Picasso源码解析

我一般看源码都是从它的使用入手,从它的使用来遍历它的执行过程.Picasso的使用只需一行代码,所以从这行代码开始.
这行代码就不用我写了吧.反正肯定要进入load方法.进入load方法一看,还要进入RequestCreator 类.

RequestCreator类如下:
从这个变量Request.Builder data和它的一系列方法可以看出,这个类是对图片设置一些个人的请求,而这些请求被包装成一个Request类.这个类与一些其他的设置将一并传入一个类(这个类是啥了?).配置好之后一定会进入into方法.
方法流程如下:
1.createRequest,创建请求
2.createKey,创建请求键值
3.看缓存中是否有图片,有的话setBitmap
4.没有的话,就将request和一些其他的配置传入action类.
5.submit,对action这个任务进行执行.

此时我们进入Picasso类中看submit这个方法,发现最终要使用Dispatcher这个类.

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;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()) {      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 || target.isLayoutRequested()) {        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)) {      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) {      setPlaceholder(target, getPlaceholderDrawable());    }    Action action =        new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,            errorDrawable, requestKey, tag, callback, noFade);    picasso.enqueueAndSubmit(action);  }

Dispatcher类如下:
此类是一个调度器,对任务进行调度与分发.我们将action传入dispatchSubmit方法里,它将会将这个任务通过handler分发到handlerThread线程中执行,也就是执行performSubmit这个方法.下面是这个方法的执行流程:
1.hunterMap.get(action.getKey()),此操作从map中取出之前之前的BitmapHunter,如果没有,说明此任务是一个新的任务
2.forRequest(action.getPicasso(), this, cache, stats, action),创建请求
3.service.submit(hunter),调用executor执行线程.
4.hunterMap.put(action.getKey(), hunter),把已经执行的任务放入map中,避免重复操作.

在执行完submit后如果成功了,就要进入到performComplete中了,此方法中又将进入到batch方法中,batch是一个存储hunter的集合,它将hunter加入其中,而不是立即就将得到后的图片显示出来.再看它后面的这步操作handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);寻着它我们要进入到performBatchComplete方法,可以看到它将hunter集合通过mainHandler传到主线程中了.mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));此时我们回到Picasso类中找到这个mainHandler,可以看到这个变量:static final Handler HANDLER.在这个变量中有这个操作,hunter.picasso.complete(hunter);再次进入到Picasso中的complete方法,再进入到此方法的deliverAction方法中,你会方向这步操作action.complete(result, from);此时你到action的具体实现类找这个方法,它有一步操作PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);终于现身了,真是众里寻她千百度啊.

这其中涉及到几个重要的类BitmapHunter,Action,等.下面会进行详细的分析.

void dispatchSubmit(Action action) {    handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));}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);    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());    }  }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");    }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);    }  }void performBatchComplete() {    List<BitmapHunter> copy = new ArrayList<>(batch);    batch.clear();    mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));    logBatch(copy);  }private static class DispatcherHandler extends Handler {    private final Dispatcher dispatcher;    public DispatcherHandler(Looper looper, Dispatcher dispatcher) {      super(looper);      this.dispatcher = dispatcher;    }    @Override public void handleMessage(final Message msg) {      switch (msg.what) {        case REQUEST_SUBMIT: {          Action action = (Action) msg.obj;          dispatcher.performSubmit(action);          break;        }        case REQUEST_CANCEL: {          Action action = (Action) msg.obj;          dispatcher.performCancel(action);          break;        }        case TAG_PAUSE: {          Object tag = msg.obj;          dispatcher.performPauseTag(tag);          break;        }        case TAG_RESUME: {          Object tag = msg.obj;          dispatcher.performResumeTag(tag);          break;        }        case HUNTER_COMPLETE: {          BitmapHunter hunter = (BitmapHunter) msg.obj;          dispatcher.performComplete(hunter);          break;        }        case HUNTER_RETRY: {          BitmapHunter hunter = (BitmapHunter) msg.obj;          dispatcher.performRetry(hunter);          break;        }        case HUNTER_DECODE_FAILED: {          BitmapHunter hunter = (BitmapHunter) msg.obj;          dispatcher.performError(hunter, false);          break;        }        case HUNTER_DELAY_NEXT_BATCH: {          dispatcher.performBatchComplete();          break;        }        case NETWORK_STATE_CHANGE: {          NetworkInfo info = (NetworkInfo) msg.obj;          dispatcher.performNetworkStateChange(info);          break;        }        case AIRPLANE_MODE_CHANGE: {          dispatcher.performAirplaneModeChange(msg.arg1 == AIRPLANE_MODE_ON);          break;        }        default:          Picasso.HANDLER.post(new Runnable() {            @Override public void run() {              throw new AssertionError("Unknown handler message received: " + msg.what);            }          });      }    }  }

BitmapHunter类如下:
run()方法执行过程:
1.hunt(), 加载图片
2.加载为空,dispatchFailed,发送失败事件
3.得到图片,dispatchComplete,在上面的dispatcher类中分析过.

hunt()方法执行过程:
1.shouldReadFromMemoryCache(memoryPolicy),判断是否在缓存中.
2.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
不在缓存中,就到网上找呗.这里的RequestHandler类对不同uri进行解析,最后返回result.

@Override public void run() {    try {      updateThreadName(data);      if (picasso.loggingEnabled) {        log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));      }      result = hunt();      if (result == null) {        dispatcher.dispatchFailed(this);      } else {        dispatcher.dispatchComplete(this);      }    } 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);    }  }  Bitmap hunt() throws IOException {    Bitmap bitmap = null;    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;      }    }    networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;    RequestHandler.Result result = requestHandler.load(data, networkPolicy);    if (result != null) {      loadedFrom = result.getLoadedFrom();      exifOrientation = result.getExifOrientation();      bitmap = result.getBitmap();      // If there was no Bitmap then we need to decode it from the stream.      if (bitmap == null) {        Source source = result.getSource();        try {          bitmap = decodeStream(source, data);        } finally {          try {            //noinspection ConstantConditions If bitmap is null then source is guranteed non-null.            source.close();          } catch (IOException ignored) {          }        }      }    }    //........................    //........................    return bitmap;  }

Action类:
此类是一个接口,它定义了你获取一个图片应该要传的值,包括request,imageview,和一些其他的配置.它就像是一个你将要操作的行为一样,携带了你该要带的东西,只为最后获取到图片.这里边对target进行弱引用,便于gc回收.它的一个强大的地方就是使用泛型,你可以指定不同的view来显示图片.里边的俩个虚拟方法,留给特定的子类实现,任你对图片为所欲为.
abstract void complete(Bitmap result, Picasso.LoadedFrom from);
abstract void error(Exception e);

RequestHandler类:
此类是一个接口,定义了不同类型uri的加载方式,通过继承它来实现不同的加载.
NetWorkRequestHandler:
继承RequestHandler类的子类,对网络图片进行加载,Picasso内部的网络加载用的是OKhttp.它的ImageDownload的默认实现就是OKhttp3download.