Custom NSButton with semi-transparent background

Question

I'm trying to create a custom NSButton with a 50% opaque black background and white text. To do this I've subclassed NSButton and overloaded DrawRect:

- (void) drawRect:(NSRect)dirtyRect
{

    [self setBordered:NO];

    //REMED since it has same effect as NSRectFill below
    //[[self cell] setBackgroundColor:[NSColor colorWithCalibratedRed:0 green:0 blue:0 alpha:0.2]];

    NSColor* backgroundColor = [NSColor colorWithCalibratedWhite:0 alpha:0.3f];
    [backgroundColor setFill];
    NSRectFill(dirtyRect);

    [super drawRect:dirtyRect];
}

The white text appears fine but the button's background is always 100% opaque. The alpha value is not interpreted.

Any ideas? Thanks!

Answer 1

The default operation of NSRectFill() is copy which is not what you want. Replace it with

NSRectFillUsingOperation(dirtyRect, NSCompositeSourceAtop);

Answer 2

Another solution I found was to keep my code the same but turn on the Core Animation Layer for each button in Interface Builder. I don't know enough about Core Animation Layer to know why this worked. I had previously turned CAL off because it was making my fonts look very jagged.