// _placeholderLabel
    UILabel *placeHolderLabel = [[UILabel alloc] init];
    placeHolderLabel.text = @"請輸入內容";
    placeHolderLabel.numberOfLines = 0;
    placeHolderLabel.textColor = [UIColor lightGrayColor];
    [placeHolderLabel sizeToFit];
    [textView addSubview:placeHolderLabel];

    // same font
    textView.font = [UIFont systemFontOfSize:13.f];
    placeHolderLabel.font = [UIFont systemFontOfSize:13.f];

    [textView setValue:placeHolderLabel forKey:@"_placeholderLabel"];