修改UITextField的占位符的颜色

    

    注意:必须先设置placeholder

    self.valueTextField.placeholder = model.title;

    [self.valueTextField setValue:DefaultLineViewColor forKeyPath:@"_placeholderLabel.textColor"];

    [self.valueTextField setValue:[UIFont boldSystemFontOfSize:14] forKeyPath:@"_placeholderLabel.font"];