invalid context 0x0

今天碰到一个问题，自定义了一个CALayer，并且在layer的drayInContext绘制直线，但是绘制线的时候，log中打印出以下错误提示：

CGContextSetStrokeColorWithColor: invalid context 0x0. This is a serious error. This application, or a library it uses, is using an invalid context  and is thereby contributing to an overall degradation of system stability and reliability. This notice is a courtesy: please fix this problem. It will become a fatal error in an upcoming update.

只需要在drawInContext:(CGContextRef)ctx的最上方加上UIGraphicsPushContext(ctx)即可。