UITextField占位符动画
1.导入#HNBouncyPlaceholderTextField.h
2.实例化对象
HNBouncyPlaceholderTextField *bouncyTextField = [[HNBouncyPlaceholderTextField alloc] initWithFrame:<#CGRect#>];
3.设置占位符
bouncyTextField.placeholder = @"placeholder";
4.设置动画完成时的占位符(可选)
bouncyTextField.rightPlaceholder = @"placeholder";