'invalid context 0x0'错误时使用CGContext *函数 [英] ‘invalid context 0x0’ error when using CGContext* functions

问题描述

/*  Adding the Path */
UserGraphBuff = UIGraphicsGetCurrentContext();

CGContextSetRGBStrokeColor(UserGraphBuff,5,10,0,1);
CGContextSetLineWidth(UserGraphBuff, 2 );

CGContextBeginPath(UserGraphBuff);  

//line to last user point
CGContextAddLineToPoint(UserGraphBuff, (*xVal)[sizeof xVal / sizeof *xVal - 1], (*yNewVal)[sizeof yNewVal / sizeof *yNewVal - 1]);
//line to rest of user points in reverse order
for (int i = sizeof xVal / sizeof *xVal - 1; i > -1; i--){
    CGContextAddLineToPoint(UserGraphBuff, (*xVal)[i], (*yNewVal)[i]);
}

//EOFill
CGContextEOFillPath(UserGraphBuff);

上面是我想要通过的代码。它应该做什么CGContext说它，但我没有得到任何东西绘制。
我一直收到此错误：

Above is the code I am trying to work through. Its supposed to do what CGContext says it does but I am not getting any thing drawn. I keep getting this error:

Fri Oct 28 13:18:40 case.app testApplication[4127] <Error>: CGContextSetRGBStrokeColor: invalid context 0x0
Fri Oct 28 13:18:40 case.app testApplication[4127] <Error>: CGContextSetLineWidth: invalid context 0x0
Fri Oct 28 13:18:40 case.app testApplication[4127] <Error>: CGContextBeginPath: invalid context 0x0
Fri Oct 28 13:18:40 case.app testApplication[4127] <Error>: CGContextAddLineToPoint: invalid context 0x0
Fri Oct 28 13:18:40 case.app testApplication[4127] <Error>: CGContextDrawPath: invalid context 0x0

我引用我的头文件中的CGContextRef。

I reference the CGContextRef in my header file.

我不认为我理解CGContext够好，也不知道CGContextRef应该是什么。

i don't think i understand CGContext well enough nor do i know what CGContextRef should be.

推荐答案

为了让代码起作用，需要在 UIView 的 drawRect $ c> subclass（并且你不直接调用 drawRect - 当需要UIView来渲染自己时，它被操作系统调用）。你可能试图从触摸事件或从 viewDidLoad 或类似的东西运行此代码。

For your code to work, it needs to be executed in the drawRect method of your UIView subclass (and you don't call drawRect directly - this is called by the OS when it needs the UIView to render itself). You're presumably trying to run this code from a touch event or from viewDidLoad or something like that.

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