CGContextClearRect 在 objective c 中的 touchesMoved 事件中无法正常工作

Question

我使用了这段代码，它部分有效，但图像裁剪不正确 output seen like this

-(void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:_img.superview];

UIGraphicsBeginImageContext(self.img.frame.size);
[_img.image drawInRect:CGRectMake(0, 0, _img.frame.size.width, _img.frame.size.height)];

CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 10);
CGContextBeginPath(UIGraphicsGetCurrentContext());

CGContextClearRect (UIGraphicsGetCurrentContext(), CGRectMake(currentPoint.x, currentPoint.y, 15, 15));
CGContextStrokePath(UIGraphicsGetCurrentContext());
_img.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();   
}

Answer 1

我早些时候遇到了同样的问题，然后我尝试了下面的代码，它对我有用

UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:self.img_main];
// NSLog(@"Current ponint : %f,%f",currentPoint.x,currentPoint.y);

self.img_main.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
self.img_main.contentMode = UIViewContentModeScaleAspectFit;

UIGraphicsBeginImageContextWithOptions(self.img_main.bounds.size, NO, 0.0);
[_img_main.image drawInRect:CGRectMake(0,0, self.img_main.bounds.size.width,self.img_main.bounds.size.height)];
CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);

//For clearing Size and Mode
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush_Size );
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeDestinationOut);
//For Clearing Opacity        
CGContextSetAlpha(UIGraphicsGetCurrentContext(), brush_opacity);
CGContextStrokePath(UIGraphicsGetCurrentContext());

_img_main.image = UIGraphicsGetImageFromCurrentImageContext();

CGContextFlush(UIGraphicsGetCurrentContext());
UIGraphicsEndImageContext();