在ios 7的模拟器中，选择一个输入框准备输入时，会触发这个错误，以下是出错详细日志：
<Error>: CGContextSetBlendMode: 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.
本来以为是我程序的问题，后面在网上查了下，这是ios7 模拟器的一个bug。大家不必纠结！
参考网址：
http://stackoverflow.com/questions/19276882/error-appears-when-i-type-on-uitextfield-with-physical-keyboard-in-ios7