firebase - Flutter Firebase setData()函数在异步函数完成之前将空值分配给字段

Question

我正在尝试为用户上载化身到Firestore存储并返回图像URL，以将其保存为用户实例上用户的avatarUrl属性。我可以将图片成功上传到Firestore。问题与URL的接收有关。异步函数会延迟，因此前面的用户实例化函数会使用avatarUrl属性的空值进行处理。
这是函数启动

                    if(_formKey.currentState.validate()){
                      setState(() {
                        loading=true;
                      });
                      await uploadFile();
                      print('Print before async function complete...');
                      print(_avatarUrl);
                      dynamic result=  await SellerDatabaseService(uid:user.uid).sellerProfileSetup(
                        shopName: shopName,
                        phone: phone,
                        bio: bio,
                        location: location,
                        avatarUrl: _avatarUrl,
                        rating: 0,
                        sales: 0,
                        credit: 0,
                        posts: 0,
                        joinedDate:DateTime.now(),
                        //todo remove null from profile image
                      );
                      if(result==null){
                        setState(() {
                          loading=false;
                          error='Please supply correct details';
                        });
                      }
                    }
                  },

这是上传功能

 String _avatarUrl;
  Future uploadFile() async {
    StorageReference storageReference = FirebaseStorage.instance
        .ref()
        .child('avatars/${_image.path}');
    StorageUploadTask uploadTask = storageReference.putFile(_image);
    await uploadTask.onComplete;
    print('File Uploaded');
    storageReference.getDownloadURL().then((fileURL) {
      setState(() {
        _avatarUrl = fileURL;
        print('Print after async function completes...');
        print(_avatarUrl);

      });
    });
  }

这是输出

I/flutter ( 7332): Print before async function complete...
I/flutter ( 7332): null
I/flutter ( 7332): Print after async function completes...
I/flutter ( 7332): https://firebasestorage.googleapis.com/v0/b/sakahapa-77a09.appspot.com/o/avatars%2Fdata%2Fuser%2F0%2Fcom.sakahapa.sakaHapa%2Fcache%2Fimage_picker246664303.jpg?alt=media&token=5868d6a5-b1e3-4aa2-8e64-72dc6e9b0a0f

如图所示，URL在SellerDatabaseService(uid:user.uid).sellerProfileSetup()之后返回
功能已运行。所以我在avatarUrl属性上得到一个空值。我该如何预防？
提前致谢。

Answer 1

更改此:

    storageReference.getDownloadURL().then((fileURL) {
      setState(() {
        _avatarUrl = fileURL;
        print('Print after async function completes...');
        print(_avatarUrl);

      });
    });

到这个:

    var fileURL = await storageReference.getDownloadURL();
      setState(() {
        _avatarUrl = fileURL;
        print('Print after async function completes...');
        print(_avatarUrl);

      });
    });

使用await而不是then()。当前，您正在输入方法uploadFile()，但是由于getDownloadURL()是异步的，因此即使在获取URL之前，print()方法也会被执行。