Repeat a semi-transparent NSImage

In a project I'm working on, I have 3 images: top, middle, and bottom. Top and bottom are fixed height, and middle should be repeated in between the two. (The window size will be changing.) They all are tinted with a color from the user preferences, then need to have their alpha set using a value from the preferences.

I can do pretty much everything. The part I get stuck at is drawing the middle. I decided using [NSColor +colorWithPaternImage:] would be the easiest thing to use. There's a lot of code that makes the actually images and colors, so just assume they exist and are not nil.

int problem; // just to help explain things

float alpha;
NSImage *middleTinted;

NSRect drawRect = [self bounds];
drawRect.size.height = self.bounds.size.height - topTinted.size.height - bottomTinted.size.height;
drawRect.origin.y = topTinted.size.height;

NSColor* colorOne = [NSColor colorWithPatternImage:middleTinted];
NSColor* colorTwo = [color开发者_如何转开发One colorWithAlphaComponent:alpha];

if(problem == 1)
{
    [colorOne set];
}
else if(problem == 2)
{
    [colorTwo set];
}

[NSBezierPath fillRect:drawRect];

Assuming problem == 1, it draws the correct image, in the correct location and with the correct size, but no alpha. (Obviously, since I didn't specify one.)

When problem == 2, I'd expect it to do the same thing, but have the correct alpha value. Instead of this, I get a black box.

Is there a solution that will repeat the image with the correct alpha? I figure I could just draw the image manually in a loop, but I'd prefer a more reasonable solution if one exists.

I expect the problem is that pattern colors don't support -colorWithAlphaComponent:.

NSCell.h contains a method called NSDrawThreePartImage that does the work of drawing end caps and a tiled image in between. It also has an alphaFraction parameter that should meet your needs.

If that doesn't work for you, then you might get the pattern color approach to work by re-rendering your middleTinted image into a new NSImage, using the desired alpha value. (See NSImage's draw... methods.)