uitextview/uiwebview中的掩码文本

对我终于明白了。我不知道这是否是苹果的方式,但它是有效的。也许他们有机会使用一些私有API。不管怎样,这是一种伪算法,它是如何工作的:

1) 获取窗口的屏幕截图

2) 使用CGImageCreateWithImageInRect裁剪所需的矩形

3) 应用渐变遮罩(摘自Apple反射示例代码)

4) 使用新创建的图像创建UIImageView

我还注意到,即使在最低的设备上,它也不会影响性能。 希望这会有帮助! 这是一个收获的结果( link text )

只需制作一个示例(仅支持横向,请参阅下面的变换,仅支持顶部遮罩)。在本例中,我覆盖了需要屏蔽的表的didmovetown:

- (void)didMoveToWindow {
    if (self.window) {

        UIImageView *reflected = (UIImageView *)[self.superview viewWithTag:TABLE_SHADOW_TOP];
        if (!reflected) {
            UIImage *image = [UIImage screenshot:self.window];

            //
            CGRect croppedRect = CGRectMake(480-self.frame.size.height, self.frame.origin.x, 16, self.frame.size.width);
            CGImageRef cropImage = CGImageCreateWithImageInRect(image.CGImage, croppedRect);
            UIImage *reflectedImage = [UIImage imageMaskedWithGradient:cropImage];
            CGImageRelease(cropImage);

            UIImageView *reflected = [[UIImageView alloc] initWithImage:reflectedImage];
            reflected.transform = CGAffineTransformMakeRotation(-(M_PI/2));
            reflected.tag = TABLE_SHADOW_TOP;
            CGRect adjusted = reflected.frame;
            adjusted.origin = self.frame.origin;
            reflected.frame = adjusted;
            [self.superview addSubview:reflected];
            [reflected release];
        }
    }
}

这是uiimage类别:

CGImageRef CreateGradientImage(int pixelsWide, int pixelsHigh)
{
    CGImageRef theCGImage = NULL;

    // gradient is always black-white and the mask must be in the gray colorspace
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

    // create the bitmap context
    CGContextRef gradientBitmapContext = CGBitmapContextCreate(NULL, pixelsWide, pixelsHigh,
                                                               8, 0, colorSpace, kCGImageAlphaNone);

    // define the start and end grayscale values (with the alpha, even though
    // our bitmap context doesn't support alpha the gradient requires it)
    CGFloat colors[] = {0.0, 1.0, 1.0, 1.0};

    // create the CGGradient and then release the gray color space
    CGGradientRef grayScaleGradient = CGGradientCreateWithColorComponents(colorSpace, colors, NULL, 2);
    CGColorSpaceRelease(colorSpace);

    // create the start and end points for the gradient vector (straight down)
    CGPoint gradientStartPoint = CGPointZero;
    //  CGPoint gradientStartPoint = CGPointMake(0, pixelsHigh);
    CGPoint gradientEndPoint = CGPointMake(pixelsWide/1.75, 0);

    // draw the gradient into the gray bitmap context
    CGContextDrawLinearGradient(gradientBitmapContext, grayScaleGradient, gradientStartPoint,
                                gradientEndPoint, kCGGradientDrawsAfterEndLocation);
    CGGradientRelease(grayScaleGradient);

    // convert the context into a CGImageRef and release the context
    theCGImage = CGBitmapContextCreateImage(gradientBitmapContext);
    CGContextRelease(gradientBitmapContext);

    // return the imageref containing the gradient
    return theCGImage;
}

CGContextRef MyCreateBitmapContext(int pixelsWide, int pixelsHigh)
{
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    // create the bitmap context
    CGContextRef bitmapContext = CGBitmapContextCreate (NULL, pixelsWide, pixelsHigh, 8,
                                                        0, colorSpace,
                                                        // this will give us an optimal BGRA format for the device:
                                                        (kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst));
    CGColorSpaceRelease(colorSpace);

    return bitmapContext;
}

+ (UIImage *)imageMaskedWithGradient:(CGImageRef)image {

    UIDeviceOrientation deviceOrientation = [UIDevice currentDevice].orientation;
    DEBUG(@"need to support deviceOrientation: %i", deviceOrientation);

    float width  = CGImageGetWidth(image);
    float height = CGImageGetHeight(image);

    // create a bitmap graphics context the size of the image
    CGContextRef mainViewContentContext = MyCreateBitmapContext(width, height);

    // create a 2 bit CGImage containing a gradient that will be used for masking the 
    // main view content to create the 'fade' of the reflection.  The CGImageCreateWithMask
    // function will stretch the bitmap image as required, so we can create a 1 pixel wide gradient
    CGImageRef gradientMaskImage = CreateGradientImage(width, 1);

    // create an image by masking the bitmap of the mainView content with the gradient view
    // then release the  pre-masked content bitmap and the gradient bitmap
    CGContextClipToMask(mainViewContentContext, CGRectMake(0.0, 0.0, width, height), gradientMaskImage);
    CGImageRelease(gradientMaskImage);

    // draw the image into the bitmap context
    CGContextDrawImage(mainViewContentContext, CGRectMake(0, 0, width, height), image);

    // create CGImageRef of the main view bitmap content, and then release that bitmap context
    CGImageRef reflectionImage = CGBitmapContextCreateImage(mainViewContentContext);
    CGContextRelease(mainViewContentContext);

    // convert the finished reflection image to a UIImage 
    UIImage *theImage = [UIImage imageWithCGImage:reflectionImage];

    // image is retained by the property setting above, so we can release the original
    CGImageRelease(reflectionImage);

    return theImage;

}

希望有帮助。