java - 将 RealmObjectChangeListener 添加到异步下载的 RealmObject

Question

假设异步API调用返回Member Data，它是一个RealmObject，并且Member被保存到realm中。

public class Member extends RealmObject { 
    String name;
}

现在我想将 RealmObjectChangeListener 添加到成员对象(例如成员)，以便我们可以在数据可用时更新 UI。但是，首先，Member 表为空，member 为 null，因为用于获取成员数据的 API 调用是异步的。将 RealmObjectChangeListener 添加到成员将导致 NullPointerException。遇到这种情况该如何处理呢？

Answer 1

如果您正在监听所有成员，那么很简单:保留对成员的字段引用(在 RecyclerView 的 ViewHolder 中很容易)并将 RealmObjectChangeListener 添加到从 results.get() 获取的成员中，绑定(bind)到 ViewHolder。

void bind(Member member) {
    if(this.member != null) {
        this.member.removeAllChangeListeners();
    }
    this.member = member;
    this.member.addChangeListener(...);

<小时/>

假设您正在等待具有特定名称的特定成员，并且您只关心该成员:

private final Handler handler = new Handler(Looper.getMainLooper());

private RealmResults<Member> objectResults;
private RealmChangeListener<RealmResults<Member>> objectTracker = (results) -> {
    if(member != null && member.isValid()) {
        return;
    }
    if(realm.isInTransaction()) {
        handler.post(() -> {
            setupMember(results);
        });
    } else {
        setupMember(results);
    }
};

private void setupMember(RealmResults<Member> members) {
    if(member != null && member.isValid()) {
        member.removeAllChangeListeners();
    }
    member = members.first(null);
    if(member != null) {
        member.addChangeListener((member, fieldChangeSet) -> {
            // do whatever
        }); 
    }
}

private Member member;

private Realm realm;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(...);
    realm = Realm.getDefaultInstance();
    objectResults = realm.where(Member.class).equalTo("name", name).findAllAsync();
    objectResults.addChangeListener(objectTracker);
}

@Override
protected void onDestroy() {
    super.onDestroy();
    if(member != null) {
        member.removeAllChangeListeners();
    }
    objectResults.removeAllChangeListeners();
    realm.close();
}

<小时/>

编辑:如果您不需要字段级更改:

private RealmResults<Member> objectResults;
private RealmChangeListener<RealmResults<Member>> objectTracker = (results) -> {
    setupMember(results);
};

private void setupMember(RealmResults<Member> members) {
    Member member = members.first(null);
    if(member != null) {
        updateViews(member); // update views with member
    } else {
        // member doesn't exist or was deleted
    }
}

private Realm realm;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(...);
    realm = Realm.getDefaultInstance();
    objectResults = realm.where(Member.class).equalTo("name", name).findAllAsync();
    objectResults.addChangeListener(objectTracker);
}

@Override
protected void onDestroy() {
    super.onDestroy();
    objectResults.removeAllChangeListeners();
    realm.close();
}