iOS绘图出现的错误

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-(void)createCircle
{
CGPoint layerCenter = CGPointMake(CGRectGetWidth(self.view.frame)/2, CGRectGetWidth(self.view.frame)/2);
CAShapeLayer *layer = [CAShapeLayer layer];
layer.frame = self.view.bounds;
layer.lineWidth = 6.0;
layer.strokeColor = [UIColor redColor].CGColor;
layer.fillColor = [UIColor whiteColor].CGColor;
self.path = [UIBezierPath bezierPath];
[self.path addArcWithCenter:layerCenter radius:50 startAngle:0.0 endAngle:M_PI*2 clockwise:YES];
[self.path stroke];
[self.path fill];
layer.path = self.path.CGPath;
[self.view.layer addSublayer:layer];
}

错误日志如下：

Sep  9 16:09:01  Test1[28072] : CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetLineWidth: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetLineJoin: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetLineCap: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetMiterLimit: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextAddPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextAddPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Sep  9 16:09:01  Test1[28072] : CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable

上网查找错误的原因，大部分都说是IOS9适配问题，设置app的状态栏样式的使用使用了旧的方式在IOS9中不兼容。但是我的工程中并没有设置状态栏，并且按照其解决方法更改后也没有解决这个问题。后来看到这篇文章 H含金量 iOS绘图及贝塞尔曲线关键知识 才知道问题所在。

因为绘图不在drawRect:方法中操作导致绘图时没有当前的图形上下文(context)可设置。所以应该在drawRect:中执行图形绘制。修改后代码如下：

-(void)createCircle
{
CGPoint layerCenter = CGPointMake(CGRectGetWidth(self.view.frame)/2, CGRectGetWidth(self.view.frame)/2);
CAShapeLayer *layer = [CAShapeLayer layer];
layer.frame = self.view.bounds;
layer.lineWidth = 6.0;
layer.strokeColor = [UIColor redColor].CGColor;
layer.fillColor = [UIColor whiteColor].CGColor;
self.path = [UIBezierPath bezierPath];
[self.path addArcWithCenter:layerCenter radius:50 startAngle:0.0 endAngle:M_PI*2 clockwise:YES];
//    [self.path stroke];
//    [self.path fill];
layer.path = self.path.CGPath;
[self.view.layer addSublayer:layer];
[self.view setNeedsDisplay];
}
- (void) drawRect:(CGRect)rect {
[self.path stroke];
[self.path fill];
}

或者改成这样：

-(void)createCircle
{
    CGPoint layerCenter = CGPointMake(CGRectGetWidth(self.view.frame)/2, CGRectGetWidth(self.view.frame)/2);
    CAShapeLayer *layer = [CAShapeLayer layer];
    layer.frame = self.view.bounds;
    layer.lineWidth = 6.0;
    layer.strokeColor = [UIColor redColor].CGColor;
    layer.fillColor = [UIColor whiteColor].CGColor;

    self.path = [UIBezierPath bezierPath];
    [self.path addArcWithCenter:layerCenter radius:50 startAngle:0.0 endAngle:M_PI*2 clockwise:YES];
        layer.path = self.path.CGPath;

    UIGraphicsBeginImageContext(self.view.bounds.size);
    [self.path stroke];
    [self.path fill];
    UIGraphicsEndImageContext();

    [self.view.layer addSublayer:layer];
}

这样就不会再出现错误了。其实我这里的path是UIBezierPath，可以直接去掉[self.path stroke];和[self.path fill];这两处的。