问题

调试程序时发现控制台输出了这么一大串错误，核心错误是CGContextXXX: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable。
详细错误如下：

CGContextSetFillColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddCurveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddCurveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddCurveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddCurveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextClosePath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
...

经过仔细排查后，出现问题的核心代码如下：

...
NSBezierPath* path = [NSBezierPath bezierPathWithRoundedRect:frame xRadius:radius yRadius:radius];

path.lineWidth = lineWidth;
path.flatness = 0.0;
    
//这里出现问题
[fillColor setFill];  
[path fill];
...

错误原因

个人理解，控制台打印错误的含义为：当前绘图的上下文为空，因此无法绘制。即就是说：只有在drawRect函数内部才提供了绘制图形上下文环境，不能在非drawRect函数内部通过NSBezierPath或者CGPathRef绘制内容。（个人经验所得，若有谬误，请及时指出。）

解决方案

把所有与NSBezierPath或者CGPathRef绘图相关的代码移至drawRect函数内。
（以上错误也暴露出，NSBezierPath的fill方法内部的具体实现原理。）