用CGContext绘图 [英] Drawing with CGContext

问题描述

我正在尝试通过 touchesMove：方法画线。

I'm trying to draw lines through touchesMove: method.

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，屏幕上什么都没有显示。

The touchesMoved: is invoked; However nothing is shown on the screen.

我丢失了什么？

已添加 >

self是UIImageView的子类。

self is a subclass of UIImageView.

推荐答案

好，我发现了为什么它不起作用。我在每个触摸移动事件中都创建了 CGContext 。我将 UIGraphicsBeginImageContext（self.frame.size）; 行移至 init 方法和 UIGraphicsEndImageContext （）; 到 dealloc 。

OK, I found why it wasn't working. I created CGContext every touch move event. I moved the line UIGraphicsBeginImageContext(self.frame.size); to init method and UIGraphicsEndImageContext(); to dealloc.

这是我绘制代码的方式。

Here's the code how I drawing.

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;
}

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