flutter - Dart 断言构造函数中是否不为 null

Question

如何在 Dart 构造函数中断言参数不为 null？

const CardStackCarousel({
    Key key,
    this.itemBuilder,
    this.itemCount,
    int backgroundItemCount,
    this.controller,
    this.offsetAboveBackcards: 10.0,
    this.minScale: 0.8,
    this.verticalAxis: false,
  })  : backgroundItemCount = backgroundItemCount == null ? itemCount - 1 : backgroundItemCount,
        assert(backgroundItemCount < itemCount, 'background item must be less than itemCount'),
        super(key: key);

使用上面的代码，我收到错误:

The method '<' was called on null.
Receiver: null

当用户未指定backgroundItemCount属性时。所以我的想法是，也许我们应该只断言该属性不为空。但我不知道该怎么做

Answer 1

这是因为您没有断言 CardStackCarousel属性(property) - 可能有 backgroundItemCount同样，您断言通过构造函数接收的参数，根据您的逻辑，该参数可能为 null。

当您调用时:

backgroundItemCount = backgroundItemCount == null ? itemCount - 1 : backgroundItemCount

您不会为收到的输入分配新值，您可能会将其保存到本地属性。您在上面一行中实际执行的操作是:

this.backgroundItemCount = backgroundItemCount == null ? itemCount - 1 : backgroundItemCount

这就是断言失败的原因，因为它没有检查 this.backgroundItemCount ，它检查通过构造函数接收的属性， backgroundItemCount .

我会引用Dart official documentation中的一句话:

Warning: The right-hand side of an initializer does not have access to this.

...

During development, you can validate inputs by using assert in the initializer list.

这解释了为什么您无法检查 this ，因为它仍然是“静态”验证您的输入 - 在右侧评估时，它还没有实例。

---

如果您仍需要确保backgroundItemCount < itemCount ，你应该简单地assert(backgroundItemCount == null || backgroundItemCount < itemCount, 'If background item is supplied, it must be less than itemCount') .

像这样:

const CardStackCarousel({
    Key key,
    this.itemBuilder,
    this.itemCount,
    int backgroundItemCount,
    this.controller,
    this.offsetAboveBackcards: 10.0,
    this.minScale: 0.8,
    this.verticalAxis: false,
  })  : backgroundItemCount = backgroundItemCount == null ? itemCount - 1 : backgroundItemCount,
        assert(backgroundItemCount == null || backgroundItemCount < itemCount, 'If background item is supplied, it must be less than itemCount'),
        super(key: key);