1. 修改textField的placeholder的字体颜色、大小
[textField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];
[textField setValue:[UIFont boldSystemFontOfSize:18] forKeyPath:@"_placeholderLabel.font"];
[textField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];
[textField setValue:[UIFont boldSystemFontOfSize:18] forKeyPath:@"_placeholderLabel.font"];