相关疑难解决方法(0)

我正在升级一个基于 Flutter 框架的个人包。我在 Flutter Text 小部件源代码中注意到这里有一个空检查：

if (textSpan != null) {
  properties.add(textSpan!.toDiagnosticsNode(name: 'textSpan', style: DiagnosticsTreeStyle.transition));
}

但是，textSpan!仍然使用!运算符。不textSpan应该在不必使用!运算符的情况下提升为不可为空的类型吗？但是，尝试删除运算符会出现以下错误：

An expression whose value can be 'null' must be null-checked before it can be dereferenced.
Try checking that the value isn't 'null' before dereferencing it.

Run Code Online (Sandbox Code Playgroud)

这是一个独立的示例：

An expression whose value can be 'null' must be null-checked before it can be dereferenced.
Try checking that the value isn't 'null' before dereferencing it.

我收到一个编译时错误：

错误：“字符串？”类型的值 …

如果过期不为空，我想渲染文本小部件。所以我检查了 null

但出现错误

class TileButton extends StatelessWidget {
  final DateTime? expires;

  const TileButton({
    Key? key,
    this.expires,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        Text(
          'Hello Flutter',
        ),
        if (expires != null)
          Text(
            expires.day.toString(),
          ),
      ],
    );
  }
}