iOS Core Graphics 如何删除绘图

Question

我正在按照我的涂鸦应用程序教程进行操作。

http://www.raywenderlich.com/18840/how-to-make-a-simple-drawing-app-with-uikit

他的删除功能是使用白色绘图。但是我正在使用图像背景，当我删除时，我真的需要删除。

我试过了

CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);

但它不起作用。这是我的代码:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }

    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height) blendMode:kCGBlendModeNormal alpha:_alpha];


    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}

Answer 1

通过在触摸开始时交换 tempImageView 和 drawImageView 解决了这个问题，如果它正在删除

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];

    if (_isErasing) {
        self.tempImageView.image = self.drawImageView.image;
        self.drawImageView.image = nil;
    }

}



- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}





- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        if (_isErasing) {
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
        }
        else {
            CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
        }

        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:_alpha];
    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}