擦除在图像视图上绘制的绘图

Question

所以，我正在开发一个虚拟壁画应用程序。

我可以通过在图像视图（其中包含图像）上移动手指来绘制一些随机线。现在我正试图擦除在它上面制作的图纸，但到目前为止还没有成功。

我用谷歌搜索找到了一些解决方案，下面这行是完整的推荐：-

CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeClear);

使用此绘图可以擦除，但它也会擦除完成绘图的图像。有什么办法可以防止图像被删除？只擦掉上面画的图？

以下是我用来擦除图纸的代码：-

在touchesMoved:-

UITouch *touch = [touches anyObject];
CGPoint previousPoint = [touch locationInView:self.upperImageView];
UIGraphicsBeginImageContext(self.upperImageView.frame.size);
[self.upperImageView.image drawInRect:CGRectMake(0, 0, self.upperImageView.frame.size.width, self.upperImageView.frame.size.height)];

CGContextSaveGState(UIGraphicsGetCurrentContext());
CGContextSetShouldAntialias(UIGraphicsGetCurrentContext(),YES);
CGContextSetLineCap(UIGraphicsGetCurrentContext(), eraserCap);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 45.0);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.25, 0.25, 0.25, 1.0);
CGMutablePathRef pathB = CGPathCreateMutable();
CGPathMoveToPoint(pathB,nil,location.x,location.y);
CGPathAddLineToPoint(pathB,nil,previousPoint.x,previousPoint.y);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
CGContextAddPath(UIGraphicsGetCurrentContext(),pathB);
CGContextStrokePath(UIGraphicsGetCurrentContext());
upperImageView.image = UIGraphicsGetImageFromCurrentImageContext();
CGContextRestoreGState(UIGraphicsGetCurrentContext());
UIGraphicsEndImageContext();
location = previousPoint;

在touchesEnded:-

UITouch *touch = [touches anyObject];
CGPoint previousPoint = [touch locationInView:self.upperImageView];
UIGraphicsBeginImageContext(self.upperImageView.frame.size);
[self.upperImageView.image drawInRect:CGRectMake(0, 0, self.upperImageView.frame.size.width, self.upperImageView.frame.size.height)];
CGContextSaveGState(UIGraphicsGetCurrentContext());
CGContextSetShouldAntialias(UIGraphicsGetCurrentContext(), YES);
CGContextSetLineCap(UIGraphicsGetCurrentContext(), eraserCap);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0,1.0,1.0, 0.0);
CGMutablePathRef pathB = CGPathCreateMutable();
CGPathMoveToPoint(pathB, nil, location.x, location.y);
CGPathAddLineToPoint(pathB, nil, previousPoint.x, previousPoint.y);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
CGContextAddPath(UIGraphicsGetCurrentContext(),pathB);
CGContextStrokePath(UIGraphicsGetCurrentContext());
self.upperImageView.image = UIGraphicsGetImageFromCurrentImageContext();
// CGContextRestoreGState(UIGraphicsGetCurrentContext());
UIGraphicsEndImageContext();
previousPoint = location;

我做错了什么吗？请在这里指导我正确的方向，任何帮助将不胜感激。

Answer 1

您可以为图像和绘图使用不同的视图。 或者将绘制的路径保留在数组中，并分别删除每条路径。

Answer 2

将所有绘图移动到覆盖UIImageView 的单独UIView。这样，当你去“清除”内容时，原来的UIImageView下面仍然完好无损。

这样的事情会让你做好准备：

- (void)viewWillAppear {
    // Assumes you add a property of type UIView
    self.drawingView = [[UIView alloc] initWithFrame:self.upperImageView.bounds];
    [self.upperImageView addSubview:drawingView];

}

然后，您的绘图代码只需更改为这个（注意您不再需要重新绘制图像）：

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint previousPoint = [touch locationInView:self.drawingView];
    UIGraphicsBeginImageContext(self.drawingView.frame.size);
    CGContextSaveGState(UIGraphicsGetCurrentContext());
    CGContextSetShouldAntialias(UIGraphicsGetCurrentContext(),YES);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), eraserCap);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 45.0);
    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.25, 0.25, 0.25, 1.0);
    CGMutablePathRef pathB = CGPathCreateMutable();
    CGPathMoveToPoint(pathB,nil,location.x,location.y);
    CGPathAddLineToPoint(pathB,nil,previousPoint.x,previousPoint.y);
    CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    CGContextAddPath(UIGraphicsGetCurrentContext(),pathB);
    CGContextStrokePath(UIGraphicsGetCurrentContext());
    upperImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    CGContextRestoreGState(UIGraphicsGetCurrentContext());
    UIGraphicsEndImageContext();
    location = previousPoint;
}

在touchesEnded: 方法中进行类似的更改。