Android-Glide源码解析（一）调用流程

之前在做一个视频播放器项目时，由于该项目的时间比较长，代码也是经过了很多人手。这个项目的重点之一就是图片的加载。每个人对图片加载框架的喜好以及熟悉程度都不一样，导致了该项目里面有好几个ImageLoader。这样对系统资源是一个很大的浪费，维护起来也很麻烦。 接手之后第一件事就是统一图片加载框架。图片加载框架比较火的有Glide、Fresco和Picasso。试过用Fresco去加载一个几十张图片的ListView，虽然很快，但是内存抖动非常厉害。再用Glide实现同样的功能 ，内存的曲线很平滑。Picasso这个框架我了解的比较少，由于我有加载Gif图的需求，这是Picasso无法实现的，因此最终选用了Glide作为我的图片框架。

接下来的几篇文章将会对Glide的源码做一个分析总结。

• Android-Glide源码解析（一）调用流程
• Android-Glide源码解析（二）线程管理
• Android-Glide源码解析（三）图片处理
• 本系列文章基于Glide v4.8.0

Glide的使用方法如下

// For a simple view:
@Override public void onCreate(Bundle savedInstanceState) {
  ...
  ImageView imageView = (ImageView) findViewById(R.id.my_image_view);

  Glide.with(this).load("http://goo.gl/gEgYUd").into(imageView);
}

Glide.with(this)调用了下面的方法。返回的是一个RequestManager对象。

  //Glide.java
  @NonNull
  public static RequestManager with(@NonNull FragmentActivity activity) {
    return getRetriever(activity).get(activity);
  }
  
  @NonNull
  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的时候会先去获取Glide的实例。如果还没有初始化，则会对Glide初始化。
    return Glide.get(context).getRequestManagerRetriever();
  }
  /**
   * Get the singleton.
   *
   * @return the singleton
   */
  @NonNull
  public static Glide get(@NonNull Context context) {
  //单例模式
    if (glide == null) {
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context);
        }
      }
    }

    return glide;
  }


  private static void checkAndInitializeGlide(@NonNull Context context) {
    // In the thread running initGlide(), one or more classes may call Glide.get(context).
    // Without this check, those calls could trigger infinite recursion.
    if (isInitializing) {
      throw new IllegalStateException("You cannot call Glide.get() in registerComponents(),"
          + " use the provided Glide instance instead");
    }
    isInitializing = true;
    initializeGlide(context);
    isInitializing = false;
  }
  private static void initializeGlide(@NonNull Context context) {
    initializeGlide(context, new GlideBuilder());
  }

/**初始化Glide的方法*/
  @SuppressWarnings("deprecation")
  private static void initializeGlide(@NonNull Context context, @NonNull GlideBuilder builder) {
    Context applicationContext = context.getApplicationContext();
    //获取GlideModule注解
    GeneratedAppGlideModule annotationGeneratedModule = getAnnotationGeneratedGlideModules();
    List<com.bumptech.glide.module.GlideModule> manifestModules = Collections.emptyList();
    //解析AndroidManifest文件，查看是否设置了GlideModule。
    //这个ManifestParser已经标记为过时了。新版的Glide不需要在Manifest文件里面设置。
    if (annotationGeneratedModule == null || annotationGeneratedModule.isManifestParsingEnabled()) {
      manifestModules = new ManifestParser(applicationContext).parse();
    }
    if (annotationGeneratedModule != null
        && !annotationGeneratedModule.getExcludedModuleClasses().isEmpty()) {
      Set<Class<?>> excludedModuleClasses =
          annotationGeneratedModule.getExcludedModuleClasses();
      Iterator<com.bumptech.glide.module.GlideModule> iterator = manifestModules.iterator();
      while (iterator.hasNext()) {
        com.bumptech.glide.module.GlideModule current = iterator.next();
        if (!excludedModuleClasses.contains(current.getClass())) {
          continue;
        }
        if (Log.isLoggable(TAG, Log.DEBUG)) {
          Log.d(TAG, "AppGlideModule excludes manifest GlideModule: " + current);
        }
        iterator.remove();
      }
    }

    if (Log.isLoggable(TAG, Log.DEBUG)) {
      for (com.bumptech.glide.module.GlideModule glideModule : manifestModules) {
        Log.d(TAG, "Discovered GlideModule from manifest: " + glideModule.getClass());
      }
    }

    RequestManagerRetriever.RequestManagerFactory factory =
        annotationGeneratedModule != null
            ? annotationGeneratedModule.getRequestManagerFactory() : null;
    builder.setRequestManagerFactory(factory);
    for (com.bumptech.glide.module.GlideModule module : manifestModules) {
      module.applyOptions(applicationContext, builder);
    }
//如果存在自定义的GlideModule，加载自定义配置
    if (annotationGeneratedModule != null) {
      annotationGeneratedModule.applyOptions(applicationContext, builder);
    }
    //使用建造者模式，创建Glide实例
    Glide glide = builder.build(applicationContext);
    for (com.bumptech.glide.module.GlideModule module : manifestModules) {
      module.registerComponents(applicationContext, glide, glide.registry);
    }
    if (annotationGeneratedModule != null) {
      annotationGeneratedModule.registerComponents(applicationContext, glide, glide.registry);
    }
    applicationContext.registerComponentCallbacks(glide);
    Glide.glide = glide;
  }

  @NonNull
  Glide build(@NonNull Context context) {
    if (sourceExecutor == null) {
      sourceExecutor = GlideExecutor.newSourceExecutor();//网络请求的线程池
    }

    if (diskCacheExecutor == null) {
      diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();//磁盘缓存的线程池
    }

    if (animationExecutor == null) {
      animationExecutor = GlideExecutor.newAnimationExecutor();//加载动画的线程池
    }

    if (memorySizeCalculator == null) {
      memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();//内存缓存的Size计算器。里面包括计算Bitmap的缓存池大小，解码图片时的缓存数组大小等
    }

    if (connectivityMonitorFactory == null) {
      connectivityMonitorFactory = new DefaultConnectivityMonitorFactory();//检查是否有网络权限
    }

//创建bitmap池子
    if (bitmapPool == null) {
      int size = memorySizeCalculator.getBitmapPoolSize();
      if (size > 0) {
        bitmapPool = new LruBitmapPool(size);
      } else {
        bitmapPool = new BitmapPoolAdapter();
      }
    }
//创建解码时的临时数组池子
    if (arrayPool == null) {
      arrayPool = new LruArrayPool(memorySizeCalculator.getArrayPoolSizeInBytes());
    }

    if (memoryCache == null) {
      memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());//内存缓存
    }

    if (diskCacheFactory == null) {
      diskCacheFactory = new InternalCacheDiskCacheFactory(context);//磁盘缓存工厂对象
    }

    if (engine == null) {//负责管理加载操作、内存和缓存资源的对象
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(),
              GlideExecutor.newAnimationExecutor(),
              isActiveResourceRetentionAllowed);
    }

//RequestManager的管理器。Glide.with()方法就是通过这个类拿到了RequstManager对象。
    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptions.lock(),
        defaultTransitionOptions);
  }

分析完了Glide.with()方法，我们再来看下load()方法做了什么工作。

  /**
   * Equivalent to calling {@link #asDrawable()} and then {@link RequestBuilder#load(String)}.
   *
   * @return A new request builder for loading a {@link Drawable} using the given model.
   */
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder<Drawable> load(@Nullable String string) {
  //这个asDrawable方法表示最终这张图片要加载到的目标是一个drawable里面。
    return asDrawable().load(string);
  }
  @NonNull
  @CheckResult
  public RequestBuilder<Drawable> asDrawable() {
    return as(Drawable.class);
  }
  @NonNull
  @CheckResult
  public <ResourceType> RequestBuilder<ResourceType> as(
      @NonNull Class<ResourceType> resourceClass) {
      //这个RequestBuilder是一个设置加载图片选项和启动加载的类。
    return new RequestBuilder<>(glide, this, resourceClass, context);
  }


  @Override
  public RequestBuilder<TranscodeType> load(@Nullable Object model) {
    return loadGeneric(model);
  }

/**asDrawable().load()方法最终执行到这里，返回一个ReuqestBuilder对象*/
  @NonNull
  private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
    this.model = model;
    isModelSet = true;
    return this;
  }

接下来就到了Glide.with(this).load("http://goo.gl/gEgYUd").into(imageView);的into方法了。into是RequestBuilder的一个方法。

  /**
   * Sets the {@link ImageView} the resource will be loaded into, cancels any existing loads into
   * the view, and frees any resources Glide may have previously loaded into the view so they may be
   * reused.
   *
   * @see RequestManager#clear(Target)
   *
   * @param view The view to cancel previous loads for and load the new resource into.
   * @return The
   * {@link com.bumptech.glide.request.target.Target} used to wrap the given {@link ImageView}.
   */
  @NonNull
  public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
    Util.assertMainThread();//确保是在主线程里面调用的，如