我正试图从Firebase Storage文件构建URL,但我构建的Future<String>
似乎总是返回null
。这是我呼叫的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 ${docRef.id}');
}
}
有问题的线路是:
return Future.value(
'https://storage.googleapis.com/xxxxxxxxx.appspot.com/${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
值返回到我的调用代码中,现在看起来是这样的:
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);
}
};
我尝试了许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
});
};
由于某些原因,Future
总是返回null
。我在这里做错了什么?
您在then()
回调中返回Future
值,它本质上是从回调本身而不是从getUrlFromStorageRefFromDocumentRef()
函数返回该值。在此之前,您只需要添加一个返回语句:
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>
(或任何通用类型的占位符(,这些占位符也需要返回才能使其可用