你需要在位图上下文中创建图像的副本，通过绘制叠加或以某种方式处理它来修改副本，然后将修改后的图像分配给按钮的 UIControlStateSelected 状态。

You need to create a copy of the image in a bitmap context, modify the copy by drawing an overlay or processing it in some way, and then assign the modified image to the button's UIControlStateSelected state.

下面是一些示例代码，用于创建上下文并在图像上为按钮的选定状态绘制50％的黑色覆盖图，使其变暗：

Here is some sample code that creates the context and draws a 50% black overlay on the image for the selected state of the button, which darkens it:

//assume UIImage* image exists

//get the size of the image
CGFloat pixelsHigh = image.size.height;
CGFloat pixelsWide = image.size.width;

//create a new context the same size as the image
CGContextRef    cx = NULL;
CGColorSpaceRef colorSpace;
void *          bitmapData;
int             bitmapByteCount;
int             bitmapBytesPerRow;

bitmapBytesPerRow   = (pixelsWide * 4);
bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

colorSpace = CGColorSpaceCreateDeviceRGB();
bitmapData = malloc( bitmapByteCount );
if (bitmapData == NULL)
{
    fprintf (stderr, "Memory not allocated!");
    return;
}
cx = CGBitmapContextCreate (bitmapData,
                                 pixelsWide,
                                 pixelsHigh,
                                 8,      // bits per component
                                 bitmapBytesPerRow,
                                 colorSpace,
                                 kCGImageAlphaPremultipliedLast);
if (cx == NULL)
{
    free (bitmapData);
    fprintf (stderr, "Context not created!");
    return;
}
CGColorSpaceRelease( colorSpace );
CGRect imageRect = CGRectMake(0, 0, pixelsWide, pixelsHigh);

//draw the existing image in the context
CGContextDrawImage(cx, imageRect, image.CGImage);

//draw a 50% black overlay
CGContextSetRGBFillColor(cx, 0, 0, 0, 0.5);
CGContextFillRect(cx, imageRect);

//create a new image from the context
CGImageRef newImage = CGBitmapContextCreateImage(cx);
UIImage* secondaryImage = [UIImage imageWithCGImage:newImage];
CGImageRelease(newImage);
CGContextRelease(cx);

//assign the images to the button
[button setBackgroundImage:image forState:UIControlStateNormal];
[button setBackgroundImage:secondaryImage forState:UIControlStateSelected];