[dart]颤抖着如何等待map.forEach在Future< void>中完成功能? [英] [dart]flutter how to wait map.forEach to complete in Future<void> function?
问题描述
示例代码
Map<String,String> gg={'gg':'abc','kk':'kojk'};
Future<void> secondAsync() async {
await Future.delayed(const Duration(seconds: 2));
print("Second!");
gg.forEach((key,value) async{await Future.delayed(const Duration(seconds: 5));
print("Third!");
});
}
Future<void> thirdAsync() async {
await Future<String>.delayed(const Duration(seconds: 2));
print('third');
}
void main() async {
secondAsync().then((_){thirdAsync();});
}
输出
Second!
third
Third!
Third!
如您所见，我要等到地图的foreach循环完成后再打印third
预期产量
as you can see i want to use to wait until foreach loop of map complete to complete then i want to print third
expected Output
Second!
Third!
Third!
third
推荐答案
Map.forEach(类似地，Iterable.forEach)用于在集合的每个元素上执行一些代码，以消除副作用 .返回值将被忽略.因此，如果提供返回Future的函数，则该Future将会丢失，并且在完成时将不会通知您.
Map.forEach (and similarly, Iterable.forEach) is meant to execute some code on each element of a collection for side effects. The return values are ignored. Therefore if you supply a function that returns a Future, that Future is lost, and you will not be able to be notified when it completes.
不要在async回调中使用.forEach.
Don't use .forEach with async callbacks.
相反，如果要依次等待每个async回调 ，请使用常规的for循环:
Instead, if you want to wait for each async callback sequentially, just use a normal for loop:
for (var mapEntry in gg.entries) {
await Future.delayed(const Duration(seconds: 5));
}
，或者如果您真的更喜欢使用.forEach语法，则可以使用Future.forEach:
or if you really prefer using .forEach syntax, you could use Future.forEach:
await Future.forEach([
for (var mapEntry in gg.entries)
Future.delayed(const Duration(seconds: 5)),
]);
如果要允许您的async回调可能并行运行，则可以使用Future.wait:
If you want to allow your async callbacks to possibly run in parallel, you can use Future.wait:
await Future.wait([
for (var mapEntry in gg.entries)
Future.delayed(const Duration(seconds: 5)),
]);
类似的问题:
- Flutter async/await doesn't work inside forEach
- My async call is returning before list is populated in forEach loop
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