android-lifecycle笔记

摘要：的职责是为或者管理数据如何获取一个继承或者如果你需要在中使用上下文对象可以继承如果继承构造函数必须有一个类型的参数并且只能是而不是其子类在或者中获取实例对象的唯一性如果一个未被销毁那么多次获取得到的是同一个实例对象从源码中去分析这个逻辑先看

ViewModel的职责是为Activity或者Fragment管理数据.

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AndroidViewModel
class 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)

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 get(@NonNull Class 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 get(@NonNull String key, @NonNull Class 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.java
public 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.

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.

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

由图可看出,Activity重建前后,ViewModel保持不变.

LiveData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.

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.

v4包下的SupportActivity和Fragment默认都实现了LifecycleOwner(API27+),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();
}

//supportActivity
public 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 {
 //.........
}

如果你使用的是java8,可以通过添加以下依赖来使用DefaultLifecycleObserver

android.arch.lifecycle:common-java8:

然后实现DefaultLifecycleObserver

 class TestObserver implements DefaultLifecycleObserver {
     @Override
     public void onCreate(LifecycleOwner owner) {
          // your code
      }
  }

如果使用的是java7,通过注解来监听生命周期事件.并且可以给对用的生命周期方法添加一个LifecycleOwner参数.如果方法被Lifecycle.Event.ON_ANY注解,那么该方法可以额外添加一个Lifecycle.Event.ON_ANY参数.

 class TestObserver implements LifecycleObserver {
     @OnLifecycleEvent(ON_CREATE)
    void onCreated(LifecycleOwner source) {}
     @OnLifecycleEvent(ON_ANY)
    void onAny(LifecycleOwner source, Event event) {}
  }

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.