Glide是如何绑定生命周期的？

android

前言

前面我们讲了Glide的一个优点就是请求可以绑定Activity/Fragment的生命周期。onStart()时重新加载，在onStop()时暂停加载。
那么具体是怎么实现的呢？下面我们来一步步分析。

具体分析

    Glide.with(context).load(pngUrl).into(imageView);

看看Glide.with()方法

    public static RequestManager with(Activity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }

RequestManager是用来管理图片加载请求的。它实现了LifecycleListener接口。

public interface LifecycleListener {
    void onStart();
    void onStop();
    void onDestroy();
}

这个方法里面，onStart（）会在Activity/Fragment的onStart()调用时被回调，onStop（）在Activity/Fragment的onStop()调用时回调，onDestroy（）在Activity/Fragment的onDestroy()调用时回调。（后面慢慢说明）

在RequestManager中对应实现如下：

    @Override
    public void onStart() {
        Util.assertMainThread();
        // 启动请求
        requestTracker.resumeRequests();
    }
    @Override
    public void onStop() {
        Util.assertMainThread();
        // 暂停请求
        requestTracker.pauseRequests();
    }
    @Override
    public void onDestroy() {
        // 清空请求
        requestTracker.clearRequests();
    }

onStart()和onStop()当然必须是要在UI线程，跟生命周期的执行一致。
RequestTracker是用来取消、启动请求的。

现在我们来看下RequestManagerRetriever，为什么要重新取一下

   // 对应上面的 return retriever.get(activity);
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public RequestManager get(Activity activity) {
        if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
            return get(activity.getApplicationContext());
        } else {
            assertNotDestroyed(activity);
            // 关键获取了这个Activity的FragmentManager
            android.app.FragmentManager fm = activity.getFragmentManager();
            return fragmentGet(activity, fm);
        }
    }

首先我们获取到了这个Activity的FragmentManager，再看fragmentget();

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    RequestManager fragmentGet(Context context, android.app.FragmentManager fm) {
        // 通过RequestManagerFragment，绑定到Activity上
        RequestManagerFragment current = getRequestManagerFragment(fm);
        // 生成已经绑定生命周期的RequsetManager
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

然后通过生成一个特定的Fragment去依附Activity，完成生命周期的绑定

    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    RequestManagerFragment getRequestManagerFragment(final android.app.FragmentManager fm) {
        RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
        if (current == null) {
            current = pendingRequestManagerFragments.get(fm);
            if (current == null) {
             // 实例化RequestManagerFragment
                current = new RequestManagerFragment();
                pendingRequestManagerFragments.put(fm, current);
                // 完成生命周期绑定
                fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
                handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
            }
        }
        return current;
    }

再看RequestManagerFragment的实现

    public RequestManagerFragment() {
        this(new ActivityFragmentLifecycle());
    }
    // For testing only.
    @SuppressLint("ValidFragment")
    RequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
        this.lifecycle = lifecycle;
    }

ActivityFragmentLifecycle用来对Activity的生命周期函数进行跟踪和通知

class ActivityFragmentLifecycle implements Lifecycle {
    private final Set<LifecycleListener> lifecycleListeners =
            Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
    private boolean isStarted;
    private boolean isDestroyed;
    @Override
    public void addListener(LifecycleListener listener) {
        lifecycleListeners.add(listener);
        if (isDestroyed) {
            listener.onDestroy();
        } else if (isStarted) {
            listener.onStart();
        } else {
            listener.onStop();
        }
    }
    void onStart() {
        isStarted = true;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStart();
        }
    }
    void onStop() {
        isStarted = false;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStop();
        }
    }
    void onDestroy() {
        isDestroyed = true;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onDestroy();
        }
    }
}

前面我们讲过RequestManager实现了LifecycleListener。这里用了Set集合存储RequestManager。

再回到RequestManagerFragment

    @Override
    public void onStart() {
        super.onStart();
        lifecycle.onStart();
    }
    @Override
    public void onStop() {
        super.onStop();
        lifecycle.onStop();
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
        lifecycle.onDestroy();
    }

生命周期真正的回调，对应执行了RequestManager的回调。
还有许多细节就需要自己去探索了，相信你也大半懂了基本原理。

小结

关键在于获取Activity的FragmentManager（另外还支持support.v4中的FragmentManager类，这也是为什么要依赖这个包），生成特定的Fragment去依附Activity，监听到生命周期函数。