Android mvvm使用流程

Android mvvm使用流程
一.几种常见架构模式对比
1.MVC（Model-View-Controller）
MVC模式的优点在于模块化、可扩展性和可维护性，但缺点是控制器和视图之间的耦合度较高。
2.MVP（Model-View-Presenter）模式
Presenter同时持有Model和View对象，缺点是V层与P层还是有一定的耦合度
3.MVVM（Model-View-ViewModel）
ViewModel作为中间层，将数据从Model传递给View，
ViewModel 可以将数据与 UI 组件的生命周期进行绑定。
ViewModel 类提供了一种保存和管理界面相关数据的机制，并确保这些数据在配置更改（如设备旋转）时不会丢失或重新创建。
二.使用方法
1.model 层
getTestLiveData 返回LiveData提供 数据给ViewModel，ViewModel 及view层获取到LiveData即监听数据的变化。
创建model类

public class TestLiveDataRepositoryImpl {
    private static final String TAG = AppConstants.APP_TAG + "TestLiveDataRepositoryImpl";
    private static volatile TestLiveDataRepositoryImpl mInstance;
    private MutableLiveData<String> mTestLiveData1 = new MutableLiveData<>();
    private MutableLiveData<String> mTestLiveData2 = new MutableLiveData<>();
    //在创建一个聚合类MediatorLiveData
    private MediatorLiveData<String> mMediatorLiveData = new MediatorLiveData<>();
 
    private Handler mHandler = new Handler();
 
    public TestLiveDataRepositoryImpl() {
        //分别把mTestLiveData1和mTestLiveData2合并到mediatorLiveData中，只要有一个变化都会触发mMediatorLiveData变化
        mMediatorLiveData.addSource(mTestLiveData1, new Observer<String>() {
            @Override
            public void onChanged(String testLiveData1Str) {
                Log.i(TAG, "mMediatorLiveData.onChanged mTestLiveData1 testLiveData1Str:" + testLiveData1Str);
                mMediatorLiveData.postValue(testLiveData1Str);
            }
        });
        mMediatorLiveData.addSource(mTestLiveData2, new Observer<String>() {
            @Override
            public void onChanged(String testLiveData2Str) {
                Log.i(TAG, "mMediatorLiveData.onChanged mTestLiveData2 testLiveData2Str:" + testLiveData2Str);
                mMediatorLiveData.postValue(testLiveData2Str);
            }
        });
    }
 
    public static TestLiveDataRepositoryImpl getInstance() {
        if (mInstance == null) {
            synchronized (TestLiveDataRepositoryImpl.class) {
                if (mInstance == null) {
                    mInstance = new TestLiveDataRepositoryImpl();
                }
            }
        }
        return mInstance;
    }
 
    public LiveData<String> getTestLiveData() {
//        testDelaySendLiveData1(3);
//        testDelaySendLiveData2(6);
        return mMediatorLiveData;
    }
 
    //模拟数据1变化
    private void testDelaySendLiveData1(int second) {
        Log.i(TAG, "testDelaySendLiveData1 11 second:" + second);
        mHandler.postDelayed(() -> {
            Log.i(TAG, "testDelaySendLiveData1 22 call setValue second:" + second);
            //如果当前在子线程中必须使用postValue
            mTestLiveData1.setValue("delay " + second + " seconds");
        }, 1000 * second);
    }
    //模拟数据2变化
    private void testDelaySendLiveData2(int second) {
        Log.i(TAG, "testDelaySendLiveData2 11 second:" + second);
        mHandler.postDelayed(() -> {
            Log.i(TAG, "testDelaySendLiveData2 22 call setValue second:" + second);
            //如果当前在子线程中必须使用postValue
            mTestLiveData2.setValue("delay " + second + " seconds");
        }, 1000 * second);
    }
 
    /**
     *  模拟数据变化调用，可以通过点击按钮调用模拟触发
     */
    public void delaySetValue() {
        Log.i(TAG, "delaySetValue ");
        testDelaySendLiveData1(3);
        testDelaySendLiveData2(6);
    }
}

2.ViewModel 层
2.1 创建ViewModel 类MyViewModel

public class MyViewModel extends ViewModel {
    private static final String TAG = "MyViewModel ";
    public int count = 0;
 
    public MyViewModel() {
        count = 0;
        Log.i(TAG, "MyViewModel() ");
    }
 
 
    public LiveData<String> getTestLiveData() {
        return TestLiveDataRepositoryImpl.getInstance().getTestLiveData();
    }
 
    public void delaySetValue() {
        Log.i(TAG, "delaySetValue ");
        TestLiveDataRepositoryImpl.getInstance().delaySetValue();
    }
}

2.2 创建 ViewModelProvider.Factory 类

public class MyViewModelFactory implements ViewModelProvider.Factory {
    private static final String TAG = "MyViewModelFactory ";
    public MyViewModelFactory() {
    }
 
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        Log.i(TAG, "create() modelClass:" + modelClass);
        if (modelClass == MyViewModel.class) {
            return (T) new MyViewModel();
        }
 
        return null;
    }
}

3.创建view类UiTestActivity
ViewModelProvider初始化时如果不传递 ViewModelProvider.Factory,则ViewModelProvider内部会使用默认的 Factory,
如果配置修改比如修改语言，activity 被销毁，再次打开activity获取的还是销毁前mMyViewModel对象。
即销毁前count被设置了3,重新打开activity打印mMyViewModel.count还是3。

public class UiTestActivity extends AppCompatActivity implements View.OnClickListener {
    private static final String TAG = AppConstants.APP_TAG + "UiTestActivity ";
    private Button mBtnOne;
    private final Handler mHandler = new Handler();
    private MyViewModel mMyViewModel;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.test_activity);
 
        //如果不传递 ViewModelProvider.Factory,则内部会使用默认的 Factory,
        //如果配置修改比如修改语言，activity 被销毁，再次打开activity获取的还是销毁前mMyViewModel对象。
        //即销毁前count被设置了3,重新打开activity打印mMyViewModel.count还是3。
        //mMyViewModel = new ViewModelProvider(this).get(MyViewModel.class);
 
        mMyViewModel = new ViewModelProvider(this, new MyViewModelFactory()).get(MyViewModel.class);
 
        final Observer<String> testObserver = new Observer<String>() {
            @Override
            public void onChanged(String testStr) {
                //更新ui
                Log.d(TAG, "testObserver.onChanged testStr:" + testStr);
            }
        };
        Log.d(TAG, "onCreate ** UiTestActivity is mMyViewModel.count:" + mMyViewModel.count
                + " mMyViewModel:" + mMyViewModel);
        mMyViewModel.getTestLiveData().observe(this, testObserver);
    }
 
    @Override
    protected void onStop() {
        super.onStop();
        Log.d(TAG, "onStop");
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy");
    }
 
    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.test_btn1:
                mMyViewModel.count++;
                Log.d(TAG, "test_btn1.onClick() mMyViewModel.count:" + mMyViewModel.count);
                //通过点击按钮模拟model数据变化
                mMyViewModel.delaySetValue();
                break;
            default:
                break;
        }
    }
}