Xamarin.Forms:依赖部服务以获取Firebase Android数据

Question

我正在使用xamarin.forms实现相同的UI对iOS和Android实现，但是我必须实现单独读取和写入数据的功能(它们是不同的(。

我想将所有数据从firebase放在观察力的情况下，这样：

ObservableCollection<Post> posts;
posts = DependencyService.Get<IFeed>().getPosts();

和所谓的Android代码为：

    ObservableCollection<Post> posts = new ObservableCollection<Post>();
    public FeedAndroid() 
    { 
        database = FirebaseDatabase.GetInstance(MainActivity.app);
        dataRef = database.Reference;
        postsRef = dataRef.Child("posts");
        posts.Clear();
        postsRef.AddChildEventListener(this);
    }
    public void OnChildAdded(DataSnapshot snapshot, string previousChildName)
    {
        Post newPost = new Post { Title = snapshot.Child("title")?.GetValue(true)?.ToString(),
        Desc = snapshot.Child("desc")?.GetValue(true)?.ToString(),
        Img = snapshot.Child("image")?.GetValue(true)?.ToString()};
        posts.Add(newPost);
    }
    public ObservableCollection<Post> getPosts()
    {
        return posts;
    }

但这不起作用。有什么想法？

Answer 1

我想您的FeedAndroid()是实现IFeed接口的类的构造函数。您的FirebaseDatabase实例是在此构造函数中创建的，这可能是这里的问题。我建议在MainActivity中实现它。例如：

public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity, IChildEventListener
{
    public DatabaseReference postsRef;
    public static ObservableCollection<Post> collection = new ObservableCollection<Post>();
    protected override void OnCreate(Bundle bundle)
    {
        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;
        base.OnCreate(bundle);
        global::Xamarin.Forms.Forms.Init(this, bundle);
        LoadApplication(new App());
    }
    protected override void OnResume()
    {
        base.OnResume();
        FirebaseApp.InitializeApp(this);
        FirebaseAuth mAuth = FirebaseAuth.Instance;
        FirebaseUser user = mAuth.CurrentUser;
        if (user == null)
        {
            var result = mAuth.SignInAnonymously();
        }
        postsRef = FirebaseDatabase.Instance.Reference.Child("posts");
        postsRef.AddChildEventListener(this);
    }
    public void OnCancelled(DatabaseError error)
    {
        //TODO:
    }
    public void OnChildAdded(DataSnapshot snapshot, string previousChildName)
    {
        collection.Add(new Post() {//Your Data here});
    }
    public void OnChildChanged(DataSnapshot snapshot, string previousChildName)
    {
        //TODO:
    }
    public void OnChildMoved(DataSnapshot snapshot, string previousChildName)
    {
        //TODO:
    }
    public void OnChildRemoved(DataSnapshot snapshot)
    {
        //TODO:
    }
}

，在您的FeedAndroid类中，只需返回此collection：

public ObservableCollection<Post> getPosts()
{
    return MainActivity.collection;
}