Picasso源码解析

前言

---

Picasso和OKHttp同属大名鼎鼎的Square公司，是一款优秀的图片处理框架。而一款图片处理框架无外乎流的加载，内存和硬盘缓存等内容，学习这些优秀的图片框架，可以增强我们对这方面的理解。

使用方法

---

//Picasso支持File，path，Uri，resourceId 4种加载图片
//最简单的显示方式
imageView = (ImageView) findViewById(R.id.img);
Picasso.with(this).load("https://www.baidu.com/img/bd_logo1.png").into(imageView);

//还可以预加载
//预加载一张图片
Picasso.with(this).load("https://www.baidu.com/img/bd_logo1.png").fetch();

效果图

关系图

---

这是来自Skykai
的关系类图，很详尽的展示了Picasso的类之间的关系

但是往往太详尽的东西都不太好理解，所以贴一张我整理的流程图，便于大家理解

源码分析

---

我们来看看

Picasso.with(this).load("https://www.baidu.com/img/bd_logo1.png").into(imageView);

这句简单的代码是如何把一张远程图片显示到ImageView上的

首先来看看Picasso.with(this)干了些什么

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

显然Picasso是一个单例模式，向外提供服务，同时通过构造器的模式产生实例。

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

      if (downloader == null) {
        downloader = new OkHttp3Downloader(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);
    }

可以看到默认的情况下Picasso的下载器是使用OkHttp3Downloader，毕竟是自己家的产品。缓存是使用LRU，内部也就是维护了一个LinkedHashMap，线程池是自己写的一个PicassoExecutorService。使用构建器可以很灵活的替换掉这几部分，体现了组件化开发的优势。

再来看下Picasso的构造函数干了些什么

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<>();
    this.cleanupThread = new CleanupThread(referenceQueue, HANDLER);
    this.cleanupThread.start();
  }

可以看到主要是将构造器的参数赋值给自己，然后还干了一件很重要的事，保存了几个RequestHandler的实例，用于处理不同类的图片请求，并且采用了责任链的设计模式去处理。


接着看看load("https://www.baidu.com/img/bd_logo1.png")做了些什么

public RequestCreator load(@Nullable Uri uri) {
    return new RequestCreator(this, uri, 0);
  }

public RequestCreator load(@Nullable 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));
  }

就简单的产生了一个RequestCreator对象


看来主要的逻辑都在into(imageView)里了

public void into(ImageView target, Callback callback) {
    //首先记录下请求时间
    long started = System.nanoTime();
    //然后确认了下是不是在主线程
    checkMain();

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

    //对image的url合法性进行校验
    if (!data.hasImage()) {
      picasso.cancelRequest(target);
      if (setPlaceholder) {
        setPlaceholder(target, getPlaceholderDrawable());
      }
      return;
    }

    //这里是根据配置信息决定是否需要重新测量尺寸
    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 || target.isLayoutRequested()) {
        if (setPlaceholder) {
          setPlaceholder(target, getPlaceholderDrawable());
        }
        picasso.defer(target, new DeferredRequestCreator(this, target, callback));
        return;
      }
      data.resize(width, height);
    }

    //这里就是真正的请求开始
    //主要记录了请求的时间，id和一些裁剪和缓存的策略
    Request request = createRequest(started);
    String requestKey = createKey(request);

    //根据缓存策略，决定是否读取缓存
    if (shouldReadFromMemoryCache(memoryPolicy)) {
        //如果缓存存在的话，直接显示缓存内容就可以了，取消请求
      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;
      }
    }
    //这里是决定是否显示默认的占位图
    if (setPlaceholder) {
      setPlaceholder(target, getPlaceholderDrawable());
    }

    //如果缓存不存在，就组装一个Action，加入请求队列
    Action action =
        new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
            errorDrawable, requestKey, tag, callback, noFade);
    //这里重新回到picasso
    picasso.enqueueAndSubmit(action);
  }

可以看到RequestCreator主要起到验证缓存，组装请信息的作用

然后回到Picasso看看

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);
      targetToAction.put(target, action);
    }
    submit(action);
  }

  void submit(Action action) {
    dispatcher.dispatchSubmit(action);
  }

这里也非常简单检验了下target的合法性，一般tag都是context然后就交给dispatcher了

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

'''省略一大段代码'''
case REQUEST_SUBMIT: {
          Action action = (Action) msg.obj;
          dispatcher.performSubmit(action);
          break;
        }
'''省略一大段代码'''     


void performSubmit(Action action) {
    performSubmit(action, true);
  }

//真正执行业务的就是这个方法了
  void performSubmit(Action action, boolean dismissFailed) {
      //在请求前，检查下这个请求是否被中断了
    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;
    }

    //得到hunter对象，对于第一次请求的，hunter都为null，这里主要为失败后重新请求服务
    BitmapHunter hunter = hunterMap.get(action.getKey());
    if (hunter != null) {
      hunter.attach(action);
      return;
    }

    //检查线程池是否正常
    if (service.isShutdown()) {
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
      }
      return;
    }

    //通过forRequest得到hunter对象，hunter本身是一个Runnable,所以可以通过线程池的submit执行
    hunter = forRequest(action.getPicasso(), this, cache, stats, action);
    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());
    }
  }

从上面代码结合注释来看，dispatcher只是起到了分发的作用，和它的名字和符合。真正的业务处理应该在hunter中。

首先是如何产生BitmapHunter这个对象

static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
      Action action) {
    Request request = action.getRequest();
    //这里的requestHandlers 就是Picasso的构造函数中生成的那些，根据条件来挑选可以执行当前请求的handler
    //对于网络请求，采用的应该是NetworkRequestHandler
    List requestHandlers = picasso.getRequestHandlers();

    // Index-based loop to avoid allocating an iterator.
    //noinspection ForLoopReplaceableByForEach
    for (int i = 0, count = requestHandlers.size(); i < count; i++) {
      RequestHandler requestHandler = requestHandlers.get(i);
      if (requestHandler.canHandleRequest(request)) {
        return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
      }
    }

    return new BitmapHunter(picasso, dispatcher, cache, stats, action, ERRORING_HANDLER);
  }

然后看看hunter的run方法

@Override public void run() {
    try {
    //首先更新ThreadLocal中该线程对应的名字
      updateThreadName(data);

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

    //通过hunt()这个方法得到Bitmap
      result = hunt();

    //如果result不为空，就执行complete方法，否则调用dispatchFailed
      if (result == null) {
        dispatcher.dispatchFailed(this);
      } else {
        dispatcher.dispatchComplete(this);
      }
    //各种异常对应的处理方法
    } catch (NetworkRequestHandler.ResponseException e) {
      if (!NetworkPolicy.isOfflineOnly(e.networkPolicy) || e.code != 504) {
        exception = e;
      }
      dispatcher.dispatchFailed(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);
    }
  }

从上面的代码来看主要的逻辑应该在hunt()这个方法里

Bitmap hunt() throws IOException {
    Bitmap bitmap = null;
    //跟之前一直，还是根据策略检查下内存缓存
    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;
      }
    }
    //如果失败，重试的次数
    networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
    //使用requestHandler执行网络请求，得到结果
    RequestHandler.Result result = requestHandler.load(data, networkPolicy);
    if (result != null) {
      loadedFrom = result.getLoadedFrom();
      exifOrientation = result.getExifOrientation();
      bitmap = result.getBitmap();

        //首先看下得到的Bitmap是否为空，如果为空就需要对流进行decode
      // If there was no Bitmap then we need to decode it from the stream.
      if (bitmap == null) {
        Source source = result.getSource();
        try {
        //通过流得到bitmap
          bitmap = decodeStream(source, data);
        } finally {
          try {
            //noinspection ConstantConditions If bitmap is null then source is guranteed non-null.
            source.close();
          } catch (IOException ignored) {
          }
        }
      }
    }
    //根据预设规则对bitmap进行裁剪
    if (bitmap != null) {
      if (picasso.loggingEnabled) {
        log(OWNER_HUNTER, VERB_DECODED, data.logId());
      }
      stats.dispatchBitmapDecoded(bitmap);
      if (data.needsTransformation() || exifOrientation != 0) {
        synchronized (DECODE_LOCK) {
          if (data.needsMatrixTransform() || exifOrientation != 0) {
            bitmap = transformResult(data, bitmap, exifOrientation);
            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;
  }

这里可以得出结论hunt()这个方法还是不涉及到网络请求，只是根据结果对流进行处理，有兴趣的同学可以自行看下decodeStream这个方法

我们只能接着看下NetworkRequestHandler里的load方法了

@Override public Result load(Request request, int networkPolicy) throws IOException {
    //看到这，终于找到了网络请求的处理了
    //在Picasso的Bulid中我们看到downloader默认是OkHttp3Downloader
    //所以这里使用OkHttp3Downloader进行请求和硬盘的缓存
    //缓存的地址是/data/data/package name/cache/picasso-cache
    //之后的文章中可以详细讨论下OKHttp的请求实现，这里我们得到一个结论即可
    okhttp3.Request downloaderRequest = createRequest(request, networkPolicy);
    Response response = downloader.load(downloaderRequest);
    ResponseBody body = response.body();

    if (!response.isSuccessful()) {
      body.close();
      throw new ResponseException(response.code(), request.networkPolicy);
    }

    // Cache response is only null when the response comes fully from the network. Both completely
    // cached and conditionally cached responses will have a non-null cache response.
    Picasso.LoadedFrom loadedFrom = response.cacheResponse() == null ? NETWORK : DISK;

    // Sometimes response content length is zero when requests are being replayed. Haven't found
    // root cause to this but retrying the request seems safe to do so.
    if (loadedFrom == DISK && body.contentLength() == 0) {
      body.close();
      throw new ContentLengthException("Received response with 0 content-length header.");
    }
    if (loadedFrom == NETWORK && body.contentLength() > 0) {
      stats.dispatchDownloadFinished(body.contentLength());
    }
    return new Result(body.source(), loadedFrom);
  }

再得到bitmap后，我们最后看下dispatcher的performComplete方法

void performComplete(BitmapHunter hunter) {
    //这里根据缓存策略，决定是否存储到内存，也就是LRU对应的map
    if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
      cache.set(hunter.getKey(), hunter.getResult());
    }
    //清除这个请求对应的hunter
    hunterMap.remove(hunter.getKey());
    batch(hunter);
    if (hunter.getPicasso().loggingEnabled) {
      log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
    }
  }

总结

---

结合流程图，我们可以清楚的了解到Picasso的网络请求(OKHttp),内存缓存(LRU)和硬盘缓存(OKHttp代为执行)

也可以通过源代码发现Picasso默认是不会把硬盘上的缓存加载进来，因为LRUcache在初始化的时候是new出来的。我们需要通过Build构建一个我们自己的Picasso对象，然后执行setSingletonInstance方法，设置我们自己的单例。

Cache cache = DO SOMETHING//把硬盘里的缓存加载进来
Picasso picasso = new Picasso.Builder(this).memoryCache(cache ).build();
        Picasso.setSingletonInstance(picasso);

从Picasso中我们总结出灵活这个词，每个模块都可以被调用方定制，替换。Picasso只制定流程，不关心细节，这点我们在设计流程的时候可以大大参考一下！