关于CGContextRef无效的问题

• 在实现CGContextRef制图的过程用，因为直接调用“[self drawRect:self.bounds]”出现啦如下错误：

Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextSaveGState: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextBeginPath: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextSetLineWidth: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextSetLineCap: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextSetLineJoin: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextSetRGBStrokeColor: 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.
Nov  7 15:22:09 7777 PasswordDemo[15969] <Error>: CGContextMoveToPoint: 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.

🐷解决办法：
其实在实现的制图过程中，我们不需要直接调用“[self drawRect:self.bounds]”，只要调用“[self setNeedsDisplay]”即可。