textField设置placeholder的字体大小，颜色等

textField.placeholder = @"请输入密码";

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

[textField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];

setValue: forKeyPath:    利用kvc的机制，设置placeholder的属性