android-lifecycle笔记

ViewModel

ViewModel的职责是为Activity或者Fragment管理数据.

如何获取一个ViewModel?

1. 继承ViewModel或者AndroidViewModel

class MyViewModel : ViewModel(){
     
 }
 
 //若是你须要在ViewModel中使用上下文对象,能够继承AndroidViewModel
class MyViewModel2(var application : Application) : AndroidViewModel(){
    
}

若是继承 AndroidViewModel,构造函数必须有一个 Application类型的参数,而且只能是 Application,而不是其子类.

1. 在Activity或者Fragment中获取MyViewModel实例对象

val myViewModel = ViewModelProviders.of(activity/fragment).get(MyViewModel::class.java)

ViewModel的惟一性

1. 若是一个Activity/Fragment未被销毁,那么屡次获取ViewModel,获得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;
    }

从这里能够看到是从ViewModelStore中获取ViewModel对象.在ViewModelStore中封装了一个HashMap对象.若是HashMap对象中存储了ViewModel对象,就直接复用,若是ViewModel对象未建立,就从新建立后存储在HashMap中.

要保证ViewModel是惟一的,ViewModelStore就必须是惟一的.看一下ViewModelStore是若是获取到的.
在给ViewModelProvider传参数时调用了ViewModelStores.of(activity),看一下这一步作了什么?

//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,天然要实现其方法.

//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;
        }
    }

能够看出,对于一个未销毁的Activity或者Fragment,其ViewModelStore对象是惟一的.那么其存储的ViewModel对象也是惟一的.

ViewModel的特色

由上面可知,Activity或者Fragment未被销毁,ViewModel是惟一的,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,仍旧能够复用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,好比Activity或者Fragment.能够和LiveData结合使用

生命周期

由图可看出,Activity重建先后,ViewModel保持不变.

LiveData

LiveData持有数据,并可在给定的生命周期内被观测.
继承关系以下:
[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setValue

postValue能够从后台线程更新数据,setValue只能在主线程中更新数据.若是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是经过ArchTaskExecutor来实如今主线程中更新数据.在ArchTaskExecutor中使用主线程的Handler.

observeForever与removeObserver

若是一个观察者经过observeForever被添加,这个观察者会被认为一直处于激活状态,任何数据更新都会当即通知到它.所以须要手动调用removeObserver去移除它.

onActive与onInactive

Lifecycle

v4包下的SupportActivity和Fragment默认都实现了LifecycleOwner(<font color='#ff0000' size='6px'>API27+</font>),而且都默认初始化了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 {
 //.........
}

LifecycleObserver

若是你使用的是java8,能够经过添加如下依赖来使用DefaultLifecycleObserver

android.arch.lifecycle:common-java8:<version>

而后实现DefaultLifecycleObserver

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

若是使用的是java7,经过注解来监听生命周期事件.而且能够给对用的生命周期方法添加一个LifecycleOwner参数.若是方法被Lifecycle.Event.ON_ANY注解,那么该方法能够额外添加一个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) {}
  }

google官方MVVM架构图

由图能够看出,无论数据从哪里来,最终要汇聚到数据仓库,LiveData观察的是数据仓库的变化.而且ViewModel中能够定义多个LiveData.