用CGContext绘图

Question

我正在尝试通过touchesMove:方法画线。

下面是我的touchesMoved:。

UIGraphicsBeginImageContext(self.frame.size);
CGContextRef context = UIGraphicsGetCurrentContext();

// context setting
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineJoin(context, kCGLineJoinRound);
CGContextSetLineWidth(context, 2.0);
CGContextSetRGBStrokeColor(context, 255, 0, 0, 0.5);
CGContextSetBlendMode(context, kCGBlendModeNormal);

// drawing
CGContextMoveToPoint(context, lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(context, currentPoint.x, currentPoint.y);

CGContextStrokePath(context);
CGContextFlush(context);
self.image = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

调用touchesMoved:，但是屏幕上没有显示任何内容。

我遗漏了什么？

添加了

self是UIImageView的一个子类。

Answer 1

好吧，我知道为什么它不起作用了。我创建了CGContext每一个触摸移动事件。我将行UIGraphicsBeginImageContext(self.frame.size);移到init方法，将UIGraphicsEndImageContext();移到dealloc。

这是我画的代码。

static CGPoint lastPoint;
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch_ = [touches anyObject];
    CGPoint point_  = [touch_ locationInView:self];

    lastPoint = point_;

    CGContextRef context = UIGraphicsGetCurrentContext();

    // context setting
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineJoin(context, kCGLineJoinRound);
    CGContextSetLineWidth(context, 2.0);
    CGContextSetRGBStrokeColor(context, 255, 0, 0, 0.5);
    CGContextSetBlendMode(context, kCGBlendModeNormal);
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    lastPoint = CGPointZero;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self];

    CGContextRef context = UIGraphicsGetCurrentContext();

    // drawing
    CGContextMoveToPoint(context, lastPoint.x, lastPoint.y);
    CGContextAddLineToPoint(context, currentPoint.x, currentPoint.y);

    CGContextStrokePath(context);
    CGContextFlush(context);
    self.image = UIGraphicsGetImageFromCurrentImageContext();

    lastPoint = currentPoint;
}