这是实现方法

          final StorageReference storageReference =
                FirebaseStorage.instance.ref().child('images/$name');
            StorageUploadTask uploadTask = storageReference.putData(asset);

            final StreamSubscription<StorageTaskEvent> streamSubscription =
                uploadTask.events.listen((event) {
              // You can use this to notify yourself or your user in any kind of way.
              // For example: you could use the uploadTask.events stream in a StreamBuilder instead
              // to show your user what the current status is. In that case, you would not need to cancel any
        // subscription as StreamBuilder handles this automatically.
     });
         await uploadTask.onComplete;
            streamSubscription.cancel();
//get your ref url    

            String docUrl = await (await uploadTask.onComplete).ref.getDownloadURL();
//docUrl is your url as a string
            Firestore.instance
                .collection('collectiion')
                .document()
        .updateData({"url": docUrl});