一、业务逻辑持有者

在此之前，无论是MVC模式，还是MVP模式，在视图层，都会产生对业务逻辑层的依赖，从而导致出现内存泄漏的情况。那ViewModel又是如何处理业务逻辑层和视图层的依赖，从而避免了常见的内存泄漏的问题的呢？
在这里插入图片描述

依赖Lifecycle对宿主生命周期的感知，处理对Viewmodel的依赖

1.1、应用

NormalViewModel

package com.anniljing.viewmodelcorestudy;

import android.app.Application;

import java.util.ArrayList;
import java.util.List;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;

public class NormalViewModel extends AndroidViewModel {
    public NormalViewModel(@NonNull Application application) {
        super(application);
    }

    public List<Integer> getdatas() {
        List<Integer> data = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            data.add(i);
        }
        return data;
    }
}

NormalActivity

package com.anniljing.viewmodelcorestudy;

import android.os.Bundle;

import com.anniljing.viewmodelcorestudy.databinding.ActivityNormalBinding;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

public class NormalActivity extends AppCompatActivity {
    private ActivityNormalBinding mBinding;
    private NormalViewModel mViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBinding = ActivityNormalBinding.inflate(getLayoutInflater());
        setContentView(mBinding.getRoot());
        mViewModel = new ViewModelProvider(this).get(NormalViewModel.class);
    }
}

ComponentActivity源码

          getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });

此处在Activity中Lifecycle的ON_DESTROY事件中，清除了ViewModel。

1.2、源码解析

在这里插入图片描述

1.2.1 创建ViewModelStore

public ComponentActivity() {
        Lifecycle lifecycle = getLifecycle();
        //noinspection ConstantConditions
        if (lifecycle == null) {
            throw new IllegalStateException("getLifecycle() returned null in ComponentActivity's "
                    + "constructor. Please make sure you are lazily constructing your Lifecycle "
                    + "in the first call to getLifecycle() rather than relying on field "
                    + "initialization.");
        }
        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });
        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                ensureViewModelStore();
                getLifecycle().removeObserver(this);
            }
        });
    }

在ComponentActivity的构造方法中，注册了多个Lifecycle的观察者，其中有两个涉及ViewModelStore的，一个是收到宿主的生命周期事件后，就创建了ViewModelStore,随后就移除了该观察者；另一个则是监听到宿主的ON_DESTROY事件后，清空ViewModelStore的ViewModel

1.2.2、创建Factory

1.2.2.1、创建ViewModel的时候，第一步我们创建了ViewModelProvider，调用了其中一个构造方法

 public constructor(
        owner: ViewModelStoreOwner
    ) : this(owner.viewModelStore, defaultFactory(owner), defaultCreationExtras(owner))

1.2.2.2、ViewModelProvider的构造方法中，调用了默认的Factory

public companion object {
            internal fun defaultFactory(owner: ViewModelStoreOwner): Factory =
                if (owner is HasDefaultViewModelProviderFactory)
                    owner.defaultViewModelProviderFactory else instance

            internal const val DEFAULT_KEY = "androidx.lifecycle.ViewModelProvider.DefaultKey"
        }

1.2.2.3、由于ComponentActivity实现了HasDefaultViewModelProviderFactory接口，所以Factory的实际对象是SavedStateViewModelFactory

  @NonNull
    @Override
    public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
    }

1.2.3、创建ViewModel

public open operator fun <T : ViewModel> get(key: String, modelClass: Class<T>): T {
        val viewModel = store[key]
        if (modelClass.isInstance(viewModel)) {
            (factory as? OnRequeryFactory)?.onRequery(viewModel)
            return viewModel as T
        } else {
            @Suppress("ControlFlowWithEmptyBody")
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        val extras = MutableCreationExtras(defaultCreationExtras)
        extras[VIEW_MODEL_KEY] = key
        // AGP has some desugaring issues associated with compileOnly dependencies so we need to
        // fall back to the other create method to keep from crashing.
        return try {
            factory.create(modelClass, extras)
        } catch (e: AbstractMethodError) {
            factory.create(modelClass)
        }.also { store.put(key, it) }
    }

应用层通过ViewModelProvider的get方法创建ViewModel,并且保存到ViewModelStore中。

二、屏幕状态的持有者

该功能主要处理当设备配置发生变化时，界面会重新绘制，导致数据丢失的问题。
在这里插入图片描述
屏幕发生旋转的时候，会调用onDestroy方法，界面会重新绘制。

2.1、ViewModel之前

2.1.1、manifest中配置

在这里插入图片描述
在activity里面的configChanges属性里面配置orientation|screenSize，则屏幕旋转的时候，界面就不会重新绘制。

2.1.2、onSaveInstanceState和onRestoreInstanceState

如果manifest里面没有配置configChanges，则屏幕旋转的时候，在onDestroy之前会调用onSaveInstanceState，在恢复界面的时候，会调用onRestoreInstanceState，这样我们就可以在以上两个方法中做保存和恢复数据的逻辑。

package com.anniljing.viewmodelcorestudy;

import android.os.Bundle;
import android.util.Log;

import com.anniljing.viewmodelcorestudy.databinding.ActivitySaveStateBinding;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

public class SaveStateActivity extends AppCompatActivity {
    private static final String TAG = "SaveStateActivity";
    private ActivitySaveStateBinding mBinding;
    private NormalState mNormalState;
    private static final String KEY="NormalState";
    private String saveString="";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.e(TAG, "onCreate");
        if (savedInstanceState !=null){
            saveString=savedInstanceState.getString(KEY,"");
        }
        mBinding = ActivitySaveStateBinding.inflate(getLayoutInflater());
        setContentView(mBinding.getRoot());
        mNormalState = new NormalState();
        mNormalState.setString(saveString);
        Log.e(TAG, mNormalState.getString());
        mBinding.tvState.setOnClickListener((view) -> {
            mNormalState.setString("NormalState");
            Log.e(TAG,"Click after:"+ mNormalState.getString());
        });
    }

    @Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
        super.onSaveInstanceState(outState);
        Log.e(TAG, "onSaveInstanceState");
        outState.putString(KEY,"NormalState");
    }

    @Override
    protected void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        Log.e(TAG, "onRestoreInstanceState");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.e(TAG, "onDestroy");
    }
}

保存数据：
在这里插入图片描述
重新绘制的时候，取出数据：

在onSaveInstanceState里面增加保存逻辑，在onCreate方法里面就可以恢复以前的数据了。

2.2、使用ViewModel

ViewModelSaveState：

package com.anniljing.viewmodelcorestudy;

import android.app.Application;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;

public class ViewModelSaveState extends AndroidViewModel {
    private String mString;

    public ViewModelSaveState(@NonNull Application application) {
        super(application);
    }

    public String getString() {
        return mString;
    }

    public void setString(String string) {
        mString = string;
    }
}

package com.anniljing.viewmodelcorestudy;

import android.os.Bundle;
import android.util.Log;

import com.anniljing.viewmodelcorestudy.databinding.ActivitySaveStateBinding;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

public class SaveStateActivity extends AppCompatActivity {
    private static final String TAG = "SaveStateActivity";
    private ActivitySaveStateBinding mBinding;
    private ViewModelSaveState mViewModelSaveState;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.e(TAG, "onCreate");
        mBinding = ActivitySaveStateBinding.inflate(getLayoutInflater());
        setContentView(mBinding.getRoot());
        mViewModelSaveState = new ViewModelProvider(this).get(ViewModelSaveState.class);
        Log.e(TAG,"Before click:"+mViewModelSaveState.getString());
        mBinding.tvState.setOnClickListener((view) -> {
            mViewModelSaveState.setString("ViewModelState");
            Log.e(TAG, "Click after:" + mViewModelSaveState.getString());
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.e(TAG,"onDestroy");
    }
}

在这里插入图片描述
可以看到，引入ViewModel以后，不需要在onSaveInstanceState方法中增加额外的保存逻辑，重绘后就可以恢复以前的数据。

无论是ViewModel,还是重写onSaveInstanceState方法，当内存有限导致界面退出重绘时，就无法恢复以前的数据了。
我们可以通过以下操作来模拟内存不足时，导致应用杀死的效果：
1、启用开发者模式
2、打开“不保留活动”
3、打开应用后，按home键，退出应用
4、重新打开应用。
在这里插入图片描述

2.3、使用SavedStateHandle

ViewModelWithHandleSaveState

package com.anniljing.viewmodelcorestudy;

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.SavedStateHandle;
import androidx.lifecycle.ViewModel;

public class ViewModelWithHandleSaveState extends ViewModel {
    private static final String KEY = "ViewModelWithHandleSaveState";
    private SavedStateHandle mStateHandle;

    public ViewModelWithHandleSaveState(SavedStateHandle stateHandle) {
        mStateHandle = stateHandle;
    }

    public MutableLiveData<String> getLiveData() {
        return mStateHandle.getLiveData(KEY);
    }
}

1、构造方法里面添加SavedStateHandle参数。
2、获取的时候通过mStateHandle。
SaveStateActivity

package com.anniljing.viewmodelcorestudy;

import android.os.Bundle;
import android.util.Log;

import com.anniljing.viewmodelcorestudy.databinding.ActivitySaveStateBinding;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

public class SaveStateActivity extends AppCompatActivity {
    private static final String TAG = "SaveStateActivity";
    private ActivitySaveStateBinding mBinding;
    private ViewModelWithHandleSaveState mWithHandleSaveState;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.e(TAG, "onCreate");
        mBinding = ActivitySaveStateBinding.inflate(getLayoutInflater());
        setContentView(mBinding.getRoot());
        mWithHandleSaveState = new ViewModelProvider(this).get(ViewModelWithHandleSaveState.class);
        Log.e(TAG, "Before click:" + mWithHandleSaveState.getLiveData().getValue());
        mWithHandleSaveState.getLiveData().observe(this, s -> {
            Log.e(TAG, "After click:" + mWithHandleSaveState.getLiveData().getValue());
        });
        mBinding.tvState.setOnClickListener((view) -> mWithHandleSaveState.getLiveData().postValue("ViewModelWithHandleSaveState"));
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.e(TAG, "onDestroy");
    }
}