xcode 为什么会出现无效的上下文错误？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/2570846/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why there is an invalid context error?
提问by Tattat
Here is the code I use to draw:
这是我用来绘制的代码：
- (void) drawSomething
{
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 1, 0, 0, 1);
CGContextSetLineWidth(context, 6.0);
CGContextMoveToPoint(context, 100.0f, 100.0f);
CGContextAddLineToPoint(context, 200.0f, 200.0f);
CGContextStrokePath(context);
NSLog(@"draw");
}
But I got the error like this:
但我得到了这样的错误：
[Session started at 2010-04-03 17:51:07 +0800.]
Sat Apr 3 17:51:09 MacBook.local MyApp[12869] <Error>: CGContextSetRGBStrokeColor: invalid context
Sat Apr 3 17:51:09 MacBook.local MyApp[12869] <Error>: CGContextSetLineWidth: invalid context
Sat Apr 3 17:51:09 MacBook.local MyApp[12869] <Error>: CGContextMoveToPoint: invalid context
Sat Apr 3 17:51:09 MacBook.local MyApp[12869] <Error>: CGContextAddLineToPoint: invalid context
Sat Apr 3 17:51:09 MacBook.local MyApp[12869] <Error>: CGContextDrawPath: invalid context
Why it prompt me to say that the context is invalided?
为什么它提示我说上下文无效？
回答by Thomas Joulin
Like the documentationsays :
就像文档说的：
The current graphics context is nil by default. Prior to calling its drawRect: method, view objects push a valid context onto the stack, making it current.
默认情况下，当前图形上下文为零。在调用它的 drawRect: 方法之前，视图对象将一个有效的上下文推送到堆栈上，使其成为当前的。
So you need to put this code in the drawRect method
所以需要把这段代码放在drawRect方法中
回答by Brad Larson
From my answerto this similar question:
If this is to be drawn to the screen, you'll need to locate your drawing code within the -drawRect:method of a UIView (or –drawInContext:of a CALayer). To update its contents, you'd need to call -setNeedsDisplayon the UIView or CALayer. Attempting drawing at any other time will cause the "invalid context" error you're seeing.
如果要将其绘制到屏幕上，则需要在-drawRect:UIView（或–drawInContext:CALayer）的方法中找到绘制代码。要更新其内容，您需要调用-setNeedsDisplayUIView 或 CALayer。在任何其他时间尝试绘制都会导致您看到的“无效上下文”错误。
See also this question.
另请参阅此问题。
