Zzz个人博客
0%

ViewModel

Posted on Edited on In
Symbols count in article: 15k Reading time ≈ 13 mins.

ViewModel

ViewModelProviders

1
2
3
class TestViewModel extends ViewModel

TestViewModel test = ViewModelProviders.of(this).get(TestViewModel.class);

在ViewModelProviders中会创建ViewModelProvider，通过ViewModelProvider来管理ViewModel。

1
2
3
4
5
6
7
8
9
10
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory) {
    Application application = checkApplication(activity);
    if (factory == null) {
        factory = AndroidViewModelFactory.getInstance(application);
    }

    return new ViewModelProvider(ViewModelStores.of(activity), (Factory)factory);
}

ViewModelProvider中的核心要素只有ViewModelProvider.Factory和ViewModelStore。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class ViewModelProvider {
    private final ViewModelProvider.Factory mFactory;
    private final ViewModelStore mViewModelStore;
    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        ViewModel viewModel = this.mViewModelStore.get(key);
        if (modelClass.isInstance(viewModel)) {
            return viewModel;
        } else {
            if (viewModel != null) {
            }

            viewModel = this.mFactory.create(modelClass);
            this.mViewModelStore.put(key, viewModel);
            return viewModel;
        }
    }
}

在ViewModelProvider.get时，先从mViewModelStore中取，取不到就通过mFactory创建。下面分别是2个
Factory中的创建方式，当有包含Application构造函数时，注入mApplication，不包含则直接无参数构造。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
public static class NewInstanceFactory implements ViewModelProvider.Factory{
    @NonNull
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        try {
            return (ViewModel)modelClass.newInstance();
        } catch (InstantiationException var3) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, var3);
        } catch (IllegalAccessException var4) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, var4);
        }
    }
}
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
    @NonNull
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
            try {
                return (ViewModel)modelClass.getConstructor(Application.class).newInstance(this.mApplication);
            } catch (NoSuchMethodException var3) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, var3);
            } catch (IllegalAccessException var4) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, var4);
            } catch (InstantiationException var5) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, var5);
            } catch (InvocationTargetException var6) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, var6);
            }
        } else {
            return super.create(modelClass);
        }
    }
}

通过下图可看到ViewModelStore可来源于FragmentActivity，非FragmentActivity可以通过ViewModelStores._of_(activity) 中的HolderFragment 来注入，原理同后文的ReportFragment。
#### LiveData.postValue
可在子线程中触发，最终使用的是setValue，如果发送数据过快，存在数据丢失可能，每次从mPendingData中取最新数据。
在post时将数据复制到mPendingData，送入主循环队列，后面发送数据时，从mPendingData缓存中取数据。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
protected void postValue(T value) {
    boolean postTask;
    synchronized(this.mDataLock) {
        postTask = this.mPendingData == NOT_SET;
        this.mPendingData = value;
    }

    if (postTask) {
        ArchTaskExecutor.getInstance().postToMainThread(this.mPostValueRunnable);
    }
}

this.mPostValueRunnable = new Runnable() {
    public void run() {
        Object newValue;
        synchronized(LiveData.this.mDataLock) {
            newValue = LiveData.this.mPendingData;
            LiveData.this.mPendingData = LiveData.NOT_SET;
        }

        LiveData.this.setValue(newValue);
    }
};

LiveData.setValue

在主线程中执行。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
@MainThread
protected void setValue(T value) {
    assertMainThread("setValue");
    ++this.mVersion;
    this.mData = value;
    this.dispatchingValue((LiveData.ObserverWrapper)null);
}

private void dispatchingValue(@Nullable LiveData<T>.ObserverWrapper initiator) {
    if (this.mDispatchingValue) {
        this.mDispatchInvalidated = true;
    } else {
        this.mDispatchingValue = true;

        do {
            this.mDispatchInvalidated = false;
            if (initiator != null) {
                this.considerNotify(initiator);
                initiator = null;
            } else {
                IteratorWithAdditions iterator = this.mObservers.iteratorWithAdditions();

                while(iterator.hasNext()) {
                    this.considerNotify((LiveData.ObserverWrapper)((Entry)iterator.next()).getValue());
                    if (this.mDispatchInvalidated) {
                        break;
                    }
                }
            }
        } while(this.mDispatchInvalidated);

        this.mDispatchingValue = false;
    }
}
private void considerNotify(LiveData<T>.ObserverWrapper observer) {
    if (observer.mActive) {
        if (!observer.shouldBeActive()) {
            observer.activeStateChanged(false);
        } else if (observer.mLastVersion < this.mVersion) {
            observer.mLastVersion = this.mVersion;
            observer.mObserver.onChanged(this.mData);
        }
    }
}

setValue主要分3步：

  • ++this.mVersion;
  • 取出mObservers分发值
  • 根据当前条件分发值

分发值的条件：

  • observer处于Active
  • observer.mLastVersion < this.mVersion
  • 至少处于started之后的生命周期状态，observer.shouldBeActive()，也就是started和resumed状态下。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
boolean shouldBeActive() {
    return this.mOwner.getLifecycle().getCurrentState().isAtLeast(State.STARTED);
}
public static enum State {
    DESTROYED,
    INITIALIZED,
    CREATED,
    STARTED,
    RESUMED;

    private State() {
    }

    public boolean isAtLeast(@NonNull Lifecycle.State state) {
        return this.compareTo(state) >= 0;
    }
}

LiveData.observe和observeForever

二者都是将observer包装后放入mObservers中。不同的是observe使用的是LifecycleBoundObserver，当生命周期状态为DESTROYED时，将自己从mObservers移除；observeForever使用的是AlwaysActiveObserver，并将激活状态一直设置为true。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
    if (owner.getLifecycle().getCurrentState() != State.DESTROYED) {
        LiveData<T>.LifecycleBoundObserver wrapper = new LiveData.LifecycleBoundObserver(owner, observer);
        LiveData<T>.ObserverWrapper existing = (LiveData.ObserverWrapper)this.mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
            throw new IllegalArgumentException("Cannot add the same observer with different lifecycles");
        } else if (existing == null) {
            owner.getLifecycle().addObserver(wrapper);
        }
    }
}

@MainThread
public void observeForever(@NonNull Observer<T> observer) {
    LiveData<T>.AlwaysActiveObserver wrapper = new LiveData.AlwaysActiveObserver(observer);
    LiveData<T>.ObserverWrapper existing = (LiveData.ObserverWrapper)this.mObservers.putIfAbsent(observer, wrapper);
    if (existing != null && existing instanceof LiveData.LifecycleBoundObserver) {
        throw new IllegalArgumentException("Cannot add the same observer with different lifecycles");
    } else if (existing == null) {
        wrapper.activeStateChanged(true);
    }
}

Lifecycle

在SupportActivity中：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
public class SupportActivity{
    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
    }
    public Lifecycle getLifecycle() {
        return this.mLifecycleRegistry;
    }
}

public class ReportFragment extends Fragment {
    public static void injectIfNeededIn(Activity activity) {
        FragmentManager manager = activity.getFragmentManager();
        if (manager.findFragmentByTag("android.arch.lifecycle.LifecycleDispatcher.report_fragment_tag") == null) {
            manager.beginTransaction().add(new ReportFragment(), "android.arch.lifecycle.LifecycleDispatcher.report_fragment_tag").commit();
            manager.executePendingTransactions();
        }

    }
}

SupportActivity onCreate时注入ReportFragment，用来监听生命周期。
在ReportFragment中，通过添加Fragment到Activity中来实现对Activity生命周期的捕捉（类似的有权限申请）。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
public class ReportFragment extends Fragment {
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        this.dispatchCreate(this.mProcessListener);
        this.dispatch(Event.ON_CREATE);
    }

    public void onStart() {
        super.onStart();
        this.dispatchStart(this.mProcessListener);
        this.dispatch(Event.ON_START);
    }

    public void onResume() {
        super.onResume();
        this.dispatchResume(this.mProcessListener);
        this.dispatch(Event.ON_RESUME);
    }

    public void onPause() {
        super.onPause();
        this.dispatch(Event.ON_PAUSE);
    }

    public void onStop() {
        super.onStop();
        this.dispatch(Event.ON_STOP);
    }

    public void onDestroy() {
        super.onDestroy();
        this.dispatch(Event.ON_DESTROY);
        this.mProcessListener = null;
    }
    private void dispatch(Event event) {
        Activity activity = this.getActivity();
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner)activity).getLifecycle().handleLifecycleEvent(event);
        } else {
            if (activity instanceof LifecycleOwner) {
                Lifecycle lifecycle = ((LifecycleOwner)activity).getLifecycle();
                if (lifecycle instanceof LifecycleRegistry) {
                    ((LifecycleRegistry)lifecycle).handleLifecycleEvent(event);
                }
            }

        }
    }
}

最终生命周期的处理实现在LifecycleRegistryOwner中，也就是 SupportActivity中的mLifecycleRegistry。
​

在LifecycleRegistryOwner中，当前后状态相同是，不会触发状态变更。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
public class LifecycleRegistry extends Lifecycle {
    public void handleLifecycleEvent(@NonNull Event event) {
        State next = getStateAfter(event);
        this.moveToState(next);
    }

    private void moveToState(State next) {
        if (this.mState != next) {
            this.mState = next;
            if (!this.mHandlingEvent && this.mAddingObserverCounter == 0) {
                this.mHandlingEvent = true;
                this.sync();
                this.mHandlingEvent = false;
            } else {
                this.mNewEventOccurred = true;
            }
        }
    }

     private void sync() {
        LifecycleOwner lifecycleOwner = (LifecycleOwner)this.mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            Log.w("LifecycleRegistry", "LifecycleOwner is garbage collected, you shouldn't try dispatch new events from it.");
        } else {
            while(!this.isSynced()) {
                this.mNewEventOccurred = false;
                if (this.mState.compareTo(((LifecycleRegistry.ObserverWithState)this.mObserverMap.eldest().getValue()).mState) < 0) {
                    this.backwardPass(lifecycleOwner);
                }

                Entry<LifecycleObserver, LifecycleRegistry.ObserverWithState> newest = this.mObserverMap.newest();
                if (!this.mNewEventOccurred && newest != null && this.mState.compareTo(((LifecycleRegistry.ObserverWithState)newest.getValue()).mState) > 0) {
                    this.forwardPass(lifecycleOwner);
                }
            }

            this.mNewEventOccurred = false;
        }
    }
}

ProcessLifecycleOwnerInitializer

1
2
3
4
5
6
7
8
9
10
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    public ProcessLifecycleOwnerInitializer() {
    }

    public boolean onCreate() {
        LifecycleDispatcher.init(this.getContext());
        ProcessLifecycleOwner.init(this.getContext());
        return true;
    }
}

LifecycleDispatcher

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
class LifecycleDispatcher {
    static void init(Context context) {
        if (!sInitialized.getAndSet(true)) {
            ((Application)context.getApplicationContext()).registerActivityLifecycleCallbacks(new LifecycleDispatcher.DispatcherActivityCallback());
        }
    }

}

static class DispatcherActivityCallback{
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            if (activity instanceof FragmentActivity) {
                ((FragmentActivity)activity).getSupportFragmentManager().registerFragmentLifecycleCallbacks(this.mFragmentCallback, true);
            }

            ReportFragment.injectIfNeededIn(activity);
        }
}

通过对application生命周期注册，在onActivityCreated时注入ReportFragment。
如果是FragmentActivity，还会registerFragmentLifecycleCallbacks。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
static class FragmentCallback extends FragmentLifecycleCallbacks {
    FragmentCallback() {
    }

    public void onFragmentCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {
        LifecycleDispatcher.dispatchIfLifecycleOwner(f, Event.ON_CREATE);
        if (f instanceof LifecycleRegistryOwner) {
            if (f.getChildFragmentManager().findFragmentByTag("android.arch.lifecycle.LifecycleDispatcher.report_fragment_tag") == null) {
                f.getChildFragmentManager().beginTransaction().add(new LifecycleDispatcher.DestructionReportFragment(), "android.arch.lifecycle.LifecycleDispatcher.report_fragment_tag").commit();
            }

        }
    }

    public void onFragmentStarted(FragmentManager fm, Fragment f) {
        LifecycleDispatcher.dispatchIfLifecycleOwner(f, Event.ON_START);
    }

    public void onFragmentResumed(FragmentManager fm, Fragment f) {
        LifecycleDispatcher.dispatchIfLifecycleOwner(f, Event.ON_RESUME);
    }
}

主要是将onFragmentCreated、onFragmentStarted、onFragmentResumed分别分发为ON_CREATE、ON_START、ON_RESUME。
​

ProcessLifecycleOwner

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
public class ProcessLifecycleOwner{
    static void init(Context context) {
        sInstance.attach(context);
    }
    void attach(Context context) {
        this.mHandler = new Handler();
        this.mRegistry.handleLifecycleEvent(Event.ON_CREATE);
        Application app = (Application)context.getApplicationContext();
        app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                ReportFragment.get(activity).setProcessListener(ProcessLifecycleOwner.this.mInitializationListener);
            }

            public void onActivityPaused(Activity activity) {
                ProcessLifecycleOwner.this.activityPaused();
            }

            public void onActivityStopped(Activity activity) {
                ProcessLifecycleOwner.this.activityStopped();
            }
        });
    }
}

主要用处是将ProcessListener（ActivityInitializationListener）注入到对应activity的ReportFragment中。而ActivityInitializationListener中主要包含常用的3个生命周期状态。当ReportFragment中处理什么周期时，会回调这3个方法。

1
2
3
4
5
interface ActivityInitializationListener {
    void onCreate();
    void onStart();
    void onResume();
}

总结

ViewModel

  • ViewModel使用反射的形式创建，获取时先从ViewModelStores缓存中获取，没找到则通过Factory创建
  • 非FragmentActivity类或没实现ViewModelStoreOwner，可通过HolderFragment注入ViewModelStores
  • LiveData中postValue可在子线程使用，最终会在主线程调用setValue来分发数据
  • LiveData中postValue时，执行setValue时，每次取mPendingData，因此如果postValue过快，会导致前面的数据会被后面的覆盖。
  • 分发数据时的条件有三个
    • observer处于Active
    • observer.mLastVersion < livedata的mVersion
    • 当前生命周期处于started或resumed下
  • 注册的observer可与生命后期绑定，也可以是全局的，与生命周期绑定的observer会在destory后反注册

Lifecycle

  • SupportActivity及其子类的生命周期是通过ReportFragment管理的，在onCreated是注入ReportFragment
  • 非SupportActivity及其子类的，可以通过全局监听各个Activity的生命周期在onCreated是注入ReportFragment
  • 非SupportActivity及其子类的生命周期管理是通过ProcessLifecycleOwner，ProcessLifecycleOwner在Activity onCreated时设置ReportFragment中监听来感知生命周期变化。可通过静态方法获取ProcessLifecycleOwner对象的LifecycleOwner实现对非SupportActivity及其子类的的感知。
  • Lifecycle的状态分为DESTROYED、INITIALIZED、CREATED、STARTED、RESUMED 5种，在ViewModel中可以设置数据的是STARTED、RESUMED

技巧

  • 通过注入Fragment来感知Activity的生命周期和一些Fragment以Activity绑定的一些方法，应用案例入ReportFragment、HolderFragment，另外权限管理和Activiyt跳转的返回值也可以通过这种方式
  • 通过ContentProvider来初始化SDK，ContentProvider会在Application的attachContext之后，onCreate之前执行onCreate方法，在AndroidManifest.xml注册后，会自动初始化ContentProvider。但是如果需要对启动进行管理，或者sdk依赖于其他的sdk，则不适合通过ContentProvider来初始化SDK。