我正在尝试从 Firebase 存储文件构建 URL,但我构建的 Future<String>
似乎总是返回 null
。这是我正在呼叫的 Future
:
Future<String> getUrlFromStorageRefFromDocumentRef(
DocumentReference docRef) async {
try {
docRef.get().then((DocumentSnapshot documentSnapshot) {
if (documentSnapshot.exists) {
String filename = documentSnapshot.get('file');
firebase_storage.Reference ref = firebase_storage
.FirebaseStorage.instance
.ref()
.child('/flamelink/media/$filename');
if (ref == null) {
return Future.error("Storage Reference is null");
} else {
print(ref.fullPath);
return Future.value(
'https://storage.googleapis.com/xxxxxxxxx.appspot.com/${ref.fullPath}');
}
} else {
return Future.error('No Snapshot for DocumentReference ${docRef.id}');
}
});
} catch (e) {
print(e);
return Future.error('No DocumentReference for ID ${docRef.id}');
}
}
有问题的行是:
return Future.value(
'https://storage.googleapis.com/xxxxxxxxx.appspot.com/${ref.fullPath}');
值得注意的是,String
是从 Firebase Storage
路径生成的,在返回值之前一切看起来都很完美。
它应该将 String
值返回给我的调用代码,目前看起来像这样:
DocButtonCallback docCallback = () async {
bool isKidsDoc = item.screenId == StringsManager.instance.screenIdKids;
try {
// first we need to get the URL for the document ...
var url = await AssetManager.instance
.getUrlFromStorageRefFromDocumentRef(isKidsDoc
? feature.relatedDocumentKidsRef
: feature.relatedDocumentRef);
String urlString = url.toString();
canLaunch(urlString).then((value) {
launch(urlString);
}).catchError((error) {
// TODO: open alert to tell user
});
} catch (error) {
print(error);
}
};
我尝试了很多不同的方法来获得String
,包括:
DocButtonCallback docCallback = () async {
bool isKidsDoc = item.screenId == StringsManager.instance.screenIdKids;
await AssetManager.instance
.getUrlFromStorageRefFromDocumentRef(isKidsDoc
? feature.relatedDocumentKidsRef
: feature.relatedDocumentRef)
.then((urlString) {
canLaunch(urlString).then((value) {
launch(urlString);
}).catchError((error) {
// TODO: open alert to tell user
});
}).catchError((error) {
// TODO: open alert to tell user
});
};
出于某种原因,Future
总是返回 null
。我在这里做错了什么?
答案 0 :(得分:1)
您将在 Future
回调中返回 then()
值,它本质上是从回调本身而不是从您的 getUrlFromStorageRefFromDocumentRef()
函数返回此值。在那里你只需要在这之前添加一个 return 语句:
当前:
docRef.get().then((DocumentSnapshot documentSnapshot) {
if (documentSnapshot.exists) {
...
之后:
/// Adding the return statement here to return the actual value
/// returned internally by the then callback
return docRef.get().then((DocumentSnapshot documentSnapshot) {
if (documentSnapshot.exists) {
...
如果您将鼠标悬停在 then()
回调上,您的 IDE 应该向您显示此回调将返回 Future<T>
(或任何通用类型占位符),为了使其可用,还需要返回该回调