图解Picasso源码

以下是picasso调用的图解：

1.当我们在调用Picasso.with()方法的时候，会生成一个单例的Picasso

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

首先是全局的一个单例对象，其次是通过Builder模式生成对应的Picasso对象。

2.我们在调用完上面的方法之后，会继续调用load方法：

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));
  }

支持loadFile，Uri，path等，在这里会其实最终都是调用load(uri)的方法，生成RequestCreator，接下来的placeHolder，errImage，设置图片等，通过链式调用实现。

3.在创建完成RequestCreator之后，我们会执行RequestCreator的into方法：

public void into(ImageView target, Callback callback) {
    long started = System.nanoTime();
    checkMain();

    if (target == null) {
      throw new IllegalArgumentException("Target must not be null.");
    }

    if (!data.hasImage()) {//1
      picasso.cancelRequest(target);
      if (setPlaceholder) {
        setPlaceholder(target, getPlaceholderDrawable());
      }
      return;
    }

    if (deferred) {
      if (data.hasSize()) {//2
        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());
        }
        picasso.defer(target, new DeferredRequestCreator(this, target, callback));
        return;
      }
      data.resize(width, height);
    }

    Request request = createRequest(started);
    String requestKey = createKey(request);

    if (shouldReadFromMemoryCache(memoryPolicy)) {//3
      Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
      if (bitmap != null) {
        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) {//4
      setPlaceholder(target, getPlaceholderDrawable());
    }

    Action action =
        new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
            errorDrawable, requestKey, tag, callback, noFade);

    picasso.enqueueAndSubmit(action);//5
  }

在这里主要就是执行图片的获取，转换大小以及图片的显示的操作。

3.1 上面的第一步主要是判断有没有资源文件设置或者本地路径等，如果有那么直接显示，这里主要是变相地实现了Picasso的本地图片的加载的实现。

3.2 第二步设置要显示的ImageView控件的大小，以便计算图片和控件的大小比，以进行加载时候内存的优化

3.3 第三步会从内存中去读取图片，读取到图片增加命中的计算器

3.4 第四步是设置空白图片

3.5 第五步主要就是执行图片的获取操作，picasso.enqueueAndSubmit(Action)方法：

4.picasso.enqueueAndSubmit(Action)方法：

void enqueueAndSubmit(Action action) {
    Object target = action.getTarget();//因为在上面的我们初始化的ImageView的Action，所以target是要加载的ImageView。
    if (target != null && targetToAction.get(target) != action) {
      // This will also check we are on the main thread.
      cancelExistingRequest(target);
      targetToAction.put(target, action);
    }
    submit(action);
  }

在这里我们会调用submit方法，该方法主要是将action时间分发给对应的Dispatcher，dispatcher.dispatchSubmit(action);

void dispatchSubmit(Action action) {
    handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
  }

对应的会通过Handler发送一个REQUEST_SUBMIT的消息，这个Handler是DispatcherHandler的实例对象，执行对应的HandleMessage方法，

case REQUEST_SUBMIT: {
          Action action = (Action) msg.obj;
          dispatcher.performSubmit(action);
          break;
        }

5.我们来看看Dispatcher类的performSubmit方法：

void performSubmit(Action action, boolean dismissFailed) {
    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;
    }

    if (service.isShutdown()) {
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
      }
      return;
    }

    hunter = forRequest(action.getPicasso(), this, cache, stats, action);//1
    hunter.future = service.submit(hunter);
    hunterMap.put(action.getKey(), hunter);
    if (dismissFailed) {
      failedActions.remove(action.getTarget());
    }

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

注释1中会一次调用各种RequestHandler的canHandleRequest方法，来选择能处理的RequestHandler
piassco在初始化的时候，默认会添加如下的RequestHandler

List<RequestHandler> allRequestHandlers =
        new ArrayList<RequestHandler>(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));

6.上述找到对应能处理的RequestHandler之后，创建对应的bitmapHunter，BitmapHunder其实是Runnable的实现。

@Override
  public Future<?> submit(Runnable task) {
    PicassoFutureTask ftask = new PicassoFutureTask((BitmapHunter) task);
    execute(ftask);
    return ftask;
  }

创建对应的Runnable完成之后，加入线程池中执行

@Override public void run() {
    try {
      updateThreadName(data);

      if (picasso.loggingEnabled) {
        log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
      }

      result = hunt();//1

      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);
    }
  }

注释1是获取图片的逻辑

Bitmap hunt() throws IOException {
    Bitmap bitmap = null;

    if (shouldReadFromMemoryCache(memoryPolicy)) {//1
      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;
    RequestHandler.Result result = requestHandler.load(data, networkPolicy);
    if (result != null) {//2
      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) {//3
      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) {
            bitmap = transformResult(data, bitmap, exifRotation);
            if (picasso.loggingEnabled) {
              log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId());
            }
          }
          if (data.hasCustomTransformations()) {
            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;
  }

上述注释1就是从内存中获取图片，如果取不到图片注释2由RequestHandler.load拿到图片数据的流的结果对象，然后解析成图片，注释3在拿到图片之后，对图片根据控件的大小进行转换。拿到图片之后，发送消息code为HUNTER_COMPLETE的消息到MessageQueue中，

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

接着dispatcherHandler执行

case HUNTER_COMPLETE: {
          BitmapHunter hunter = (BitmapHunter) msg.obj;
          dispatcher.performComplete(hunter);
          break;
        }

void performComplete(BitmapHunter hunter) {
    if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
      cache.set(hunter.getKey(), hunter.getResult());
    }
    hunterMap.remove(hunter.getKey());
    batch(hunter);//1
    if (hunter.getPicasso().loggingEnabled) {
      log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
    }
  }

进行内存中保存。完成图片的内存缓存之后，就是对图片设置到ImageView上，Dispatch的batch方法

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);
    }
  }

case HUNTER_DELAY_NEXT_BATCH: {
          dispatcher.performBatchComplete();
          break;
        }

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

其实上面的操作是将io线程切换到主线程上来，到绑定主线程Looper的handler接收到消息，会执行

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;
        }

void complete(BitmapHunter hunter) {
    Action single = hunter.getAction();
    List<Action> joined = hunter.getActions();

    boolean hasMultiple = joined != null && !joined.isEmpty();
    boolean shouldDeliver = single != null || hasMultiple;

    if (!shouldDeliver) {
      return;
    }

    Uri uri = hunter.getData().uri;
    Exception exception = hunter.getException();
    Bitmap result = hunter.getResult();
    LoadedFrom from = hunter.getLoadedFrom();

    if (single != null) {
      deliverAction(result, from, single);//1
    }

    if (hasMultiple) {
      //noinspection ForLoopReplaceableByForEach
      for (int i = 0, n = joined.size(); i < n; i++) {
        Action join = joined.get(i);
        deliverAction(result, from, join);
      }
    }

    if (listener != null && exception != null) {
      listener.onImageLoadFailed(this, uri, exception);
    }
  }

注释1就是将图片设置到Image的操作。

private void deliverAction(Bitmap result, LoadedFrom from, Action action) {
    if (action.isCancelled()) {
      return;
    }
    if (!action.willReplay()) {
      targetToAction.remove(action.getTarget());
    }
    if (result != null) {
      if (from == null) {
        throw new AssertionError("LoadedFrom cannot be null.");
      }
      action.complete(result, from);
      if (loggingEnabled) {
        log(OWNER_MAIN, VERB_COMPLETED, action.request.logId(), "from " + from);
      }
    } else {
      action.error();
      if (loggingEnabled) {
        log(OWNER_MAIN, VERB_ERRORED, action.request.logId());
      }
    }

通过ImageViewAction执行界面的设置操作。

总结

上面其实只是一个源码的阅读流程记录而已，其实其中的更多的是设计思想的理解，设计模式的学习等，比如这里的Action接口，多个子类实现了该接口，都会执行compelete方法，这就是策略模式的模型，会根据你传入的是ImageViewAction还是GetAction来执行对应的complete，这样就需要判断是ImageViewAction还是GetAction来执行对应的操作；还有在初始化的时候，会在list中增加各种RequestHandler，然后在根据你传入的uri，找到对应的RequestHandler这种设计，个人觉得这种设计在解决一个问题有多种串行的尝试的时候，这个还是很有用。

如果你们觉得文章对你有启示作用，希望你们帮忙点个赞或者关注下，谢谢。