iOS CGContextMoveToPoint invalid context 0x0 If you want to see th

: CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

在控制器类中 直接用使用Quartz2D画图是会报错的， Quartz2D只能在UIView类的drawRect方法中写