为什么存在无效的上下文错误？ [英] Why there is an invalid context error?

问题描述

以下是我用来绘制的代码：

Here is the code I use to draw:

- (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");

}

但是我得到了这样的错误：

But I got the error like this:

[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

为什么它会提示我说上下文无效？

Why it prompt me to say that the context is invalided?

推荐答案

喜欢文档说：

默认情况下，当前图形上下文为零。在调用drawRect：方法之前，视图对象将有效的上下文推送到堆栈上，使其成为当前的。

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.

所以你需要把这段代码放在drawRect方法中

So you need to put this code in the drawRect method

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