在ios 7的模擬器中，選擇一個輸入框准備輸入時，會觸發這個錯誤，以下是出錯詳細日志：
<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.
本來以為是我程序的問題，后面在網上查了下，這是ios7 模擬器的一個bug。見：