Android Arch Component Sample简单解析
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BasicSample
这是谷歌官方提供的一个关于Android Architecture Component 库使用的例子,通过查看源码,这个例子只简单的展示如何使用Architecture Component库中的相关构件的使用,包括LifeCycle,ViewModel,Room,LiveData等,其中还涉及了DataBinding这个库。看起来挺简单的,并且不涉及网络请求,只是单纯的对数据库进行初始化,然后加载数据库的相关数据。
接下来我们来看下具体的代码。
首先我们回顾下官方提供的架构:
从上图中我们可以看到,首先创建一个页面(Activity/Fragment),然后根据显示的页面创建ViewModel,ViewModel通过LiveData去渲染当前的页面,LiveData通过Repository来初始化数据,如果有本地数据则加载本地数据,如果没有则进行网络请求,通过网络请求获取数据,然后本地化,然后重新加载。这么一个流程下来后,页面就加载完成了。
然后我们在页面的生命周期中加入一些日志:
通过代码我们梳理了具体的流程如下:
如何使用生命感知:
一方面是通过数据的观察,一方面是对生命周期的感知,接下来上代码
虽然代码不多,我们也只看关键部分:
ProductListFragment.java
@Override public void onActivityCreated(@Nullable Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); Log.d("AAC", "ProductListFragment onActivityCreated"); final ProductListViewModel viewModel = ViewModelProviders.of(this).get(ProductListViewModel.class); subscribeUi(viewModel); }
这时为对应的页面创建ViewModel。
主要是如何订阅UI,并根据数据变更,刷新UI
private void subscribeUi(ProductListViewModel viewModel) { // Update the list when the data changes Log.d("AAC", "ProductListFragment subscribeUi"); viewModel.getProducts().observe(this, new Observer<List<ProductEntity>>() { @Override public void onChanged(@Nullable List<ProductEntity> myProducts) { Log.d("AAC", "onChanged() called with: myProducts = [" + myProducts + "]"); if (myProducts != null) { mBinding.setIsLoading(false); mProductAdapter.setProductList(myProducts); } else { mBinding.setIsLoading(true); } } }); }
通过对LiveData < List < ProductEntity > >数据的监听,通过观察数据的变化对UI进行刷新。
然后我们就看什么时候数据会变化-》
ProductListViewModel.java
public ProductListViewModel(Application application) { super(application); Log.d("AAC", "ProductListViewModel()"); final DatabaseCreator databaseCreator = DatabaseCreator.getInstance(this.getApplication()); LiveData<Boolean> databaseCreated = databaseCreator.isDatabaseCreated(); Log.d("AAC", "ProductListViewModel: databaseCreated = " + databaseCreated.getValue()); mObservableProducts = Transformations.switchMap(databaseCreated, new Function<Boolean, LiveData<List<ProductEntity>>>() { @Override public LiveData<List<ProductEntity>> apply(Boolean isDbCreated) { Log.d("AAC", "ProductListViewModel Transformations.switchMap apply() called with: isDbCreated = [" + isDbCreated + "]"); if (!Boolean.TRUE.equals(isDbCreated)) { // Not needed here, but watch out for null //noinspection unchecked return ABSENT; } else { //noinspection ConstantConditions return databaseCreator.getDatabase().productDao().loadAllProducts(); } } }); databaseCreator.createDb(this.getApplication()); Log.d("AAC", "ProductListViewModel: databaseCreated = " + databaseCreated.getValue()); }
在ProductListViewModel构造的时候,会通过对LiveData< Boolean > databaseCreated数据进行观察,通过数据的变化来初始化LiveData < List < ProductEntity > >数据。
DatabaseCreator.java
/** * Creates or returns a previously-created database. * <p> * Although this uses an AsyncTask which currently uses a serial executor, it's thread-safe. */ public void createDb(Context context) { Log.d("AAC", "DatabaseCreator createDb"); Log.d("DatabaseCreator", "Creating DB from " + Thread.currentThread().getName()); if (!mInitializing.compareAndSet(true, false)) { return; // Already initializing } mIsDatabaseCreated.setValue(false);// Trigger an update to show a loading screen. new AsyncTask<Context, Void, Void>() { @Override protected Void doInBackground(Context... params) { Log.d("AAC", "DatabaseCreator createDb doInBackground: "); Log.d("DatabaseCreator", "Starting bg job " + Thread.currentThread().getName()); Context context = params[0].getApplicationContext(); // Reset the database to have new data on every run. context.deleteDatabase(DATABASE_NAME); // Build the database! AppDatabase db = Room.databaseBuilder(context.getApplicationContext(), AppDatabase.class, DATABASE_NAME).build(); // Add a delay to simulate a long-running operation addDelay(); // Add some data to the database DatabaseInitUtil.initializeDb(db); Log.d("DatabaseCreator", "DB was populated in thread " + Thread.currentThread().getName()); mDb = db; return null; } @Override protected void onPostExecute(Void ignored) { // Now on the main thread, notify observers that the db is created and ready. Log.d("AAC", "DatabaseCreator createDb onPostExecute"); mIsDatabaseCreated.setValue(true); Log.d("AAC", "DatabaseCreator createDb onPostExecute mIsDatabaseCreated = " + mIsDatabaseCreated.getValue()); } }.execute(context.getApplicationContext()); }
我们可以看到是在创建数据库的时候,去设置databaseCreated,通过这个变量可以控制控制是否显示加载中,当数据初始化完成后,再设置为true。
就这样,所有的事件都串起来了,接下来就是核心的如何触发这些事件的代码。
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