how to darken a UIImageView

Question

I need to darken a UIImageView when it gets touched, almost exactly like icons on the springboard (home screen).

Should I be added UIView with a 0.5 alpha and black background. This seems clumsy. Should I be using Layers or something (CALayers).

Answer 1

I would let a UIImageView handle the actual drawing of the image, but toggle the image to one that's been darkened in advance. Here's some code I've used to generate darkened images with alpha maintained:

+ (UIImage *)darkenImage:(UIImage *)image toLevel:(CGFloat)level
{
    // Create a temporary view to act as a darkening layer
    CGRect frame = CGRectMake(0.0, 0.0, image.size.width, image.size.height);
    UIView *tempView = [[UIView alloc] initWithFrame:frame];
    tempView.backgroundColor = [UIColor blackColor];
    tempView.alpha = level;

    // Draw the image into a new graphics context
    UIGraphicsBeginImageContext(frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [image drawInRect:frame];

    // Flip the context vertically so we can draw the dark layer via a mask that
    // aligns with the image's alpha pixels (Quartz uses flipped coordinates)
    CGContextTranslateCTM(context, 0, frame.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextClipToMask(context, frame, image.CGImage);
    [tempView.layer renderInContext:context];

    // Produce a new image from this context
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    UIImage *toReturn = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    UIGraphicsEndImageContext();
    [tempView release];
    return toReturn;
}

Answer 2

How about subclassing UIView and adding a UIImage ivar (called image)? Then you could override -drawRect: something like this, provided you had a boolean ivar called pressed that was set while touched.

- (void)drawRect:(CGRect)rect
{
[image drawAtPoint:(CGPointMake(0.0, 0.0))];

// if pressed, fill rect with dark translucent color
if (pressed)
    {
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSaveGState(ctx);
    CGContextSetRGBFillColor(ctx, 0.5, 0.5, 0.5, 0.5);
    CGContextFillRect(ctx, rect);
    CGContextRestoreGState(ctx);
    }
}

You would want to experiment with RGBA values above. And, of course, non-rectangular shapes would require a bit more work - like a CGMutablePathRef.