面试之Glide如何绑定Activity的生命周期

Glide绑定Activity生命周期

Glide.with()

下面都是它的重载方法，Context，Activity，FragmentActivity, Fragment, android.app.Fragment fragment,View都可以作为他的参数，内容大同小异，都是先getRetriever，然后再get

@NonNullpublic static RequestManager with(@NonNull Context context) {return getRetriever(context).get(context);}@NonNullpublic static RequestManager with(@NonNull Activity activity) {return getRetriever(activity).get(activity);}@NonNullpublic static RequestManager with(@NonNull FragmentActivity activity) {return getRetriever(activity).get(activity);}@NonNullpublic static RequestManager with(@NonNull Fragment fragment) {return getRetriever(fragment.getContext()).get(fragment);}@SuppressWarnings("deprecation")@Deprecated@NonNullpublic static RequestManager with(@NonNull android.app.Fragment fragment) {return getRetriever(fragment.getActivity()).get(fragment);}@NonNullpublic static RequestManager with(@NonNull View view) {return getRetriever(view.getContext()).get(view);}

getRetriever()

原来获取的是Glide里面RequestManagerRetriever，Glide是个双重校验实现的单例

  @NonNull@NonNullprivate static RequestManagerRetriever getRetriever(@Nullable Context context) {...return Glide.get(context).getRequestManagerRetriever();}public static Glide get(@NonNull Context context) {if (glide == null) {GeneratedAppGlideModule annotationGeneratedModule =getAnnotationGeneratedGlideModules(context.getApplicationContext());synchronized (Glide.class) {if (glide == null) {checkAndInitializeGlide(context, annotationGeneratedModule);}}}return glide;}

Glide的 getRequestManagerRetriever返回了requestManagerRetriever属性

返回到第一步with方法

    return getRetriever(context).get(activity);

查看RequestManagerRetriver的get() 主要关注框出来的部分
获取到FragmentManeger 调用FragmentGet()

实例化了一个RequestManagerFragment， 然后给这个Fragment指定了一个RequestManager,最终返回这个fragment里面的requestManager；

  @Deprecated@NonNullprivate RequestManager fragmentGet(@NonNull Context context,@NonNull android.app.FragmentManager fm,@Nullable android.app.Fragment parentHint,boolean isParentVisible) {RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);RequestManager requestManager = current.getRequestManager();if (requestManager == null) {// TODO(b/27524013): Factor out this Glide.get() call.Glide glide = Glide.get(context);//实例化requsetManagerrequestManager =factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);if (isParentVisible) {requestManager.onStart();}current.setRequestManager(requestManager);}return requestManager;}

 @NonNullprivate RequestManagerFragment getRequestManagerFragment(@NonNull final android.app.FragmentManager fm, @Nullable android.app.Fragment parentHint) {RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);if (current == null) {current = pendingRequestManagerFragments.get(fm);if (current == null) {///创建Fragmentcurrent = new RequestManagerFragment();current.setParentFragmentHint(parentHint);pendingRequestManagerFragments.put(fm, current);// 将这个Frament添加到了Activity中fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();}}return current;}

有没有注意到实例化RequestManager时传入的参数，其中有一个是Fragment的LifeCycle

     requestManager =factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);

看到RequestManagerFragment里面确实例化了一个LifeCycle对象

  public RequestManagerFragment() {this(new ActivityFragmentLifecycle());}@VisibleForTesting@SuppressLint("ValidFragment")RequestManagerFragment(@NonNull ActivityFragmentLifecycle lifecycle) {this.lifecycle = lifecycle;}

在RequestManager中给这个LifeCycle注册了一个观察者

这样在Fragment 生命周期发生变化时，RequestManager也能感受到了

小结

Gilde能感受Activity生命周期，在onStop的时候停止加载，在onStart的时候继续加载，在onDestory清除任务和进行缓存的回收
原因：
整个流程调用Glide.with时首先获取到Glide单例中的RequestManagerRetriever，再调用retriever的get()，再他的get方法中创建了一个无界面的Fragment并且注册给Activity，然后将这个Fragment里面的LifeCycle传递给要返回的RequestManager，在这个RequestManager中给LifeCycle注册了一个观察者，这样就完成了生命周期在Activity->Fragment->RequestManager之间的传递；