稀有bug：当APP从后台进入前台时，键盘无法响应，输入不能

bug并不是必现，在各种型号的机子上进行测试时，不同的机子，有的会有，有的不会有，为了解决这个问题，我也是头疼了好久，最终找到方法，可以调用UIApplicationDelegate class
参考链接http://stackoverflow.com/questions/8072984/hittest-fires-when-uikeyboard-is-tapped

此处做记录，丢完就跑

- (void)applicationWillEnterForeground:(UIApplication *)application {
    // Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
    for (UIWindow *testWindow in [UIApplication sharedApplication].windows) {
        if (!testWindow.opaque && [NSStringFromClass(testWindow.class) hasPrefix:@"UIText"]) {
            BOOL wasHidden = testWindow.hidden;
            testWindow.hidden = YES;
            
            if (!wasHidden) {
                testWindow.hidden = NO;
            }
            
            break;
        }
    }
    
}