Glide原理的探索之旅

首先引入依赖

    implementation 'com.github.bumptech.glide:glide:4.12.0'
    annotationProcessor 'com.github.bumptech.glide:compiler:4.12.0'

下面一行代码，就是Glide最简单的使用方式了

        Glide.with(this).load(url).into(imageView)

首先，我们来看with，其实with的功能就是根据传入的context来获取图片请求管理器RequestManager，他是用来启动和管理图片请求的。

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

context可以传入app，activity和fragment，这关系着图片请求的生命周期。通常使用当前页面的context，这样当我们打开一个页面加载图片，然后退出页面时，图片请求会跟随页面销毁而被取消，而不是继续加载浪费资源。

当context是app时，获得的RequestManager是一个全局单例，图片请求的生命周期会跟随整个app。

注意：如果with发生在子线程，不管context是谁，都返回应用级别的RequestManager单例。

  private RequestManager getApplicationManager(@NonNull Context context) {
    // Either an application context or we're on a background thread.
    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;
  }

当context是Activity时，会创建一个无界面的fragment添加到Activity，用于感知Activity的生命周期，同时创建RequestManager给该fragment持有。

  private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      // This is a bit of hack, we're going to start the RequestManager, but not the
      // corresponding Lifecycle. It's safe to start the RequestManager, but starting the
      // Lifecycle might trigger memory leaks. See b/154405040
      if (isParentVisible) {
        requestManager.onStart();
      }
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

load方法会得到一个图片请求构建器RequestBuilder，用来创建图片请求。

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

首先是根据ImageView的ScaleType，来配置参数

  public ViewTarget into(@NonNull ImageView view) {
    Util.assertMainThread();
    Preconditions.checkNotNull(view);

    baseRequestOptions requestOptions = this;
    if (!requestOptions.isTransformationSet()
        && requestOptions.isTransformationAllowed()
        && view.getScaleType() != null) {
      // Clone in this method so that if we use this RequestBuilder to load into a View and then
      // into a different target, we don't retain the transformation applied based on the previous
      // View's scale type.
      switch (view.getScaleType()) {
        case CENTER_CROP:
          requestOptions = requestOptions.clone().optionalCenterCrop();
          break;
        case CENTER_INSIDE:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case FIT_CENTER:
        case FIT_START:
        case FIT_END:
          requestOptions = requestOptions.clone().optionalFitCenter();
          break;
        case FIT_XY:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case CENTER:
        case MATRIX:
        default:
          // Do nothing.
      }
    }

    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
         null,
        requestOptions,
        Executors.mainThreadExecutor());
  }

继续跟进into，会创建图片请求，获取Target载体已有的请求，对比两个请求，如果等效，启动异步请求。然后，图片载体绑定图片请求，也就是imageView setTag为request

  private > Y into(
      @NonNull Y target,
      @Nullable RequestListener targetListener,
      baseRequestOptions options,
      Executor callbackExecutor) {
    Preconditions.checkNotNull(target);
    if (!isModelSet) {
      throw new IllegalArgumentException("You must call #load() before calling #into()");
    }

    Request request = buildRequest(target, targetListener, options, callbackExecutor);

    Request previous = target.getRequest();
    if (request.isEquivalentTo(previous)
        && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
      // If the request is completed, beginning again will ensure the result is re-delivered,
      // triggering RequestListeners and Targets. If the request is failed, beginning again will
      // restart the request, giving it another chance to complete. If the request is already
      // running, we can let it continue running without interruption.
      if (!Preconditions.checkNotNull(previous).isRunning()) {
        // Use the previous request rather than the new one to allow for optimizations like skipping
        // setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions
        // that are done in the individual Request.
        previous.begin();
      }
      return target;
    }

    requestManager.clear(target);
    target.setRequest(request);
    requestManager.track(target, request);

    return target;
  }

继续跟进异步请求 requestManager.track(target, request)

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

  public void runRequest(@NonNull Request request) {
    requests.add(request);
    if (!isPaused) {
      request.begin();//开启图片请求
    } else {
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Paused, delaying request");
      }
      pendingRequests.add(request);//如果是暂停状态，就把请求存起来
    }
  }

到这里就启动了图片请求了，我们继续跟进request.begin()

  public void begin() {
    synchronized (requestLock) {
      //......
      if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
      //如果有尺寸，开始加载
        onSizeReady(overrideWidth, overrideHeight);
      } else {
      //如果无尺寸就先去获取
        target.getSize(this);
      }
      //......
    }
  }

然后继续瞧瞧onSizeReady

  public void onSizeReady(int width, int height) {
    stateVerifier.throwIfRecycled();
    synchronized (requestLock) {
    //......
      loadStatus =
          engine.load(
              glideContext,
              model,
              requestOptions.getSignature(),
              this.width,
              this.height,
              requestOptions.getResourceClass(),
              transcodeClass,
              priority,
              requestOptions.getDiskCacheStrategy(),
              requestOptions.getTransformations(),
              requestOptions.isTransformationRequired(),
              requestOptions.isScaleOnlyOrNoTransform(),
              requestOptions.getOptions(),
              requestOptions.isMemoryCacheable(),
              requestOptions.getUseUnlimitedSourceGeneratorsPool(),
              requestOptions.getUseAnimationPool(),
              requestOptions.getOnlyRetrieveFromCache(),
              this,
              callbackExecutor);

      //......
    }
  }

跟进engine.load

  public  LoadStatus load(
      GlideContext glideContext,
      Object model,
      Key signature,
      int width,
      int height,
      Class resourceClass,
      Class transcodeClass,
      Priority priority,
      DiskCacheStrategy diskCacheStrategy,
      Map, Transformation> transformations,
      boolean isTransformationRequired,
      boolean isScaleOnlyOrNoTransform,
      Options options,
      boolean isMemoryCacheable,
      boolean useUnlimitedSourceExecutorPool,
      boolean useAnimationPool,
      boolean onlyRetrieveFromCache,
      ResourceCallback cb,
      Executor callbackExecutor) {
    long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;

    EngineKey key =
        keyFactory.buildKey(
            model,
            signature,
            width,
            height,
            transformations,
            resourceClass,
            transcodeClass,
            options);

    EngineResource memoryResource;
    synchronized (this) {
      //从内存加载
      memoryResource = loadFromMemory(key, isMemoryCacheable, startTime);
      if (memoryResource == null) { //如果内存里没有
        return waitForExistingOrStartNewJob(
            glideContext,
            model,
            signature,
            width,
            height,
            resourceClass,
            transcodeClass,
            priority,
            diskCacheStrategy,
            transformations,
            isTransformationRequired,
            isScaleOnlyOrNoTransform,
            options,
            isMemoryCacheable,
            useUnlimitedSourceExecutorPool,
            useAnimationPool,
            onlyRetrieveFromCache,
            cb,
            callbackExecutor,
            key,
            startTime);
      }
    }
    cb.onResourceReady(
        memoryResource, DataSource.MEMORY_CACHE,  false);
    return null;
  }

  private  LoadStatus waitForExistingOrStartNewJob(
      GlideContext glideContext,
      Object model,
      Key signature,
      int width,
      int height,
      Class resourceClass,
      Class transcodeClass,
      Priority priority,
      DiskCacheStrategy diskCacheStrategy,
      Map, Transformation> transformations,
      boolean isTransformationRequired,
      boolean isScaleOnlyOrNoTransform,
      Options options,
      boolean isMemoryCacheable,
      boolean useUnlimitedSourceExecutorPool,
      boolean useAnimationPool,
      boolean onlyRetrieveFromCache,
      ResourceCallback cb,
      Executor callbackExecutor,
      EngineKey key,
      long startTime) {

    EngineJob current = jobs.get(key, onlyRetrieveFromCache);
    if (current != null) {
      current.addCallback(cb, callbackExecutor);
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Added to existing load", startTime, key);
      }
      return new LoadStatus(cb, current);
    }

    EngineJob engineJob =
        engineJobFactory.build(
            key,
            isMemoryCacheable,
            useUnlimitedSourceExecutorPool,
            useAnimationPool,
            onlyRetrieveFromCache);

    DecodeJob decodeJob =
        decodeJobFactory.build(
            glideContext,
            model,
            key,
            signature,
            width,
            height,
            resourceClass,
            transcodeClass,
            priority,
            diskCacheStrategy,
            transformations,
            isTransformationRequired,
            isScaleOnlyOrNoTransform,
            onlyRetrieveFromCache,
            options,
            engineJob);

    jobs.put(key, engineJob);

    engineJob.addCallback(cb, callbackExecutor);
    engineJob.start(decodeJob);

    if (VERBOSE_IS_LOGGABLE) {
      logWithTimeAndKey("Started new load", startTime, key);
    }
    return new LoadStatus(cb, engineJob);
  }

DecodeJob是一个Runnable，它通过一系列的调用，会来到HttpUrlFetcher的loadData方法

  public void loadData(
      @NonNull Priority priority, @NonNull DataCallback callback) {
    long startTime = LogTime.getLogTime();
    try {
      //获取输入流，此处使用的是HttpURLConnection
      InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
      //回调出去
      callback.onDataReady(result);
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
    } finally {
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));
      }
    }
  }

至此，网络请求结束，最后把图片设置上去就行了，在SingleRequest的onResourceReady方法，它会把结果回调给Target载体

 target.onResourceReady(result, animation);

继续跟进它，最终会执行setResource，把图片设置上去

  protected void setResource(@Nullable Drawable resource) {
    view.setImageDrawable(resource);
  }

with根据传入的context来获取图片请求管理器RequestManager，当传入的context是App时，图片请求的生命周期会跟随这个应用，当传入的是Activity时，会创建一个无界面的空fragment添加到Activity，用来感知Activity的生命周期。load会得到了一个图片请求构建器RequestBuilder，用来创建图片请求。into开启加载，先会根据ImageView的ScaleType来配置参数，创建图片请求，图片载体绑定图片请求，然后开启图片请求，先从内存中加载，如果内存里没有，会创建一个Runnable，通过一系列的调用，使用HttpURLConnection获取网络输入流，把结果回调出去，最后把回调结果设置上去就OK了。

Glide的缓存机制，主要分为2种缓存，一种是内存缓存，一种是磁盘缓存。
内存缓存是为了防止重复将图片读入到内存，造成内存资源浪费，磁盘缓存是为了防止重复从网络下载和读取数据。
Glide三级缓存原理：读取一张图片时，顺序是： 弱引用缓存，LruCache，磁盘缓存。

用Glide加载某张图片时，先去弱引用缓存中寻找图片，如果有则直接取出来使用，如果没有，则去LruCache中寻找，如果LruCache中有，则从中取出图片使用，并将它放入弱引用缓存中，如果都没有图片，则从磁盘缓存或网络中加载图片。

  private EngineResource loadFromMemory(
      EngineKey key, boolean isMemoryCacheable, long startTime) {
    if (!isMemoryCacheable) {
      return null;
    }

    EngineResource active = loadFromActiveResources(key); //从弱引用获取图片
    if (active != null) {
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from active resources", startTime, key);
      }
      return active;
    }

    EngineResource cached = loadFromCache(key); //从LruCache获取缓存图片
    if (cached != null) {
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from cache", startTime, key);
      }
      return cached;
    }

    return null;
  }

这里是最新的源码获取缓存的顺序，但是以前版本的Glide（如3.0以前）是先获取LruCache的，后面为什么要改成先获取弱引用缓存再获取LruCache呢？我想大概是为了在不OOM的前提下，尽可能保证图片缓存不被清理得到最大程度的复用吧，而且，弱引用缓存用的是HashMap，而LruCache用的是linkedHashMap，并且在实例化linkedHashMap时是设置了访问顺序的，所以HashMap的访问速度是要比linkedHashMap快的。

LruCache就是维护一个缓存对象列表，其中对象列表的排列方式是按照访问顺序实现的，即一直没访问的对象，将放在队尾，即将被淘汰。而最近访问的对象将放在队头，最后被淘汰。其内部维护了一个集合linkedHashMap，linkHashMap继承HashMap，在HashMap的基础上，新增了双向链表结构，每次访问数据的时候，会更新被访问数据的链表指针，该linkedHashMap是以访问顺序排序的，当调用put()方法时，就会在集合中添加元素，判断缓存是否已满，如果满了就用linkedHashMap的迭代器删除队尾元素，即近期最少访问的元素。当调用get()方法访问缓存对象时，就会调用linkedHashMap的get()方法获得对应集合元素，同时会更新该元素到队头。

那么，问题来了，如果把一个（100 * 100）的图片放到（800 * 800）的Imageview中会怎么样呢？由上可知，Glide会为每个不同尺寸的Imageview缓存一张图片，也就是说不管这张图片有没有加载过，只要Imageview的尺寸不一样，Glide就会重新加载一次，这时候，它会在加载的Imageview之前从网络上重新下载，然后再缓存。举个例子，如果一个页面的Imageview是100 * 100，另一个页Imageview是800 * 800，它俩展示同一张图片的话，Glide会下载两次图片，并且缓存两张图片，因为Glide缓存Key的生成条件之一就是控件的长宽。

除了缓存，Glide还有一点我觉得做的非常好，就是在图片加载中关闭页面，此页面也不会造成内存泄漏，因为Glide在加载资源的时候，如果是在 Activity，Fragment 这类有生命周期的组件上进行的话，会创建一个无界面的Fragment加入到FragmentManager之中，感知生命周期，当 Activity，Fragment进入不可见，或者已经销毁的时候，Glide会停止加载资源。但是如果，是在非生命周期的组件上进行时，会采用Application的生命周期贯穿整个应用，此时只有在应用程序关闭的时候才会停止加载。