在 Dart 中使用 as
进行转换有何不同?关键字并使用 cast
进行转换方法?
请看下面的例子:
import 'dart:convert';
class MyClass {
final int i;
final String s;
MyClass({required this.i, required this.s});
factory MyClass.fromJson(Map<String, dynamic> json) =>
MyClass(s: json["s"], i: json["i"]);
}
void main() {
const jsonString = '{"items": [{"i": 0, "s": "foo"}, {"i": 1, "s":"bar"}]}';
final json = jsonDecode(jsonString);
final List<MyClass> thisWorks = (json["items"] as List)
.cast<Map<String, dynamic>>()
.map(MyClass.fromJson)
.toList();
final List<MyClass> thisAlsoWorks = (json["items"] as List)
.map((json) => MyClass.fromJson(json as Map<String, dynamic>))
.toList();
final List<MyClass> thisCrashes =
(json['items'] as List<Map<String, dynamic>>)
.map(MyClass.fromJson)
.toList();
}
最后一次调用(使用 as
进行转换)导致异常:type 'List<dynamic>' is not a subtype of type 'List<Map<String, dynamic>>' in type cast
.
我希望使用 as
进行类型转换会像使用 cast
一样工作方法而不会导致异常。
最佳答案
as
执行强制转换,在执行运行时检查后,更改对象的 static
类型。它不影响对象的身份。
集合(例如List
、Map
、Set
)提供了一个.cast
返回集合的新对象(“ View ”)的方法,该集合对每个元素 执行as
转换。
void main() {
// `list1` is created as a `List<Object>` but happens to store only ints.
List<Object> list1 = <Object>[1, 2, 3];
try {
// This cast will fail because `list1` is not actually a `List<int>`.
list1 as List<int>;
} on TypeError catch (e) {
print(e); // This gets printed.
}
// `list2` is a `List`-like object that downcasts each element of `list1`
// to an `int`. `list2` is of type `List<int>`.
//
// Note that `list2` is not a copy of `list1`; mutations to `list2` will
// affect `list1`.
List<int> list2 = list1.cast<int>();
// `list3` is `list2` upcast to a `List<Object>`. If a class `Derived`
// derives from a class `Base`, then Dart also considers `List<Derived>`
// to be a derived class of `List<Base>`, so no explicit cast is necessary.
List<Object> list3 = list2;
// `list4` is `list3` downcast to `List<int>`. Since `list3` is the
// same object as `list2`, and since `list2`'s actual runtime type is
// `List<int>`, this cast succeeds (unlike `list1 as List<int>`).
List<int> list4 = list3 as List<int>;
print(identical(list1, list2)); // Prints: false
print(identical(list2, list3)); // Prints: true
print(identical(list3, list4)); // Prints: true
}
关于flutter - 使用 'cast' 方法和 'as' 关键字进行转换的区别,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/70959115/