iphone - 如何使 UIImageView 变暗

Question

我需要在触摸 UIImageView 时使它变暗，几乎就像跳板(主屏幕)上的图标一样。

我应该添加具有 0.5 alpha 和黑色背景的 UIView 吗？这看起来很笨拙。我应该使用图层还是其他东西(CALayers)。

Answer 1

我会让 UIImageView 处理图像的实际绘制，但将图像切换为提前变暗的图像。下面是我用来生成保持 alpha 值的变暗图像的一些代码:

+ (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;
}