation to the CreateWindow function in Windows):

def enumWindows
    if block_given?
        GuiTest::_enumWindows do |hwnd|
            yield newWindow( hwnd )
        end
    else
        result = []
        GuiTest::_enumWindows do |hwnd|
            result << newWindow( hwnd )
        end
        result
    end
end

Either way, our countVisibleWindows example ends up pretty much as I’d imagined:

def countVisibleWindows()
    nVisible = 0

    Win32::GuiTest.enumWindows do |window|
        nVisible += 1 if window.isWindowVisible
    end

    nVisible
end

The low-level enumWindows function that enumerates HWND values is implemented in C. The initialization code to add the enumWindows function is simply:

rb_define_module_function( mGuiTest, "_enumWindows", guitest_enumWindows, 0 );

where mGuiTest is a reference to the Win32::GuiTest module.

The guitest_enumWindows function is:

static VALUE guitest_enumWindows( VALUE self )
{
    EnumWindows( &EnumWindowsProc, 0 );
    return Qnil;
}

and the EnumWindowsProc callback is:

BOOL CALLBACK EnumWindowsProc( HWND hwnd, LPARAM lParam )
{
    rb_yield( INT2NUM((DWORD)hwnd) );
    return TRUE;
}

This shows how easy it is to extend Ruby with C code, adding functions that work just like ones written in Ruby.

So, how do all the calls and callbacks stack up when we run the countVisibleWindows function? Something like this:

countVisibleWindows
  enumWindows
    _enumWindows
      EnumWindows
        EnumWindowsProc
          rb_yield
            (code block in enumWindows)
              yield
                (code block in
                 countVisibleWindows)

In everyday use, of course, you don’t worry about that whole call stack, just the part of it you’re working with.

Submitted by Visitor on Thu, 2007-05-24 17:28.

Building win32 guitest in May of 2007 is non-trivial. I posted a recipie at http://www.kleinfelter.com/build-ruby-guitest-win32

Submitted by Frank (not verified) on Tue, 2004-08-17 15:33.

Just can’t get away from that Ruby code, eh, Mike?

Submitted by Michael Geary (not verified) on Wed, 2004-09-01 13:45.

Hi Frank! :-) Yep, I’m stuck with Ruby forever, I guess.

Anyone who wonders what the heck Frank is talking about, here is the full story.