ios修改textField的placeholder的字体颜色、大小
textField.placeholder =@"username is in here!";
[textField setValue:[UIColor redColor]forKeyPath:@"_placeholderLabel.textColor"];
[textField setValue:[UIFont boldSystemFontOfSize:16]forKeyPath:@"_placeholderLabel.font"];
这里是使用了KVC的方式
