Jetpack 是由一套库、工具和指南的集合，旨在帮助开发者快速的开发应用，Jetpack 构成如下所示：
由上图所示，Jetpack 主要由四部分组成：
-
架构 - Architecture：帮助开发者设计稳健、可测试且易维护的应用。
-
基础 - Foundation：提供了最基础的底层功能，如向后兼容性、测试、开发语言Kotlin支持，并包括多个平台开发的组件。
-
行为 - Behavior：帮助应用与标准的 Android 服务（如通知、权限、分享）相集成。
-
界面 - UI：辅助绘制界面的View类 & 各种辅助组件。
我们也可以在官网上查看具体使用。
Lifecycler 可以帮助开发者管理 Activity 和 Fragment 生命周期。
使用 MVP 例子：
//IPresenter.java
public interface IPresenter extends LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_START)
void onStart();
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void onResume();
}
//MyPresenter.java
public class MyPresenter implements IPresenter {
private static final String TAG = "TESTLIFE";
@Override
public void onStart() {
Log.d(TAG, "onStart:MyPresenter");
}
@Override
public void onResume() {
Log.d(TAG, "onResume:MyPresenter");
}
}
//MainActivity.java
//AppCompatActivity 实现 LifecycleOwner（被观察者） 接口
public class MainActivity extends AppCompatActivity {
private MyPresenter mPresenter;
private static final String TAG = "TESTLIFE";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mPresenter = new MyPresenter();
getLifecycle().addObserver(mPresenter);
}
@Override
protected void onStart() {
super.onStart();
Log.d(TAG, "onStart:MainActivity");
}
@Override
protected void onResume() {
super.onResume();
Log.d(TAG, "onResume:MainActivity");
}
}
//out:
//2020-12-30 17:37:56.611 10457-10457/com.test.testjet D/TESTLIFE: onStart:MainActivity
//2020-12-30 17:37:56.611 10457-10457/com.test.testjet D/TESTLIFE: onStart:MyPresenter
//2020-12-30 17:37:56.613 10457-10457/com.test.testjet D/TESTLIFE: onResume:MainActivity
//2020-12-30 17:37:56.614 10457-10457/com.test.testjet D/TESTLIFE: onResume:MyPresenter明显可以看出 MyPersenter 可以感知出 Activity 生命周期变化。对比传统的 MVP 我们不需要在 Activity 生命周期发生变化时，去调用 Persenter的相关方法。
Lifecycler 使用 Event 和 State 这两个枚举来跟踪其关联组件的生命周期。
- Event：Lifecycler 生命周期对应的事件
- State：Lifecycler 生命周期所处的状态
public abstract class Lifecycle {
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
@NonNull
AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();
//添加观察者
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
//移除观察者
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}从示例代码中我们可以看到如下代码：
getLifecycle().addObserver(mPresenter);
getLifecycle()，点击进去。
//ComponentActivity 是继承自 Activity 并实现了LifecycleOwner(标记类有 Android 的生命周期的)
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
...
{
//LifecycleRegistry 是 Lifecycler 实现类
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
...
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);//代码1
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
} 把生命周期改变 Lifecycler 的 State 交给 ReportFragment 处理。
代码1 ：ReportFragment#injectIfNeededIn
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
public static void injectIfNeededIn(Activity activity) {
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatchCreate(mProcessListener);
//改变 State
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);//代码1
}
private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
listener.onStart();
}
}
...
private void dispatchCreate(ActivityInitializationListener listener) {
if (listener != null) {
listener.onCreate();
}
}
}代码1：很显然这里是一个关键点。
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
if (activity instanceof LifecycleRegistryOwner) {//是否是 LifecycleRegistryOwner
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);//代码1
return;
}
if (activity instanceof LifecycleOwner) {//是否是 LifecycleOwner
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);//代码2
}
}
}LifecycleRegistryOwner 接口继承了 LifecycleOwner。LifecycleOwner# getLifecycle() 返回 Lifecycle，LifecycleRegistryOwner#getLifecycle() 返回 LifecycleRegistry。
代码1 和 代码2 调用 handleLifecycleEvent，明显是一个关键点。
//LifecycleRegistry
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event); //获取下一个状态
moveToState(next);//改变状态
}
private void moveToState(State next) {
......
sync();
......
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
//循环遍历所有观察者
while (...) {
....
//分发事件
forwardPass(lifecycleOwner);
}
}
//分发事件
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
//ObserverWithState
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
//分发事件
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}我们来看看 ObserverWithState
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}代码1：如果生命周期变了 mLifecycleObserver. onStateChanged，LifecycleEventObserver 是一个接口
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
//CallbackInfo#invokeCallbacks
mInfo.invokeCallbacks(source, event, mWrapped);//代码1
}
}CallbackInfo 其实就是 解析 OnLifecycleEcent 的注解，保存注解修饰的方法和事件，通过反射对事件的对应进行调用。
- 可观察的数据持有者，具有生命周期感知，能够在 Activity、Fragment、Service 中正确处理生命周期。
- 只有处于 Active 的观察者才会收到 LiveData 通知，如果观察者处于 Paused 或 Destroyed ，不会收到通知。
数据转换：
- map()： 可以转换 LiveData 的输出
- switchMap() ：更改被 LiveData 观察的对象
- MediatorLiveData ：提供自定义数据转换
例子：
public class MainActivity extends AppCompatActivity {
private static final String TAG = "TESTLIFE";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final MutableLiveData<String> data = new MutableLiveData<>();
data.observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.d(TAG, s);
}
});
//data.setValue("Test"); setValue 需要在主线程中调用
data.postValue("Test");
}
}@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
assertMainThread("observe");
//如果是 DESTROYED，直接 return
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
return;
}
//LifecycleBoundObserver 包装类
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
//SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers 存储
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && !existing.isAttachedTo(owner)) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
//LifecycleBoundObserver 添加
owner.getLifecycle().addObserver(wrapper);//代码1
}接下来我们看看 LifecycleBoundObserver ，LifecycleBoundObserver 是 LiveData 内部类。
class LifecycleBoundObserver extends ObserverWrapper implements LifecycleEventObserver {
...
//状态变化
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
//如果 DESTROYED 移除
removeObserver(mObserver);
return;
}
activeStateChanged(shouldBeActive());//代码1
}
...
} 代码1：调用 LiveData#activeStateChanged。
void activeStateChanged(boolean newActive) {
if (newActive == mActive) {
return;
}
mActive = newActive;
boolean wasInactive = LiveData.this.mActiveCount == 0;
LiveData.this.mActiveCount += mActive ? 1 : -1;
if (wasInactive && mActive) {
onActive();
}
if (LiveData.this.mActiveCount == 0 && !mActive) {
onInactive();
}
if (mActive) {
dispatchingValue(this);//分发
}
}
}根据 Active 状态和处于 Active 状态的组件的数量，进行方法回调，处于 Active，进行分发。
void dispatchingValue(@Nullable ObserverWrapper initiator) {
if (mDispatchingValue) {
mDispatchInvalidated = true;
return;
}
//标记是否处于分发状态
mDispatchingValue = true;
do {
mDispatchInvalidated = false;
if (initiator != null) {
//代码1
considerNotify(initiator);
initiator = null;
} else {
for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
considerNotify(iterator.next().getValue());
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}代码1：
private void considerNotify(ObserverWrapper observer) {
if (!observer.mActive) {
return;
}
if (!observer.shouldBeActive()) {
observer.activeStateChanged(false);
return;
}
if (observer.mLastVersion >= mVersion) {
return;
}
observer.mLastVersion = mVersion;
//调用 onChanged 回调
observer.mObserver.onChanged((T) mData);
}接下来看看 postValue 和 setValue 方法。
//线程
private final Runnable mPostValueRunnable = new Runnable() {
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
//noinspection unchecked
setValue((T) newValue);
}
};
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
//将 setValue 切换到主线程
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
//调用 dispatchingValue
dispatchingValue(null);
}LiveData 通常和 ViewModel 一起使用，ViewModel 感知生命周期的形式来存储和管理视图相关数据的。
例子：
public class MyViewModel extends ViewModel {
private MutableLiveData<String> name;
public MutableLiveData<String> getName() {
if (name == null) {
name = new MutableLiveData<>();
addName();
}
return name;
}
private void addName() {
name.setValue("Android");
}
}
public class MainActivity extends AppCompatActivity {
private MyPresenter mPresenter;
private static final String TAG = "TESTLIFE";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final MyViewModel myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
myViewModel.getName().observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.d(TAG, s);
}
});
}
}从例子可以看出 ViewModelProviders.of(this).get(MyViewModel.class); 是 ViewModel 入口。ViewModelProviders.of() 有许多的构造方法，这里以参数是 Activity 为例子。
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);//返回 Application
if (factory == null) {
//创建 AndroidViewModelFactory
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
//创建 ViewModelProvider
return new ViewModelProvider(activity.getViewModelStore(), factory);
} //ViewModelProvider#get
@NonNull
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
ViewModel viewModel = mViewModelStore.get(key);
if (modelClass.isInstance(viewModel)) {
//noinspection unchecked
return (T) viewModel;
} else {
//noinspection StatementWithEmptyBody
if (viewModel != null) {
// TODO: log a warning.
}
}
//反射创建 ViewModel 实现类
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
return (T) viewModel;
}创建完 ViewModel 实现类之后，就调用 observer，这部分逻辑和 LiveData 一样。