Picasso源码解析

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发布于 2022-04-03 16:55:20

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发布于 2022-04-03 16:55:20

一、介绍

一句话概括：Picasso 收到加载及显示图片的任务，创建 RequestCreator 并将它交给 Dispatcher，Dispatcher 创建 BitmapHunter （并为它找到具体的 RequestHandler）提交到线程池，BitmapHunter 调用具体 RequestHandler，任务通过 MemoryCache 及 Handler(数据获取接口) 获取图片，图片获取成功后通过 PicassoDrawable 显示到 Target 中。

我们这次源码解析，以【网络请求】，【请求成功处理】为切入点。

二、源码解析

我们来个简单案例来看其过程源码。

Picasso.with(MainActivity.this).load("http://nuuneoi.com/uploads/source/playstore/cover.jpg").into(imageView, new Callback() {
    @Override
    public void onSuccess() {
    }
    @Override
    public void onError() {
    }
});

我们主要看with、load、into这三个源码。

一、with

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

with方法是创建Picasso的实例，我们可以看到，with里面用了双重加锁的方式，保证了创建Picasso实例线程安全，而且Picasso实例是用Builder模式创建的，在多个属性情况下，Builder模式比较方便。来看下具体源码。

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);
}

这个里面主要初始化了Downloader、LruCache、PicassoExecutorService、Stats、Dispatcher，最后返回了Picasso实例。

我们大致看下他们具体内容

Downloader

static Downloader createDefaultDownloader(Context context) {
  try {
    Class.forName("com.squareup.okhttp.OkHttpClient");
    return OkHttpLoaderCreator.create(context);
  } catch (ClassNotFoundException ignored) {
  }
  return new UrlConnectionDownloader(context);
}

可以看到，先使用java反射机制查找是否含有OkHttpClient，没有的话就生成UrlConnectionDownloader。okhttp已改名okhttp3，这里肯定找不到的，所以我们download最后是UrlConnectionDownloader。

LruCache 这是个缓存相关的类

PicassoExecutorService 是个线程池

Stats 是用来保存图片相关状态信息

Dispatcher 是个控制的中心，控制线程的加载和取消、网络监听、消息处理等。这个里面参数有个HANDLER，是主线程的Handler，后面从子线程切换到主线程就是通过它实现。

最后调用了return new Picasso，我们看下源码，里面有一些关键信息

Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
    RequestTransformer requestTransformer, List<RequestHandler> 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<RequestHandler> allRequestHandlers =
      new ArrayList<RequestHandler>(builtInHandlers + extraCount);
  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);

这里我们要注意requestHandlers，后面BitmapHunter的forRequest方法中将会使用。我们这次是以网络请求为切入点，后面使用的requestHandler就是NetworkRequestHandler。

Picasso 里面还有个HANDLER，用来子线程切换主线程使用的，后面将使用。

总结下，Picasso主要就是各种初始化，以及以及一些工具api。

with就是使用Picasso前的做了一些配置准备。接下来看下load方法

load

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

他是直接返回一个RequestCreator对象，我们看看他是什么。

RequestCreator(Picasso picasso, Uri uri, int resourceId) {
  if (picasso.shutdown) {
    throw new IllegalStateException(
        "Picasso instance already shut down. Cannot submit new requests.");
  }
  this.picasso = picasso;
  this.data = new Request.Builder(uri, resourceId, picasso.defaultBitmapConfig);
}

Builder(Uri uri, int resourceId, Bitmap.Config bitmapConfig) {
  this.uri = uri;
  this.resourceId = resourceId;
  this.config = bitmapConfig;
}

我们可以看到RequestCreator，里面包含了picasso对象，还有图片相关配置等等。

load方法主要功能就是获取请求uri以及图片配置。

with和load，还比较简单，基本都是在请求前做一些准备工作。接下来看下into

into 源码

public void into(ImageView target, Callback callback) {
  long started = System.nanoTime();
  //-------------------------1-------------------
  checkMain();

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

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

  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());
      }
      picasso.defer(target, new DeferredRequestCreator(this, target, callback));
      return;
    }
    data.resize(width, height);
  }

  //------------------------------2------------------------------
  Request request = createRequest(started);
  String requestKey = createKey(request);

  if (shouldReadFromMemoryCache(memoryPolicy)) {
    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;
    }
  }

   //-----------------------------3-----------------------------
  if (setPlaceholder) {
    setPlaceholder(target, getPlaceholderDrawable());
  }
  
  Action action =
      new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
          errorDrawable, requestKey, tag, callback, noFade);

  picasso.enqueueAndSubmit(action);
}

这份代码主要分成三块。

第一块，各种检查，检查是不是在主线程上，检查图片是否有数据等等。

第二块，是判断是否从缓存获取图片，如果是，则从缓存获取图片，整个流程结束！

第三块，如果不从缓存获取图片，则准备请求网络。先看下是否设置placeholder，有的话设置placeholder，然后创建ImageViewAction，ImageViewAction整合了picasso、target、request等请求信息，同时还设置结果回调处理。最后调用了picasso.enqueueAndSubmit(action)发起请求。

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);
    targetToAction.put(target, action);
  }
  submit(action);
}
void submit(Action action) {
  dispatcher.dispatchSubmit(action);
}
void dispatchSubmit(Action action) {
  handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
}

上面我们可以发现，主要分两块。

第一块，是判断要不要把请求Action添加到targetToAction中。

第二块，是调用Dispatcher的dispatchSubmit，提交请求，最终是通过handler发送提交请求消息，发起请求。这里的handler，通过源码我们发现

this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);

这里的dispatcherThread是一个HandlerThread，那我们知道此时的this.handler是非UI线程的handler。我们继续看handler的消息是如何处理的。

@Override public void handleMessage(final Message msg) {
  switch (msg.what) {
    case REQUEST_SUBMIT: {
      Action action = (Action) msg.obj;
      dispatcher.performSubmit(action);
      break;
    }

最终是调用了

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

这里主要调用了forRequest，返回一个BitmapHunter，这个是什么？我们看源码，他是个Runnable。service是PicassoExecutorService是个线程池，那service.submit(hunter)就很容易理解啦，我们直接看BitmapHunter的run方法。

@Override public void run() {
  try {
    ...........................
    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);
  }
}

这份代码主要也是两块，

第一个是执行hunt()，应该是个请求函数，下面我们继续看。

第二个，就是不断的catch，然后通过dispatcher来处理。

我们先看下hunt()

Bitmap hunt() throws IOException {
  Bitmap bitmap = null;
  if (shouldReadFromMemoryCache(memoryPolicy)) {
    bitmap = cache.get(key);
    if (bitmap != null) {
      stats.dispatchCacheHit();
      loadedFrom = MEMORY;
      ...........
      return bitmap;
    }
  }

  data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
  RequestHandler.Result result = requestHandler.load(data, networkPolicy);
  if (result != null) {
    loadedFrom = result.getLoadedFrom();
    exifRotation = result.getExifOrientation();
    bitmap = result.getBitmap();
    if (bitmap == null) {
      InputStream is = result.getStream();
      try {
        bitmap = decodeStream(is, data);
      } finally {
        Utils.closeQuietly(is);
      }
    }
  }

  if (bitmap != null) {
    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;
}

我们可以看到，图片请求先判断是否从缓存取，如果需要，则从缓存拿数据，拿到数据后，则返回bitmap，结束。如果不从缓存拿数据，就通过requestHandler.load获取返回数据。

到目前为止，我们知道如何请求数据，接口调用必须在UI线程中，请求操作在handler子线程，请求处理我们是通过线程池来处理的。在这过程中，如果可以从缓存获取数据，就从缓存取，否则通过线程池请求图片，这个线程池能开几个线程，取决于当前网络情况，由于篇幅我这里没有写，好奇的可以自行看下代码。

好了，下面我们来继续看requestHandler.load，这个操作在线程池中执行的。这个requestHandler是哪里来的呢？这个参数是在实例BitmapHunter带过来的，我们看下是在哪里。

static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
    Action action) {
  Request request = action.getRequest();
  List<RequestHandler> requestHandlers = picasso.getRequestHandlers();
  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);
}

这里的picasso.getRequestHandlers()是在哪里生成的？这里是在Picasso实例化的时候

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));

每个RequestHandler是一个请求处理方式，有网络请求，有文件等等。。假如我们现在是从网络请求，那此时的requestHandler就是NetworkRequestHandler，我们看下其load方法

@Override public Result load(Request request, int networkPolicy) throws IOException {
  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;
  }
  if (loadedFrom == DISK && response.getContentLength() == 0) {
    Utils.closeQuietly(is);
    throw new ContentLengthException("Received response with 0 content-length header.");
  }
  if (loadedFrom == NETWORK && response.getContentLength() > 0) {
    stats.dispatchDownloadFinished(response.getContentLength());
  }
  return new Result(is, loadedFrom);
}

这里会把请求的数据包装在Result中

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

如果result！=null，那表示请求成功，我们这就看成功吧，其他状态处理基本类似。

继续看下 dispatcher.dispatchComplete(this);他会执行handler.sendMessage(handler.obtainMessage(HUNTER_COMPLETE, hunter));我们发现dispatcher处理都是在子线程中执行的。在子线程Handler中

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

继续执行dispatcher.performComplete(hunter);

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

请求成功的话，1.要不要缓存 2.把当前请求完成的hunter从hunterMap移除，不然假如后面再次网络请求这个相同图片的话，就不会处理了，原因可以看下performSubmit方法。3.执行batch

到这里，整个请求流程都处理完毕，处理数据Result也拿到。接下来就是把bitmap显示在屏幕上吧，也就是从子线程转移到UI主线程。

这个在哪执行的呢，就是在batch方法里

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);
  }
}

理解不难，这里的handler还是子线程。继续

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

继续

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

哦，看到了，这里出现了主线程中的mainThreadHandler，他项主线程MessageQueue发送了消息。我们找下mainThreadHandler。还记得创建Dispatcher时里面参数HANDLER吗？就是他。

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;
      }

HUNTER_BATCH_COMPLETE条件下，不断遍历所有BitmapHunter，继续看下picasso.complete(hunter)

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);
  }

  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);
  }
}

这里主要看下deliverAction

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);
  } else {
    action.error();
  }
}

这里主要看下action.complete(result, from);此时的action是谁，是ImageViewAction，那我们看下其complete

@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;
  PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);
  if (callback != null) {
    callback.onSuccess();
  }
}

这里应该是终结了，PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);把bitmap显示在target上了，同时调用了回调callback.onSuccess();

Picasso整个流程就是这样。看一两遍可能还是云里雾里，结合下面图看可能更快的理解。

原创声明：本文系作者授权腾讯云开发者社区发表，未经许可，不得转载。

如有侵权，请联系 cloudcommunity@tencent.com 删除。

android

原创声明：本文系作者授权腾讯云开发者社区发表，未经许可，不得转载。

如有侵权，请联系 cloudcommunity@tencent.com 删除。

android

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