改变输入框的placehould的颜色

UIColor *color = [UIColor whiteColor];

_usernameTextField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"请输入账号" attributes:@{NSForegroundColorAttributeName: color}];

附带找window的跟视图控制器

UIWindow *window = [UIApplication sharedApplication].keyWindow;

UINavigationController *nav = [[UINavigationController alloc]initWithRootViewController:loginVC];

window.rootViewController = nav;