java - Android MVVM 架构并通过 API 观察数据变化

Question

我是 Android MVVM 架构的新手。我有一个本地运行的 API，其中包含数据(“交易”)。我只想向 API 发出请求并在文本字段中显示该数据。目前，首次加载 fragment 时数据不会显示，但如果我转到另一个 Activity ，然后返回到它加载的 fragment 。

这里有 3 个重要类别。

DashboardViewModel.java:

package com.example.android_client.ui.dashboard;

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

import com.example.android_client.models.Deal;
import com.example.android_client.repository.Repository;

import java.util.List;

public class DashboardViewModel extends ViewModel {

    private MutableLiveData<String> mText;
    private Repository repository;
    private MutableLiveData<List<Deal>> deals = null;

    public void init() {
        if(this.deals == null) {
            this.repository = Repository.getInstance();
            this.deals = this.repository.getDeals();
        }
    }

    public DashboardViewModel() {
        this.mText = new MutableLiveData<>();
    }

    public LiveData<List<Deal>> getDeals() {
        return this.deals;
    }
}

DashboardFragment.java:

package com.example.android_client.ui.dashboard;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;

import com.example.android_client.R;
import com.example.android_client.models.Deal;

import java.util.List;

public class DashboardFragment extends Fragment {

    private DashboardViewModel dashboardViewModel;

    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_dashboard, container, false);
        final TextView textView = root.findViewById(R.id.text_dashboard);
        dashboardViewModel = ViewModelProviders.of(this).get(DashboardViewModel.class);
        dashboardViewModel.init();
        dashboardViewModel.getDeals().observe(this, new Observer<List<Deal>>() {
            @Override
            public void onChanged(List<Deal> deals) {
                if (deals != null && !deals.isEmpty()) {
                    System.out.println(deals.get(0).toString());
                    textView.setText(deals.get(0).toString());
                }
            }
        });
        return root;
    }
}

和Repository.java:

package com.example.android_client.repository;

import androidx.lifecycle.MutableLiveData;

import com.example.android_client.models.Deal;
import com.google.gson.Gson;

import org.jetbrains.annotations.NotNull;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.ResponseBody;

public class Repository {

    private static Repository instance;
    private ArrayList<Deal> dealsList = new ArrayList<>();
    private final OkHttpClient client = new OkHttpClient();

    public static Repository getInstance() {
        if(instance == null) {
            instance = new Repository();
        }
        return instance;
    }

    private Repository() {}

    public MutableLiveData<List<Deal>> getDeals() {
        setDeals();
        MutableLiveData<List<Deal>> deals = new MutableLiveData<>();
        deals.setValue(dealsList);
        return deals;
    }

    private void setDeals() {
        Request request = new Request.Builder()
                .url("http://10.0.2.2:8000/api/deals?<params here>")
                .build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(@NotNull Call call, @NotNull IOException e) {
                e.printStackTrace();
            }

            @Override
            public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
                try (ResponseBody responseBody = response.body()) {
                    if (!response.isSuccessful()) {
                        throw new IOException("Unexpected code " + response);
                    }
                    String jsonDeals = responseBody.string(); // can only call string() once or you'll get an IllegalStateException
                    Deal[] deals = new Gson().fromJson(jsonDeals, Deal[].class);
                    dealsList = new ArrayList<>(Arrays.asList(deals));
                }
            }
        });

    }
}

单步执行 Repository 类中的代码时，我可以看到加载 fragment 时调用了 setDeals()，并且回调中的请求已排队。第一次 getDeals() 返回时，它返回 0 个交易的列表(在 MutableLiveData 对象内)。

回调中的

onResponse 在 fragment 加载之前不会运行。调试时，我可以看到数据在对象中(所有 Gson 的东西都工作正常)，但是 onChanged 不会再次被调用(它设置 TextView )。

我是否没有正确观察交易的变化？

Answer 1

由于每当调用 getDeals() 时都会创建一个新的实时数据实例，并且将 api 响应值通知给其他实时数据实例，因此您的代码无法正常工作。您必须将 api 响应值设置为 getDeals() 返回的 MutableLiveData 的同一实例

我并不是说这是最好的架构解决方案，但如果您创建一个可变的实时数据作为类属性，并在调用 getDeals() 时返回它。也许，它会起作用。

此外，一个好的做法是返回 LiveData 而不是 MutableLiveData，以不允许外部组件修改内部值。

请看一下下面的代码。

OBS:也许有一些语法错误，因为我还没有编译它

    import com.example.android_client.models.Deal;
    import com.google.gson.Gson;
    import org.jetbrains.annotations.NotNull;
    import java.io.IOException;
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;

    import okhttp3.Call;
    import okhttp3.Callback;
    import okhttp3.OkHttpClient;
    import okhttp3.Request;
    import okhttp3.Response;
    import okhttp3.ResponseBody;

        public class Repository {

        private static Repository instance;
        private ArrayList<Deal> dealsList = new ArrayList<>();
        private final OkHttpClient client = new OkHttpClient();
        private MutableLiveData<List<Deal>> _deals = new MutableLiveData<>();                         
        private LiveData<List<Deal>> deals = _deals


        public static Repository getInstance() {
            if(instance == null) {
                instance = new Repository();
            }
            return instance;
        }

        private Repository() {}

        public LiveData<List<Deal>> getDeals() {
            setDeals();
            return deals;
        }

        private void setDeals() {
            Request request = new Request.Builder()
                    .url("http://10.0.2.2:8000/api/deals?<params here>")
                    .build();

            client.newCall(request).enqueue(new Callback() {
                @Override
                public void onFailure(@NotNull Call call, @NotNull IOException e) {
                    e.printStackTrace();
                }

                @Override
                public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
                    try (ResponseBody responseBody = response.body()) {
                        if (!response.isSuccessful()) {
                            throw new IOException("Unexpected code " + response);
                        }
                        String jsonDeals = responseBody.string(); // can only call string() once or you'll get an IllegalStateException
                        Deal[] deals = new Gson().fromJson(jsonDeals, Deal[].class);
                        dealsList = new ArrayList<>(Arrays.asList(deals));
                        _deals.setValue(dealsList);

                    }
                }
            });

        }

}
When