Android MVVM 架构和观察来自 API 的数据变化

Question

我是Android MVVM架构的新手。我有一个在本地运行的 API，其中包含数据("交易"(。我想简单地向 API 发出请求并在文本字段中显示该数据。目前，首次加载片段时数据不会显示，但是如果我转到另一个活动，然后返回到片段，则会加载。

这里有 3 个重要类别。

仪表板视图模型.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;
}
}

仪表板片段.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;
}
}

和存储库.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));
}
}
});
}
}

在单步执行 Repository 类中的代码时，我可以看到加载片段时调用了setDeals()，并且回调中的请求已排队。getDeals()第一次返回时，它返回 0 个成交的列表(在MutableLiveData对象内(。

在加载片段之前，回调中的onResponse不会运行。调试时，我可以看到数据在对象中(所有 Gson 的东西都可以正常工作(，但onChanged不会再次调用(设置文本视图(。

我没有正确观察deals的变化吗？

Answer 1

由于每当调用 getDeals(( 时都会创建新的实时数据实例，并且将 api 响应值通知其他实时数据实例，因此您的代码不起作用。您必须将 api 响应值设置为 getDeals(( 返回的 MutableLiveData 的同一实例

我并不是说它是最好的架构解决方案，但是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段。

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 

Answer 2

我认为这会有所帮助。尝试在网络调用的响应中对可变实时数据进行 postValue。 请按如下方式更改存储库类：

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();
MutableLiveData<List<Deal>> deals = new MutableLiveData<>();
public static Repository getInstance() {
if(instance == null) {
instance = new Repository();
}
return instance;
}
private Repository() {}
private MutableLiveData<List<Deal>> getDeals() {
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.postValue(dealsList);
}
}
});
return deals;
}
}

Answer 3

在函数获取交易中的存储库类中。 您正在初始化实时数据。 在后台线程中请求 URL 并在尚未从服务器接收的实时数据上发布值。

为了解决这个问题，请在存储库的构造函数中创建 livedata 实例，并在 onResponse 回调中对 livedata 进行 postvalue。

对不起，写得不好，从手机发布。