RenderInContext is incredibly slow on iOS7

Question

I am implementing "scratch" functionality in my application. User scratches the screen and sees the image "below".

On touchesMoved: I update mask image and apply it to layer. In general code is like this:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];

    UITouch *touch = [touches anyObject];
    CGPoint cPoint = [touch locationInView:self];

    UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.bounds];
    imageView.image = _maskImage;

    // ... add some subviews to imageView corresponding to touch manner

    _maskImage = [UIImage imageFromLayer:imageView.layer];

    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{
    _maskImageView.image = _maskImage;
    _viewWithOurImage.layer.mask = _maskImageView.layer;
}

I get UIImage from CALayer using code (category on UIImage):

+ (UIImage*)imageFromLayer:(CALayer*)layer
{
    UIGraphicsBeginImageContextWithOptions([layer frame].size, NO, 0);
    [layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *outputImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return outputImage;
}

This code works perfectly on iOS6 (tested on iPhone 4s and iPad2), no lags at all.

But when I run it on iOS7 (xcode4 or xcode5), it is awfully slow and laggy. I used a time profiler, it clearly points to renderInContext: row.

Then I tried following code:

...
    if (SYSTEM_VERSION_LESS_THAN(@"7.0"))
        _maskImage = [UIImage imageFromLayer:imageView.layer];
    else
        _maskImage = [UIImage imageFromViewIniOS7:imageView];
...

+ (UIImage*)imageFromViewIniOS7:(UIView*)view
{
    UIGraphicsBeginImageContextWithOptions(view.frame.size, NO, 0);
    CGContextSetInterpolationQuality(UIGraphicsGetCurrentContext(), kCGInterpolationNone);

    // actually there is NSInvocation, but I've shortened for example
    [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:YES];

    UIImage *outputImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return outputImage;
}

And it is still very slow. Tested on iPhone 4s (same as was on iOS6), new iPod5 and iPad3.

What am I doing wrong? Obviously it's a problem with iOS7...

I will appreciate any suggestions.

Answer 1

我会建议您尝试其他方法，抱歉地说， touchesMoved函数在IOS7中运行缓慢，您的代码没有错