Android 单页应用如何在 Activity 与 Fragment 中共享状态

作者：Changing Lin

2021 年 12 月 10 日
本文字数：2590 字
阅读完需：约 8 分钟

1.需求

设计师在设计某个单页应用时，嵌入了多个标签页，而且每个标签页的业务逻辑有较高的重复性；按照以往的开发思路是，每个 Fragment 独立自身的业务逻辑，且与其他 Fragment 无关联，甚至与父 Activity 都无过多的关联，这种方式会带来一定的缺点，比如 数据同步问题、重复冗余代码、每个 Fragment 显得很沉重等。
那么有没有什么方法可以解决这个问题呢？答案是肯定的，Jetpack 库的 Lifecycle 组件。

2.知识点

Jetpack 是一个由多个库组成的套件，可帮助开发者遵循最佳做法、减少样板代码并编写可在各种 Android 版本和设备中一致运行的代码，让开发者可将精力集中于真正重要的编码工作。
Lifecycle 生命周期感知型组件可执行操作来响应另一个组件（如 Activity 和 Fragment）的生命周期状态的变化。

3.源码

修改 build.gradle 来导入依赖：

dependencies {        val lifecycle_version = "2.4.0"        val arch_version = "2.1.0"
        // ViewModel        implementation("androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version")        // LiveData        implementation("androidx.lifecycle:lifecycle-livedata:$lifecycle_version")        // Lifecycles only (without ViewModel or LiveData)        implementation("androidx.lifecycle:lifecycle-runtime:$lifecycle_version")
        // Saved state module for ViewModel        implementation("androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version")
        // Annotation processor        annotationProcessor("androidx.lifecycle:lifecycle-compiler:$lifecycle_version")        // alternately - if using Java8, use the following instead of lifecycle-compiler        implementation("androidx.lifecycle:lifecycle-common-java8:$lifecycle_version")
        // optional - helpers for implementing LifecycleOwner in a Service        implementation("androidx.lifecycle:lifecycle-service:$lifecycle_version")
        // optional - ProcessLifecycleOwner provides a lifecycle for the whole application process        implementation("androidx.lifecycle:lifecycle-process:$lifecycle_version")
        // optional - ReactiveStreams support for LiveData        implementation("androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version")
        // optional - Test helpers for LiveData        testImplementation("androidx.arch.core:core-testing:$arch_version")    }

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父 Activity 的不完全代码：

public class CenterActivity extends BaseActivity {
    private ViewPager viewPager;    private BottomNavigationView bottomNavigationView;    private TextView tvTitle;    private CenterViewModel viewModel;    private EmptyViewModel emptyViewModel;
    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_pro);
        bottomNavigationView = findViewById(R.id.nav_view);        viewPager = findViewById(R.id.view_pager);
        ViewModelProvider provider = new ViewModelProvider(this); // 基于lifecycle-viewmodel来构建，并且参数是当前Activity，父类是ComponentActivity        emptyViewModel = provider.get(EmptyViewModel.class); // 构建EmptyViewModel对象				viewModel = provider.get(CenterViewModel.class); // 构建CenterViewModel对象        viewPager.setAdapter(viewModel.getAdapter(this));        viewPager.addOnPageChangeListener(onPageChangeListener);
        bottomNavigationView.setOnNavigationItemSelectedListener(this::onNavigationItemSelected);    }}

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子 Fragment 的不完全代码：

public class DashboardFragment extends Fragment {
    private DashboardViewModel dashboardViewModel;    private FragmentDashboardBinding binding;    private MapView mMapView = null;    private MapViewModel mapViewModel;    private Bundle savedInstanceState;    private EmptyViewModel emptyViewModel;    private HomeViewModel homeViewModel;
    public View onCreateView(@NonNull LayoutInflater inflater,                             ViewGroup container, Bundle savedInstanceState) {        ViewModelProvider provider = new ViewModelProvider(getActivity()); // 基于lifecycle-viewmodel来构建，并且参数是父Activity，即上面的CenterActivity        dashboardViewModel = provider.get(DashboardViewModel.class);
        binding = FragmentDashboardBinding.inflate(inflater, container, false);        View root = binding.getRoot();
        final TextView textView = binding.textDashboard;        mMapView = binding.map;        mapViewModel = provider.get(MapViewModel.class);        this.savedInstanceState = savedInstanceState;
        permissionConfigMap();
        binding.ibCenter.setOnClickListener(this::onClick);
        emptyViewModel = provider.get(EmptyViewModel.class); // 构建EmptyViewModel对象，请注意 此时的emptyViewModel对象与CenterActivity中的emptyViewModel是同一个对象        homeViewModel = provider.get(HomeViewModel.class);
        binding.positionTitleView.setAdapter(emptyViewModel.getAdapter((BaseActivity) getActivity()));
        return root;    }}

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4.验证

通过添加日志打印，我们可以在日志窗口看到如下，至此 Activity 与 Fragment 中共享状态问题得到解决，我们可以把状态存放到 ViewModel 中，便于访问。

2021-12-10 17:27:11.723 25482-25482/com.nufront.app_master E/cclin: [, , 0]:这里是CenterActivity：name.EmptyViewModel@2353d44a2021-12-10 17:27:12.158 25482-25482/com.nufront.app_master E/cclin: [, , 0]:这里是DashboardFragment页面：name.EmptyViewModel@2353d44a

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发布于: 4 小时前阅读数: 7

原文链接:【http://xie.infoq.cn/article/283278b7593603c3c01bfbb08】。文章转载请联系作者。