iphone - 为什么会出现无效上下文错误？

Question

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");

}

但是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

为什么提示我说上下文无效？

Answer 1

赞the documentation说:

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.

所以需要把这段代码放在drawRect方法中