IOS UITextField. placeholder属性这个提示符的大小和颜色

用UITextField都知道，默认有个提示，原理是kvc，监听，

textField.placeholder = @"This is textField.placeholder";  

[textField setValue:[UIColor green] forKeyPath:@"_placeholderLabel.textColor"];  

[textField setValue:[UIFont boldSystemFontOfSize:15] forKeyPath:@"_placeholderLabel.font"];