flutter - Flutter中具有默认值的CustomDrawer

Question

我对 Flutter 很陌生。我正在尝试构建一个可以根据需要导入和自定义的 CustomDrawer 类。

    import 'package:flutter/material.dart';

      class CustomDrawer extends AppBar {
            CustomDrawer ({ Key key, Widget leading, Widget title}) 
                      : super(key: key, leading: leading, title: title, actions: <Widget>[
            new IconButton(
            icon: new Icon(Icons.notifications_none),
            onPressed: () => print("Hello World"),
          ),
        ]);
}

我希望前导 Widget 在未提供参数时显示默认文本“占位符”，否则前导应显示传递给 CustomDrawer 的任何内容。
有人可以帮我吗？

Answer 1

一个简单的三元运算符可以解决这个问题。

构建方法:

@override
  Widget build(BuildContext context) {
    return Scaffold(
      /// You can try either of these.
      appBar: CustomDrawer(),
      appBar: CustomDrawer(leading: Icon(Icons.ac_unit),),
    );
  }

自定义抽屉:

class CustomDrawer extends AppBar {
  CustomDrawer({
    Key key,
    Widget leading,
    Widget title,
  }) : super(
          key: key,
          /// Adding ternary operator.
          leading: leading == null ? Placeholder() : leading,
          title: title,
          actions: <Widget>[
            IconButton(
              icon: Icon(Icons.notifications_none),
              onPressed: () => print("Hello World"),
            ),
          ],
        );
}