How to make a subview tint a portion of the screen on the iPhone?

Question

I would like to write a UIView subclass that, among other things, colors whatever's underneath it a certain color. This is what I've come up with, but it unfortunately doesn't seem to work correctly:

#import <UIKit/UIKit.h>

@class MyOtherView;
@interface MyView : UIView
{
    MyOtherView *subview;
}
@end

@implementation MyView

- (id)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
     frame.origin.x += 100.0;
     frame.origin.y += 100.0;
     frame.size.width = frame.size.height = 200.0;
     subview = [[MyOtherView alloc] initWithFrame:frame];
     [self addSubview:subview];
     [subview release];
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    // Draw a background to test out on
    UIImage *image = [UIImage imageNamed:@"somepic.png"];
    [image drawAtPoint:rect.origin];

    const CGContextRef ctx = UIGraphicsGetCurrentContext();
    [[UIColor blueColor] setFill];
    rect.size.width = rect.size.height = 200.0;
    CGContextFillRect(ctx, rect);
}

@end

@interface MyOtherView : UIView
@end

@implementation MyOtherView

- (void)drawRect:(CGRect)rect
{
    // This should tint "MyView" but it doesn't.
    const CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSaveGState(ctx);
    CGContextSetBlendMode(ctx, kCGBlendModeScreen);
    [[UIColor redColor] setFill];
    CGContextFillRect(ctx, rect);
    CGContextRestoreGState(ctx);
}

@end

I want "MyOtherView" to color "MyView" red where it overlaps, but instead it just draws an opaque red block onto it. However, this seems work fine if I copy the -drawRect: function from "MyOtherView" and append it to the one in "MyView" (this took me quite a headache to finally realize). Anyone know what I'm doing wrong? Is it even possible to do this, or should I be approaching it differently?

Answer 1

I think you're over-thinking this. Overlay one view over the other, and set the alpha of the top view to 0.5. You will also need to set opaque to NO.

If you set the background color of the view appropriately, you won't even need to override drawRect.

Answer 2

Might want to check out this SO question.