摘要：作為圖片處理的主流庫之一，非常有名，今天我們就從源碼的層面上解析他。至此，初始化的過程結束了。首先我們來看看，代碼就不貼了，基本上就是個容器類，用來存儲請求的一些信息，并且是個抽象類。

Picasso作為圖片處理的主流庫之一，非常有名，今天我們就從源碼的層面上解析他。
首先是picasso的典型用法：

Picasso.with(context)
    .load(url)
    .into(imageView); 

Picasso.with(context)
    .load(url)
    .resize(width,height)
    .centerInside().into(imageView);

Picasso .with(context)
    .load(url)
    .placeholder(R.drawable.loading)
    .error(R.drawable.error)
    .into(imageView);

可以看到，非常簡單。那么我們就從這些調用入手逐步查看代碼。

Picasso.with(@NonNull Context context)：

public static Picasso with(@NonNull Context context) {
    if (context == null) {
      throw new IllegalArgumentException("context == null");
    }
    if (singleton == null) {
      synchronized (Picasso.class) {
        if (singleton == null) {
          singleton = new Builder(context).build();
        }
      }
    }
    return singleton;
  }

可以看到幾點信息：
1.static方法，使用起來很方便，屏蔽了大量的參數，讓他們在后面逐步加入；
2.整個Picasso采用單例模式，Picasso的設計就是承擔很多工作的引擎，因此占用資源較多，沒有必要在一個app中出現多個，也為了節省資源和提高效率，采用了單例模式；
3.創建單例的實例的時候，new出了Builder，并傳入context，之后調用Builder的build來建立單例；
4.使用synchronized來保證建立單例實例的過程是線程安全的；

簡單看下build方法：

public Picasso build() {
      Context context = this.context;

      if (downloader == null) {
        downloader = Utils.createDefaultDownloader(context);
      }
      if (cache == null) {
        cache = new LruCache(context);
      }
      if (service == null) {
        service = new PicassoExecutorService();
      }
      if (transformer == null) {
        transformer = RequestTransformer.IDENTITY;
      }

      Stats stats = new Stats(cache);

      Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);

      return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
          defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
    }

這里才是Picasso對象實例真正創建的地方。那么之前的這些操作是在初始化環境：
1.downloader下載器的建立;
2.cache的建立;
3.service線程池的建立;
4.RequestTransformer的建立；
再從結構上看，可以看出Picasso的單例的創建采用的是build模式。如果哪天想修改Picasso的上述組件的設置，又不想暴露給調用者，那么就可以再寫一個build，重寫這些代碼并替換相關組件，同時在with層面上修改調用即可。

下面再來看看Picasso的構造：

Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
      RequestTransformer requestTransformer, List extraRequestHandlers, Stats stats,
      Bitmap.Config defaultBitmapConfig, boolean indicatorsEnabled, boolean loggingEnabled) {
    this.context = context;
    this.dispatcher = dispatcher;
    this.cache = cache;
    this.listener = listener;
    this.requestTransformer = requestTransformer;
    this.defaultBitmapConfig = defaultBitmapConfig;

    int builtInHandlers = 7; // Adjust this as internal handlers are added or removed.
    int extraCount = (extraRequestHandlers != null ? extraRequestHandlers.size() : 0);
    List allRequestHandlers =
        new ArrayList(builtInHandlers + extraCount);

    // ResourceRequestHandler needs to be the first in the list to avoid
    // forcing other RequestHandlers to perform null checks on request.uri
    // to cover the (request.resourceId != 0) case.
    allRequestHandlers.add(new ResourceRequestHandler(context));
    if (extraRequestHandlers != null) {
      allRequestHandlers.addAll(extraRequestHandlers);
    }
    allRequestHandlers.add(new ContactsPhotoRequestHandler(context));
    allRequestHandlers.add(new MediaStoreRequestHandler(context));
    allRequestHandlers.add(new ContentStreamRequestHandler(context));
    allRequestHandlers.add(new AssetRequestHandler(context));
    allRequestHandlers.add(new FileRequestHandler(context));
    allRequestHandlers.add(new NetworkRequestHandler(dispatcher.downloader, stats));
    requestHandlers = Collections.unmodifiableList(allRequestHandlers);

    this.stats = stats;
    this.targetToAction = new WeakHashMap();
    this.targetToDeferredRequestCreator = new WeakHashMap();
    this.indicatorsEnabled = indicatorsEnabled;
    this.loggingEnabled = loggingEnabled;
    this.referenceQueue = new ReferenceQueue