android-lifecycle笔记

ViewModelViewModel的职责是为Activity或者Fragment管理数据.如何获取一个ViewModel?继承ViewModel或者AndroidViewModel class MyViewModel : ViewModel(){ } //如果你需要在ViewModel中使用上下文对象,可以继承AndroidViewModelclass MyViewModel2(var application : Application) : AndroidViewModel(){ } 如果继承AndroidViewModel,构造函数必须有一个Application类型的参数,并且只能是Application,而不是其子类.在Activity或者Fragment中获取MyViewModel实例对象 val myViewModel = ViewModelProviders.of(activity/fragment).get(MyViewModel::class.java)ViewModel的唯一性如果一个Activity/Fragment未被销毁,那么多次获取ViewModel,得到的是同一个实例对象.从源码中去分析这个逻辑. 先看ViewModelPoviders.of()这个方法,返回一个ViewModelProvider对象 public static ViewModelProvider of(@NonNull Fragment fragment) { return of(fragment, null); } public static ViewModelProvider of(@NonNull FragmentActivity activity) { return of(activity, null); }这里只看Activity的 public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory) { Application application = checkApplication(activity); if (factory == null) { factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application); } return new ViewModelProvider(ViewModelStores.of(activity), factory); }看一下ViewModelProvider这个构造函数 public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { mFactory = factory; this.mViewModelStore = store; }当获取到ViewModelProvider对象后,调用其get方法获取到MyViewModel的实例对象看一下get方法 public <T extends ViewModel> T get(@NonNull Class<T> modelClass) { String canonicalName = modelClass.getCanonicalName(); if (canonicalName == null) { throw new IllegalArgumentException(“Local and anonymous classes can not be ViewModels”); } return get(DEFAULT_KEY + “:” + canonicalName, modelClass); } public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) { //从ViewModelStore中获取ViewModel对象 ViewModel viewModel = mViewModelStore.get(key); if (modelClass.isInstance(viewModel)) { //noinspection unchecked return (T) viewModel; } else { //noinspection StatementWithEmptyBody if (viewModel != null) { // TODO: log a warning. } } viewModel = mFactory.create(modelClass); mViewModelStore.put(key, viewModel); //noinspection unchecked return (T) viewModel; }从这里可以看到是从ViewModelStore中获取ViewModel对象.在ViewModelStore中封装了一个HashMap对象.如果HashMap对象中存储了ViewModel对象,就直接复用,如果ViewModel对象未创建,就重新创建后存储在HashMap中.要保证ViewModel是唯一的,ViewModelStore就必须是唯一的.看一下ViewModelStore是如果获取到的.在给ViewModelProvider传参数时调用了ViewModelStores.of(activity),看一下这一步做了什么?//ViewModelStores.javapublic static ViewModelStore of(@NonNull FragmentActivity activity) { if (activity instanceof ViewModelStoreOwner) { return ((ViewModelStoreOwner) activity).getViewModelStore(); } return holderFragmentFor(activity).getViewModelStore(); }查看安卓源码发现,support包下的Fragment和FragmentActivity都继承了ViewModelStoreOwner,自然要实现其方法. //FragmentActivity public ViewModelStore getViewModelStore() { if (this.getApplication() == null) { throw new IllegalStateException(“Your activity is not yet attached to the Application instance. You can’t request ViewModel before onCreate call.”); } else { if (this.mViewModelStore == null) { FragmentActivity.NonConfigurationInstances nc = (FragmentActivity.NonConfigurationInstances)this.getLastNonConfigurationInstance(); if (nc != null) { this.mViewModelStore = nc.viewModelStore; } if (this.mViewModelStore == null) { this.mViewModelStore = new ViewModelStore(); } } return this.mViewModelStore; } }可以看出,对于一个未销毁的Activity或者Fragment,其ViewModelStore对象是唯一的.那么其存储的ViewModel对象也是唯一的.ViewModel的特点由上面可知,Activity或者Fragment未被销毁,ViewModel是唯一的,那么其保存的数据也是不变的,当Activity发生了屏幕旋转等变化时,仍旧可以复用ViewModel中的数据.不要持有View层的引用ViewModel内部不要持有View层的引用,比如Activity或者Fragment.可以和LiveData结合使用生命周期由图可看出,Activity重建前后,ViewModel保持不变.LiveDataLiveData持有数据,并可在给定的生命周期内被观测.继承关系如下:[LiveData] ^- [MutableLiveData] ^- [MediatorLiveData MutableLiveData对外暴露setValue和postValue.MediatorLiveData观察别的LiveData.并能将active/inactive状态传递给它所观察的LiveData对象.postValue与setValuepostValue可以从后台线程更新数据,setValue只能在主线程中更新数据.如果同时调用这两个方法,postValue设置的数据会覆盖code>setValue设置的数据.postValue是通过ArchTaskExecutor来实现在主线程中更新数据.在ArchTaskExecutor中使用主线程的Handler.observeForever与removeObserver如果一个观察者通过observeForever被添加,这个观察者会被认为一直处于激活状态,任何数据更新都会立即通知到它.因此需要手动调用removeObserver去移除它.onActive与onInactiveLifecyclev4包下的SupportActivity和Fragment默认都实现了LifecycleOwner(<font color=’#ff0000’ size=‘6px’>API27+</font>),并且都默认初始化了lifeCycle实例 //LifecycleOwner public interface LifecycleOwner { /** * Returns the Lifecycle of the provider. * * @return The lifecycle of the provider. */ @NonNull Lifecycle getLifecycle();}//supportActivitypublic class SupportActivity extends Activity implements LifecycleOwner, Component { private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); public Lifecycle getLifecycle() { return this.mLifecycleRegistry; }}public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner, ViewModelStoreOwner { LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; }}public class LifecycleRegistry extends Lifecycle { //………}google官方MVVM架构图由图可以看出,不管数据从哪里来,最终要汇聚到数据仓库,LiveData观察的是数据仓库的变化.并且ViewModel中可以定义多个LiveData.