java将RealmObjectChangeListener添加到异步下载的RealmObject

Question

假设一个异步API调用返回的成员数据是RealmObject，并且成员被保存到realm

public class Member extends RealmObject { 
    String name;
}

现在我想将RealmObjectChangeListener添加到一个成员对象（比如，Member），以便在数据可用时更新UI。但是，首先，成员表为空，成员为null，因为获取成员数据的API调用是异步的。将RealmObjectChangeListener添加到成员将导致NullPointerException。如何处理这种情况

Answer 1

如果您正在侦听所有成员，那么很容易：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()获得的成员，绑定到ViewHolder

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

---

假设您正在等待一个特定的member和一个特定的name，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();
}

---

编辑：如果不需要更改字段级别：

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();
}