我有一个返回地图的 Future。然后,我需要使用该映射的值来等待另一个未来,然后在最后返回整个结果。问题是 dart 不能等待async
Map.forEach()
方法(参见:https://stackoverflow.com/a/42467822/15782390 https://stackoverflow.com/a/42467822/15782390).
这是我的代码:
调试控制台显示打印的项目按以下顺序排列:
flutter: getting journal entries
flutter: about to loop through pictures
flutter: getting picture
flutter: returning entries
flutter: [[....]] (Uint8List)
Future<List<JournalEntryData>> getJournalEntries() async {
List<JournalEntryData> entries = [];
print('getting journal entries');
EncryptService encryptService = EncryptService(uid);
await journal.get().then((document) {
Map data = (document.data() as Map);
print('about to loop through pictures');
data.forEach((key, value) async {
print('getting picture');
dynamic pictures = await StorageService(uid).getPictures(key);
print('done getting image');
entries.add(JournalEntryData(
date: key,
entryText: encryptService.decrypt(value['entryText']),
feeling: value['feeling'],
pictures: pictures,
));
});
});
print('returning entries');
return entries;
}
Future getPictures(String entryID) async {
try {
final ref = storage.ref(uid).child(entryID);
List<Uint8List> pictures = [];
await ref.listAll().then((result) async {
for (var picReference in result.items) {
Uint8List? pic = await ref.child(picReference.name).getData();
if (pic == null) {
// TODO make no picture found picture
var url = Uri.parse(
'https://www.salonlfc.com/wp-content/uploads/2018/01/image-not-found-scaled-1150x647.png');
var response = await http.get(url);
pic = response.bodyBytes;
}
pictures.add(pic);
}
});
return pictures;
} catch (e) {
print(e.toString());
return e;
}
}
当您需要异步行为时,特别是在地图上,必须使用 for 循环是非常烦人的,因为正如另一个答案所示,这需要您迭代条目,然后获取key
and value
像这样:
for (final mapEntry in data.entries) {
final key = mapEntry.key;
final value = mapEntry.value;
...
}
相反,您可以编写一个实用程序扩展来为您完成工作:
extension AsyncMap<K, V> on Map<K, V> {
Future<void> forEachAsync(FutureOr<void> Function(K, V) fun) async {
for (var value in entries) {
final k = value.key;
final v = value.value;
await fun(k, v);
}
}
}
然后,您可以像这样使用它:
await data.forEachAsync((key, value) async {
...
});
好多了。
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