如何使UIImageView变暗

Question

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

我是否应该添加0.5 alpha和黑色背景的UIView.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,但是将图像切换到预先变暗的图像.这是我用来生成alpha维护的暗图像的一些代码:

+ (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;
}

Answer 2

如何子类化 UIView 并添加 UIImage ivar（称为图像）？然后你可以重写 -drawRect: 类似的东西，前提是你有一个在触摸时设置的名为按下的布尔 ivar。

- (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);
    }
}

您可能想要尝试上面的 RGBA 值。当然，非矩形形状需要更多的工作 - 就像 CGMutablePathRef。