Android图片框架Glide原理深入探索

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)

with

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,用来启动和管理图片请求。

  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

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);
  }

into

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),
        /*targetListener=*/ 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, /* isLoadedFromAlternateCacheKey= */ 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);
  }

原理总结

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,获得的RequestManager是一个全局单例,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。

缓存

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;
  }

不过,这也会产生一个问题,就是Glide加载图片时,URL不变但是图片变了,这种情况会还是以前的旧图片。因为Glide加载图片会将图片缓存到本地,如果url不变则直接读取缓存不会再网络加载。

解决方案:

清除缓存让后台每次都更改图片的名字图片地址选用 ”url?key="+随机数这种格式

LruCache

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。

到此这篇关于Android Glide原理深入探索的文章就介绍到这了,更多相关Android Glide内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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