Android Glide 源码分析（一）生命周期和基本流程

Glide 依赖

implementation 'com.github.bumptech.glide:glide:4.9.0'

Glide 使用方式

Glide.with(context)
     .load(url)
     .into(view);                  

Glide -> with

//Context
public static RequestManager with(@NonNull Context context) {
    return getRetriever(context).get(context);
}

//Activity
public static RequestManager with(@NonNull Activity activity) {
    return getRetriever(activity).get(activity);
}

//FragmentActivity
public static RequestManager with(@NonNull FragmentActivity activity) {
    return getRetriever(activity).get(activity);
}

//Fragment
public static RequestManager with(@NonNull Fragment fragment) {
    return getRetriever(fragment.getActivity()).get(fragment);
}

//android.app.Fragment
public static RequestManager with(@NonNull android.app.Fragment fragment) {
    return getRetriever(fragment.getActivity()).get(fragment);
}

//View
public static RequestManager with(@NonNull View view) {
    return getRetriever(view.getContext()).get(view);
}

with 方法是一个重载方法，但最终都会调用 getRetriever 方法。

getRetriever

private static RequestManagerRetriever getRetriever(@Nullable Context context) {
    // Context could be null for other reasons (ie the user passes in null), but in practice it will
    // only occur due to errors with the Fragment lifecycle.
    Preconditions.checkNotNull(
        context,
        "You cannot start a load on a not yet attached View or a Fragment where getActivity() "
            + "returns null (which usually occurs when getActivity() is called before the Fragment "
            + "is attached or after the Fragment is destroyed).");
    return Glide.get(context).getRequestManagerRetriever();
}

public static Glide get(@NonNull Context context) {
    if (glide == null) {
        synchronized (Glide.class) {
            if (glide == null) {
                checkAndInitializeGlide(context);
            }
        }
    }
    return glide;
}

Glide 的创建使用的是双重检索（ DCL ）单例，然后通过 getRequestManagerRetriever 方法获取 RequestManagerRetriever 对象。

RequestManagerRetriever.get

//Context
public RequestManager get(@NonNull Context context) {
    if (context == null) {
      throw new IllegalArgumentException("You cannot start a load on a null Context");
    } else if (Util.isOnMainThread() && !(context instanceof Application)) {
      if (context instanceof FragmentActivity) {
        return get((FragmentActivity) context);
      } else if (context instanceof Activity) {
        return get((Activity) context);
      } else if (context instanceof ContextWrapper) {
        return get(((ContextWrapper) context).getBaseContext());
      }
    }

    return getApplicationManager(context);
}

get 方法也是一个重载方法，这里就不全部罗列了。看起来处理比较多，而实际上只处理了两种情况的参数。
一种为「 非 Application 」类型的参数，一种为「 Application 」类型的参数。

getApplicationManager

private RequestManager getApplicationManager(@NonNull Context context) {
    if (applicationManager == null) {
      synchronized (this) {
        if (applicationManager == null) {
          // Normally pause/resume is taken care of by the fragment we add to the fragment or
          // activity. However, in this case since the manager attached to the application will not
          // receive lifecycle events, we must force the manager to start resumed using
          // ApplicationLifecycle.

          // TODO(b/27524013): Factor out this Glide.get() call.
          Glide glide = Glide.get(context.getApplicationContext());
          applicationManager =
              factory.build(
                  glide,
                  new ApplicationLifecycle(),
                  new EmptyRequestManagerTreeNode(),
                  context.getApplicationContext());
        }
      }
    }
    return applicationManager;
}

ApplicationLifecycle

class ApplicationLifecycle implements Lifecycle {
  @Override
  public void addListener(@NonNull LifecycleListener listener) {
    listener.onStart();
  }

  @Override
  public void removeListener(@NonNull LifecycleListener listener) {
    // Do nothing.
  }
}

当参数为 Application 类型的参数时，会创建一个 ApplicationLifecycle 对象,此时 Glide 的生命周期就和 Application 生命周期一致。

RequestManagerRetriever.get

public RequestManager get(@NonNull FragmentActivity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      //通过当前 activity 获取 FragmentManager
      //用于创建透明 fragment
      FragmentManager fm = activity.getSupportFragmentManager();
      return supportFragmentGet(
          activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
}

supportFragmentGet

private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    //创建透明的 fragment
    SupportRequestManagerFragment current =
        getSupportRequestManagerFragment(fm, parentHint, isParentVisible);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      //将 Glide 生命周期与 fragment 绑定
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
}

getSupportRequestManagerFragment

private SupportRequestManagerFragment getSupportRequestManagerFragment(
      @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) {
    //通过 TAG 查找 fragment
    SupportRequestManagerFragment current =
        (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      //通过 FragmentManager 查找 fragment
      current = pendingSupportRequestManagerFragments.get(fm);
      //如果为空则创建一个 fragment 并加入缓存
      if (current == null) {
        current = new SupportRequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingSupportRequestManagerFragments.put(fm, current);
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
}

SupportRequestManagerFragment

public class SupportRequestManagerFragment extends Fragment {  
    ...
    @Override
    public void onStart() {
        super.onStart();
        lifecycle.onStart();
    }

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

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

当参数为非 Application 类型的参数时，会创建一个 ActivityFragmentLifecycle 对象,此时 Glide 的生命周期就和 Fragment 生命周期一致。

最终 Glide.with() 方法会返回一个 RequestManager 对象。

RequestManager -> load

public RequestBuilder load(@Nullable Bitmap bitmap) {
    return asDrawable().load(bitmap);
}

public RequestBuilder load(@Nullable Drawable drawable) {
    return asDrawable().load(drawable);
}

public RequestBuilder load(@Nullable String string) {
    return asDrawable().load(string);
}

public RequestBuilder load(@Nullable Uri uri) {
    return asDrawable().load(uri);
}

public RequestBuilder load(@Nullable File file) {
    return asDrawable().load(file);
}

public RequestBuilder load(@RawRes @DrawableRes @Nullable Integer resourceId) {
    return asDrawable().load(resourceId);
}

public RequestBuilder load(@Nullable URL url) {
    return asDrawable().load(url);
}

public RequestBuilder load(@Nullable byte[] model) {
    return asDrawable().load(model);
}

public RequestBuilder load(@Nullable Object model) {
    return asDrawable().load(model);
}

asDrawable

public RequestBuilder asDrawable() {
    return as(Drawable.class);
}

public  RequestBuilder as(
      @NonNull Class resourceClass) {
    return new RequestBuilder<>(glide, this, resourceClass, context);
}

asDrawable 方法其实是创建一个 RequestBuilder 对象

load

public RequestBuilder load(@Nullable URL url) {
    return loadGeneric(url);
}

loadGeneric

private RequestBuilder loadGeneric(@Nullable Object model) {
    this.model = model;
    isModelSet = true;
    return this;
}

load 方法流程比较简单，首先会创建一个 RequestBuilder 对象，然后将传入的资源赋值给 RequestBuilder 对象中的 model。

RequestBuilder -> into

public ViewTarget into(@NonNull ImageView view) {
    ...
    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
        /*targetListener=*/ null,
        requestOptions,
        Executors.mainThreadExecutor());
}

into

private > Y into(
      @NonNull Y target,
      @Nullable RequestListener targetListener,
      BaseRequestOptions options,
      Executor callbackExecutor) {
    ...
    requestManager.track(target, request);
    ...
}

track

synchronized void track(@NonNull Target target, @NonNull Request request) {
    targetTracker.track(target);
    requestTracker.runRequest(request);
}

runRequest

public void runRequest(@NonNull Request request) {
    //将当前 request 加入执行队列
    requests.add(request);
    //判断当前状态是否是暂停
    if (!isPaused) {
      //不是暂停则执行请求
      request.begin();
    } else {
      //暂停则清楚
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Paused, delaying request");
      }
      //加入等待队列
      pendingRequests.add(request);
    }
}

在 RequestTracker 中维护了两种队列 : requests 执行队列、pendingRequests 等待队列。（ Okhttp 中也有类似的队列，同步异步的执行就绪队列 ）

所以我们知道 into 方法最终会执行 request，那么是谁去维护处理这 2 个队列呢？

根据方法倒推

• runRequest 方法在 RequestTracker 类中
  RequestTracker -> runRequest
• track 方法执行了 runRequest 方法
  RequestManager -> track
• RequestManager 对象中持有 RequestTracker 对象
  RequestManager

所以我们重点看 RequestManager 对象

RequestManager

public class RequestManager implements LifecycleListener,
    ModelTypes> {
    ...
    
    public synchronized void onStart() {
        resumeRequests();
        targetTracker.onStart();
    }
     
    public synchronized void onStop() {
        pauseRequests();
        targetTracker.onStop();
    }
    
    public synchronized void onDestroy() {
        targetTracker.onDestroy();
        for (Target target : targetTracker.getAll()) {
            clear(target);
        }
        targetTracker.clear();
        requestTracker.clearRequests();
        lifecycle.removeListener(this);
        lifecycle.removeListener(connectivityMonitor);
        mainHandler.removeCallbacks(addSelfToLifecycle);
        glide.unregisterRequestManager(this);
    }
    ...
}

回到之前 with 方法中添加的透明 Fragment

SupportRequestManagerFragment

public class SupportRequestManagerFragment extends Fragment {  
    ...
    @Override
    public void onStart() {
        super.onStart();
        lifecycle.onStart();
    }

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

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

在页面执行 onStart 方法时会触发 lifecycle.onStart 方法，lifecycle.onStart 则会出发 RequestManager 的 onStart 方法

RequestManager -> onStart

public synchronized void onStart() {
    resumeRequests();
    targetTracker.onStart();
}

resumeRequests

public synchronized void resumeRequests() {
    requestTracker.resumeRequests();
}

resumeRequests

public void resumeRequests() {
    isPaused = false;
    for (Request request : Util.getSnapshot(requests)) {
      // We don't need to check for cleared here. Any explicit clear by a user will remove the
      // Request from the tracker, so the only way we'd find a cleared request here is if we cleared
      // it. As a result it should be safe for us to resume cleared requests.
      if (!request.isComplete() && !request.isRunning()) {
        request.begin();
      }
    }
    pendingRequests.clear();
}

所以可以想到，当 Activity 显示时会便利 request 执行队列，当 Activity 不可见时会将 request 加入到 pendingRequests 等待队列

总结

• with

1. 通过单例创建 Glide 对象
2. 当参数类型为 Application 时， Glide 生命周期与 Application 一致；当参数类型为非 Application 时，会创建一个透明的 Fragment，Glide 生命周期与 Fragment 一致
3. 最终返回一个 RequestManager 对象

• load

1. 创建一个 RequestBuilder 对象
2. 将传入的资源赋值给 RequestBuilder 对象中的 model 并返回

• into

1. 执行 request
2. 根据生命周期处理 request

Glide 源码分析生命周期和基本流程大致就介绍到这里了，如果有什么写得不对的，可以在下方评论留言，我会第一时间改正。