Picasso 是一个强大的图片加载缓存框架

一、使用

Picasso.with(this)
        .load("url")
        .placeholder(R.drawable.leak_canary_icon)//占位图
        .error(R.drawable.leak_canary_icon)//网络失败显示的图片
        .resize(480, 480)//指定图片的尺寸
        .centerCrop()//指定图片缩放类型
        .rotate(90)
        .priority(Picasso.Priority.HIGH)//指定优先级
        .tag("tag")
        .memoryPolicy(MemoryPolicy.NO_CACHE)
        .networkPolicy(NetworkPolicy.NO_CACHE)
        .into(imageView);

二、重要的类

1. Picasso: 图片加载、转换、缓存的管理类。单列模式 ，通过with方法获取实例，也是加载图片的入口。
2. RequestCreator: Request构建类，Builder 模式，采用链式设置该Request的属性(如占位图、缓存策略、裁剪规则、显示大小、优先级等等)。最后调用build()方法生成一个请求（Request）。
3. DeferredRequestCreator:当创建请求的时候还不能获取ImageView的宽和高的时候，则创建一个DeferredRequestCreator，DeferredRequestCreator里对 target 设置监听，直到可以获取到宽和高的时候重新执行请求创建。
4. Action: 请求包装类，存储了该请求和RequestCreator设置的这些属性，最终提交给线程执行下载。
5. Dispatcher:分发器，分发执行各种请求、分发结果等等。
6. PicassoExecutorService:Picasso使用的线程池，默认池大小为3。
7. LruCache:一个使用最近最少使用策略的内存缓存。
8. BitmapHunter:这是Picasso的一个核心的类，开启线程执行下载，获取结果后解码成Bitmap,然后做一些转换操作如图片旋转、裁剪等，如果请求设置了转换器Transformation,也会在BitmapHunter里执行这些转换操作。
9. NetworkRequestHandler:网络请求处理器，如果图片需要从网络下载，则用这个处理器处理。
10. FileRequestHandler:文件请求处理器，如果请求的是一张存在文件中的图片，则用这个处理器处理。
11. AssetRequestHandler: Asset 资源图片处理器，如果是加载asset目录下的图片，则用这个处理器处理。
12. ResourceRequestHandler:Resource资源图片处理器，如果是加载res下的图片，则用这个处理器处理。
13. ContentStreamRequestHandler: ContentProvider 处理器，如果是ContentProvider提供的图片，则用这个处理器处理
14. MediaStoreRequestHandler: MediaStore 请求处理器，如果图片是存在MediaStore上的则用这个处理器处理。
    15.``Response`: 返回的结果信息，Stream流或者Bitmap。
15. Request: 请求实体类，存储了应用在图片上的信息。
16. Target:图片加载的监听器接口，有3个回调方法，onPrepareLoad 在请求提交前回调，onBitmapLoaded 请求成功回调，并返回Bitmap,onBitmapFailed请求失败回调。
17. PicassoDrawable:继承BitmapDrawable，实现了过渡动画和图片来源的标识（就是图片来源的指示器，要调用 setIndicatorsEnabled(true)方法才生效），请求成功后都会包装成BitmapDrawable显示到ImageView 上。
18. OkHttpDownloader:用OkHttp实现的图片下载器，默认就是用的这个下载器。
19. UrlConnectionDownloader:使用HttpURLConnection 实现的下载器。
20. MemoryPolicy: 内存缓存策略，一个枚举类型。
21. NetworkPolicy: 磁盘缓存策略，一个枚举类型。
22. Stats: 这个类相当于日志记录，会记录如：内存缓存的命中次数，丢失次数，下载次数，转换次数等等，我们可以通过StatsSnapshot类将日志打印出来，看一下整个项目的图片加载情况。

三、源码分析

1、获取Picasso实例

Picasso采用单例模式

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

// 真正new 的地方在build()方法里
 public Picasso build() {
      Context context = this.context;

      if (downloader == null) {
        //1. 配置默认的下载器，首先通过反射获取OkhttpClient,如果获取到了，就使用OkHttpDwownloader作为默认下载器
        //如果获取不到就使用UrlConnectionDownloader作为默认下载器
        downloader = Utils.createDefaultDownloader(context);
      }
      if (cache == null) {
       // 2. 配置内存缓存，大小为手机内存的15%
        cache = new LruCache(context);
      }
      if (service == null) {
       // 3.配置Picaso 线程池，核心池大小为3
        service = new PicassoExecutorService();
      }
      if (transformer == null) {
       // 4. 配置请求转换器，默认的请求转换器没有做任何事，直接返回原请求
        transformer = RequestTransformer.IDENTITY;
      }

      Stats stats = new Stats(cache);
      //5.创建分发器 
      Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);

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

1. 配置默认的下载器

  static Downloader createDefaultDownloader(Context context) {
    try {
      // 查找项目中是否使用了OkHttp
      Class.forName("com.squareup.okhttp.OkHttpClient");
      return OkHttpLoaderCreator.create(context);
    } catch (ClassNotFoundException ignored) {
    }
    // 没有使用OkHttp则用默认的HttpURLConnection
    return new UrlConnectionDownloader(context);
  }

2. 配置内存缓存：最多手机内存的15%

  public LruCache(Context context) {
    this(Utils.calculateMemoryCacheSize(context));
  }

  public LruCache(int maxSize) {
    if (maxSize <= 0) {
      throw new IllegalArgumentException("Max size must be positive.");
    }
    this.maxSize = maxSize;
    this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);
  }

  static int calculateMemoryCacheSize(Context context) {
    ActivityManager am = getService(context, ACTIVITY_SERVICE);
    boolean largeHeap = (context.getApplicationInfo().flags & FLAG_LARGE_HEAP) != 0;
    int memoryClass = am.getMemoryClass();
    if (largeHeap && SDK_INT >= HONEYCOMB) {
      memoryClass = am.getLargeMemoryClass();
    }
    // Target ~15% of the available heap.
    return 1024 * 1024 * memoryClass / 7;
  }

3. 配置线程池

  PicassoExecutorService() {
    super(DEFAULT_THREAD_COUNT, DEFAULT_THREAD_COUNT, 0, TimeUnit.MILLISECONDS,
        new PriorityBlockingQueue<Runnable>(), new Utils.PicassoThreadFactory());
  }

4. 配置默认请求转换器

    // 空转换，请求原样返回
    RequestTransformer IDENTITY = new RequestTransformer() {
      @Override public Request transformRequest(Request request) {
        return request;
      }
    };

5. Dispatcher分发器

  Dispatcher(Context context, ExecutorService service, Handler mainThreadHandler,
      Downloader downloader, Cache cache, Stats stats) {
    // DispatcherThread继承自HandlerThread，主要做一些耗时操作 
    this.dispatcherThread = new DispatcherThread();
    this.dispatcherThread.start();
    this.context = context;
    this.service = service;
    this.hunterMap = new LinkedHashMap<String, BitmapHunter>();
    this.failedActions = new WeakHashMap<Object, Action>();
    this.pausedActions = new WeakHashMap<Object, Action>();
    this.pausedTags = new HashSet<Object>();
    // DispatcherThread子线程的handler
    this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);
    this.downloader = downloader;
    // 主线程handler
    this.mainThreadHandler = mainThreadHandler;
    this.cache = cache;
    this.stats = stats;
    this.batch = new ArrayList<BitmapHunter>(4);
    this.airplaneMode = Utils.isAirplaneModeOn(this.context);
    this.scansNetworkChanges = hasPermission(context, Manifest.permission.ACCESS_NETWORK_STATE);
    // 监听网络变化
    this.receiver = new NetworkBroadcastReceiver(this);
    receiver.register();
  }

NetworkBroadcastReceiver监听网络变化

  static class NetworkBroadcastReceiver extends BroadcastReceiver {
    static final String EXTRA_AIRPLANE_STATE = "state";

    private final Dispatcher dispatcher;

    NetworkBroadcastReceiver(Dispatcher dispatcher) {
      this.dispatcher = dispatcher;
    }

    @Override public void onReceive(Context context, Intent intent) {
      final String action = intent.getAction();
      if (ACTION_AIRPLANE_MODE_CHANGED.equals(action)) {
        if (!intent.hasExtra(EXTRA_AIRPLANE_STATE)) {
          return; 
        }
       dispatcher.dispatchAirplaneModeChange(intent.getBooleanExtra(EXTRA_AIRPLANE_STATE, false));
      } else if (CONNECTIVITY_ACTION.equals(action)) {
        ConnectivityManager connectivityManager = getService(context, CONNECTIVITY_SERVICE);
        // 监听到网络变化后，dispatcher做响应的操作
        dispatcher.dispatchNetworkStateChange(connectivityManager.getActiveNetworkInfo());
      }
    }
  }

2. 监听到网络变化后，dispatcher做响应的操作

  void performNetworkStateChange(NetworkInfo info) {
    if (service instanceof PicassoExecutorService) {
      // dispatcher 会根据网络状态调整线程池
      ((PicassoExecutorService) service).adjustThreadCount(info);
    }
    // Intentionally check only if isConnected() here before we flush out failed actions.
    if (info != null && info.isConnected()) {
      flushFailedActions();
    }
  }
 
// 根据网络状态调整线程池
 void adjustThreadCount(NetworkInfo info) {
    if (info == null || !info.isConnectedOrConnecting()) {
      setThreadCount(DEFAULT_THREAD_COUNT);
      return;
    }
    switch (info.getType()) {
      case ConnectivityManager.TYPE_WIFI:
      case ConnectivityManager.TYPE_WIMAX:
      case ConnectivityManager.TYPE_ETHERNET:
        setThreadCount(4);
        break;
      case ConnectivityManager.TYPE_MOBILE:
        switch (info.getSubtype()) {
          case TelephonyManager.NETWORK_TYPE_LTE:  // 4G
          case TelephonyManager.NETWORK_TYPE_HSPAP:
          case TelephonyManager.NETWORK_TYPE_EHRPD:
            setThreadCount(3);
            break;
          case TelephonyManager.NETWORK_TYPE_UMTS: // 3G
          case TelephonyManager.NETWORK_TYPE_CDMA:
          case TelephonyManager.NETWORK_TYPE_EVDO_0:
          case TelephonyManager.NETWORK_TYPE_EVDO_A:
          case TelephonyManager.NETWORK_TYPE_EVDO_B:
            setThreadCount(2);
            break;
          case TelephonyManager.NETWORK_TYPE_GPRS: // 2G
          case TelephonyManager.NETWORK_TYPE_EDGE:
            setThreadCount(1);
            break;
          default:
            setThreadCount(DEFAULT_THREAD_COUNT);
        }
        break;
      default:
        setThreadCount(DEFAULT_THREAD_COUNT);
    }
  }

  private void setThreadCount(int threadCount) {
    setCorePoolSize(threadCount);
    setMaximumPoolSize(threadCount);
  }

2、load生成RequestCreator

通过load方法生成一个RequestCreator，用链式api 来构建一个图片下载请求

  public RequestCreator load(String path) {
    if (path == null) {
      return new RequestCreator(this, null, 0);
    }
    if (path.trim().length() == 0) {
      throw new IllegalArgumentException("Path must not be empty.");
    }
    return load(Uri.parse(path));
  }

RequestCreator提供了很多的API 来构建请求，如展位图、大小、转换器、裁剪等等,这些API其实是为对应的属性赋值，最终会传入into方法中构建请求。

public class RequestCreator {
  private static final AtomicInteger nextId = new AtomicInteger();

  private final Picasso picasso;
  private final Request.Builder data;

  private boolean noFade;
  private boolean deferred;
  private boolean setPlaceholder = true;
  private int placeholderResId;
  private int errorResId;
  private int memoryPolicy;
  private int networkPolicy;
  private Drawable placeholderDrawable;
  private Drawable errorDrawable;
  private Object tag;
}

3、into添加View，并请求下载

into方法里面干了3件事情：

1. 判断是否设置了fit 属性，如果设置了，再看是否能够获取ImageView 的宽高，如果获取不到，生成一个DeferredRequestCreator（延迟的请求管理器），然后直接return,在DeferredRequestCreator中当监听到可以获取ImageView 的宽高的时候，再执行into方法。

2. 判断是否从内存缓存获取图片，如果没有设置NO_CACHE,则从内存获取，命中直接回调CallBack 并且显示图片。

3. 如果缓存未命中，则生成一个Action，并提交Action。

 public void into(ImageView target, Callback callback) {
    long started = System.nanoTime();
   // 检查是否在主线程
    checkMain();

    if (target == null) {
      throw new IllegalArgumentException("Target must not be null.");
    }
   //如果没有url或者resourceId 则取消请求
    if (!data.hasImage()) {
      picasso.cancelRequest(target);
      if (setPlaceholder) {
        setPlaceholder(target, getPlaceholderDrawable());
      }
      return;
    }
    //判断是否设置了fit属性
    if (deferred) {
      if (data.hasSize()) {
        throw new IllegalStateException("Fit cannot be used with resize.");
      }
      int width = target.getWidth();
      int height = target.getHeight();
      if (width == 0 || height == 0) {
        if (setPlaceholder) {
          setPlaceholder(target, getPlaceholderDrawable());
        }
       //如果获取不到宽高，生成一个DeferredRequestCreator（延迟的请求管理器），然后直接return,
      //在DeferredRequestCreator中当监听到可以获取ImageView 的宽高的时候，再执行into方法。
        picasso.defer(target, new DeferredRequestCreator(this, target, callback));
        return;
      }
      data.resize(width, height);
    }

    Request request = createRequest(started);
    String requestKey = createKey(request);
    //是否从内存缓存中获取
    if (shouldReadFromMemoryCache(memoryPolicy)) {
      Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
      if (bitmap != null) {
       //缓存命中，取消请求，并显示图片
        picasso.cancelRequest(target);
        setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
        if (picasso.loggingEnabled) {
          log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
        }
        if (callback != null) {
          callback.onSuccess();
        }
        return;
      }
    }

    if (setPlaceholder) {
      setPlaceholder(target, getPlaceholderDrawable());
    }
   //内存缓存未命中或者设置了不从内存缓存获取，则生成一个Action ，提交执行。
    Action action =
        new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
            errorDrawable, requestKey, tag, callback, noFade);

    picasso.enqueueAndSubmit(action);// 提交请求
  }

request包装类Action

abstract class Action<T> {
  static class RequestWeakReference<M> extends WeakReference<M> {
    final Action action;

    public RequestWeakReference(Action action, M referent, ReferenceQueue<? super M> q) {
      super(referent, q);
      this.action = action;
    }
  }

  final Picasso picasso;
  final Request request;
  final WeakReference<T> target;
  final boolean noFade;
  final int memoryPolicy;
  final int networkPolicy;
  final int errorResId;
  final Drawable errorDrawable;
  final String key;
  final Object tag;

  boolean willReplay;
  boolean cancelled;

  Action(Picasso picasso, T target, Request request, int memoryPolicy, int networkPolicy,
      int errorResId, Drawable errorDrawable, String key, Object tag, boolean noFade) {
    this.picasso = picasso;
    this.request = request;
    this.target =
        target == null ? null : new RequestWeakReference<T>(this, target, picasso.referenceQueue);
    this.memoryPolicy = memoryPolicy;
    this.networkPolicy = networkPolicy;
    this.noFade = noFade;
    this.errorResId = errorResId;
    this.errorDrawable = errorDrawable;
    this.key = key;
    this.tag = (tag != null ? tag : this);
  }
}

picasso.enqueueAndSubmit(action) 提交请求

  void enqueueAndSubmit(Action action) {
    Object target = action.getTarget();
    if (target != null && targetToAction.get(target) != action) {
      // This will also check we are on the main thread.
      cancelExistingRequest(target);
      //将action 保存到了一个Map 中，目标View作为key
      targetToAction.put(target, action);
    }
    submit(action);
  }
// 交给分发器分发提交请求
  void submit(Action action) {
    dispatcher.dispatchSubmit(action);
  }

Dispatcher:
 void performSubmit(Action action, boolean dismissFailed) {
    // 先查看保存暂停tag表里面没有包含Action的tag,如果包含，则将Action 存到暂停Action表里
    if (pausedTags.contains(action.getTag())) {
      pausedActions.put(action.getTarget(), action);
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_PAUSED, action.request.logId(),
            "because tag '" + action.getTag() + "' is paused");
      }
      return;
    }

    BitmapHunter hunter = hunterMap.get(action.getKey());
    if (hunter != null) {
      hunter.attach(action);
      return;
    }
   // 如果线程池北shutDown,直接return
    if (service.isShutdown()) {
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
      }
      return;
    }
    // 为请求生成一个BitmapHunter
    hunter = forRequest(action.getPicasso(), this, cache, stats, action);
    hunter.future = service.submit(hunter);//提交执行
    hunterMap.put(action.getKey(), hunter);
    if (dismissFailed) {
      failedActions.remove(action.getTarget());
    }

    if (action.getPicasso().loggingEnabled) {
      log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
    }
  }

执行请求提交

1. 先查看保存暂停tag表里面没有包含Action的tag,如果包含，则将Action 存到暂停Action表里
2. 从BitmapHunter表里查找有没有对应action的hunter,如果有直接attach
3. 为这个请求生成一个BitmapHunter，提交给线程池执行

4、BitmapHunter 下载图片

BitmapHunter继承Runnable，其实就是开启一个线程执行最终的下载

class BitmapHunter implements Runnable {
  @Override public void run() {
    try {
      updateThreadName(data);

      if (picasso.loggingEnabled) {
        log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
      }
      //  调用hunt() 方法获取最终结果
      result = hunt();

      if (result == null) {
        dispatcher.dispatchFailed(this);
      } else {
        dispatcher.dispatchComplete(this);
      }
    } catch (Downloader.ResponseException e) {
      if (!e.localCacheOnly || e.responseCode != 504) {
        exception = e;
      }
      dispatcher.dispatchFailed(this);
    } catch (NetworkRequestHandler.ContentLengthException e) {
      exception = e;
      dispatcher.dispatchRetry(this);
    } catch (IOException e) {
      exception = e;
      dispatcher.dispatchRetry(this);
    } catch (OutOfMemoryError e) {
      StringWriter writer = new StringWriter();
      stats.createSnapshot().dump(new PrintWriter(writer));
      exception = new RuntimeException(writer.toString(), e);
      dispatcher.dispatchFailed(this);
    } catch (Exception e) {
      exception = e;
      dispatcher.dispatchFailed(this);
    } finally {
      Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
    }
  }
}

当将一个bitmapHunter submit 给一个线程池执行的时候，就会执行run() 方法，run里面调用的是hunt方法来获取结果，看一下hunt方法：

Bitmap hunt() throws IOException {
    Bitmap bitmap = null;
   // 是否从内存缓存获取Bitmap
    if (shouldReadFromMemoryCache(memoryPolicy)) {
      bitmap = cache.get(key);
      if (bitmap != null) {
        stats.dispatchCacheHit();
        loadedFrom = MEMORY;
        if (picasso.loggingEnabled) {
          log(OWNER_HUNTER, VERB_DECODED, data.logId(), "from cache");
        }
        return bitmap;
      }
    }

    data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
   // 请求处理器处理请求，获取结果，Result里可能是Bitmap，可能是Stream
    RequestHandler.Result result = requestHandler.load(data, networkPolicy);
    if (result != null) {
      loadedFrom = result.getLoadedFrom();
      exifRotation = result.getExifOrientation();

      bitmap = result.getBitmap();

      // If there was no Bitmap then we need to decode it from the stream.
      if (bitmap == null) {
        InputStream is = result.getStream();
        try {
          bitmap = decodeStream(is, data);
        } finally {
          Utils.closeQuietly(is);
        }
      }
    }

    if (bitmap != null) {
      if (picasso.loggingEnabled) {
        log(OWNER_HUNTER, VERB_DECODED, data.logId());
      }
      stats.dispatchBitmapDecoded(bitmap);
      if (data.needsTransformation() || exifRotation != 0) {
        synchronized (DECODE_LOCK) {
          if (data.needsMatrixTransform() || exifRotation != 0) {
            //如果需要做转换，则在这里做转换处理，如角度旋转，裁剪等。
            bitmap = transformResult(data, bitmap, exifRotation);
            if (picasso.loggingEnabled) {
              log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId());
            }
          }
          if (data.hasCustomTransformations()) {
           // 如果配置了自定义转换器，则在这里做转换处理。
            bitmap = applyCustomTransformations(data.transformations, bitmap);
            if (picasso.loggingEnabled) {
              log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId(), "from custom transformations");
            }
          }
        }
        if (bitmap != null) {
          stats.dispatchBitmapTransformed(bitmap);
        }
      }
    }

    return bitmap;
  }

5、Downloader 下载器下载图片

hunt方法获取结果的时候，最终调用的是配置的处理器的load方法

RequestHandler.Result result = requestHandler.load(data, networkPolicy);

如果是网络图片会调用NetworkRequestHandler的load方法

@Override public Result load(Request request, int networkPolicy) throws IOException {
    //最终调用downloader的load方法获取结果
    Response response = downloader.load(request.uri, request.networkPolicy);
    if (response == null) {
      return null;
    }

    Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;

    Bitmap bitmap = response.getBitmap();
    if (bitmap != null) {
      return new Result(bitmap, loadedFrom);
    }

    InputStream is = response.getInputStream();
    if (is == null) {
      return null;
    }
    return new Result(is, loadedFrom);
  }

etworkRequestHandler最终是调用的downloader 的load方法下载图片。内置了2个Downloader，OkhttpDownloader和UrlConnectionDownloader 。我们以UrlConnectionDownloader为例，来看一下load方法：

  @Override public Response load(Uri uri, int networkPolicy) throws IOException {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
      installCacheIfNeeded(context);
    }

    HttpURLConnection connection = openConnection(uri);
    connection.setUseCaches(true);

    if (networkPolicy != 0) {
      String headerValue;

      if (NetworkPolicy.isOfflineOnly(networkPolicy)) {
        headerValue = FORCE_CACHE;
      } else {
        StringBuilder builder = CACHE_HEADER_BUILDER.get();
        builder.setLength(0);

        if (!NetworkPolicy.shouldReadFromDiskCache(networkPolicy)) {
          builder.append("no-cache");
        }
        if (!NetworkPolicy.shouldWriteToDiskCache(networkPolicy)) {
          if (builder.length() > 0) {
            builder.append(',');
          }
          builder.append("no-store");
        }

        headerValue = builder.toString();
      }

      connection.setRequestProperty("Cache-Control", headerValue);
    }

    int responseCode = connection.getResponseCode();
    if (responseCode >= 300) {
      connection.disconnect();
      throw new ResponseException(responseCode + " " + connection.getResponseMessage(),
          networkPolicy, responseCode);
    }

    long contentLength = connection.getHeaderFieldInt("Content-Length", -1);
    boolean fromCache = parseResponseSourceHeader(connection.getHeaderField(RESPONSE_SOURCE));
    // 最后获取InputStream流包装成Response返回
    return new Response(connection.getInputStream(), fromCache, contentLength);
  }

6、完成加载

在BitmapHunter获取结果后，分发器分发结果,通过Hander处理后，执行performComplete方法：

      result = hunt();
      if (result == null) {
        dispatcher.dispatchFailed(this);
      } else {
        dispatcher.dispatchComplete(this);
      }

Dispatcher：
  void performComplete(BitmapHunter hunter) {
    // 这里将结果缓存到内存
    if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
      cache.set(hunter.getKey(), hunter.getResult());
    }
    / 请求完毕，将hunter从表中移除
    hunterMap.remove(hunter.getKey());
    batch(hunter);
    }

然后将BitmapHunter添加到一个批处理列表，通过Hander发送一个批处理消息

  private void batch(BitmapHunter hunter) {
    if (hunter.isCancelled()) {
      return;
    }
    batch.add(hunter);
    if (!handler.hasMessages(HUNTER_DELAY_NEXT_BATCH)) {
      handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);
    }
  }

最后执行performBatchComplete 方法，通过主线程的Handler送处理完成的消息

  void performBatchComplete() {
    List<BitmapHunter> copy = new ArrayList<BitmapHunter>(batch);
    batch.clear();
    mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
    logBatch(copy);
  }

最后在Picasso 中handleMessage,显示图片

 static final Handler HANDLER = new Handler(Looper.getMainLooper()) {
    @Override public void handleMessage(Message msg) {
      switch (msg.what) {
        case HUNTER_BATCH_COMPLETE: {
          @SuppressWarnings("unchecked") List<BitmapHunter> batch = (List<BitmapHunter>) msg.obj;
          //noinspection ForLoopReplaceableByForEach
          for (int i = 0, n = batch.size(); i < n; i++) {
            BitmapHunter hunter = batch.get(i);
            hunter.picasso.complete(hunter);
          }
          break;
        }
      //后面代码省略
      ...
  };

回调到ImageViewAction 的complete方法显示图片

@Override public void complete(Bitmap result, Picasso.LoadedFrom from) {
    if (result == null) {
      throw new AssertionError(
          String.format("Attempted to complete action with no result!\n%s", this));
    }

    ImageView target = this.target.get();
    if (target == null) {
      return;
    }
    Context context = picasso.context;
    boolean indicatorsEnabled = picasso.indicatorsEnabled;
   //将结果包装成一个PicassoDrawable 并显示
    PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);

    if (callback != null) {
      callback.onSuccess(); 回调callback
    }
  }