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Glide通過生命週期取消加載

生命週期回調過程
onStop
--->RequestManager.onStop
-->RequestTracker.pauseRequest
--> SingleRequest.pause
--> SingleRequest.cancel
---> Engine.cancel
---> EngineJob.removeCallback
---> 如果所有的回調監聽都移除了
--> DecodeJob.cancel

//EngineJob.class
synchronized void removeCallback(ResourceCallback cb) {
   stateVerifier.throwIfRecycled();
   cbs.remove(cb);
   if (cbs.isEmpty()) {
     cancel();
     boolean isFinishedRunning = hasResource || hasLoadFailed;
     if (isFinishedRunning && pendingCallbacks.get() == 0) {
       release();
     }
   }
}

• 如果任務沒有執行，就從隊列裏移除，取消任務
• 如果任務已經執行，就利用isCancelled標記在停止流程

//DecodeJob.class
public void cancel() {
      isCancelled = true;
      DataFetcherGenerator local = currentGenerator;
      if (local != null) {
          local.cancel();
      }
}

上面最終會調用到具體的獲取圖片的任務，比如從網絡獲取圖片的HttpUrlFetcher

 //HttpUrlFetcher.class
private volatile boolean isCancelled;
@Override
public void cancel() {
    // TODO: we should consider disconnecting the url connection here, but we can't do so
    // directly because cancel is often called on the main thread.
    isCancelled = true;
}

//內部是通過HttpURLConnection來從網絡獲取數據
private InputStream loadDataWithRedirects(
      URL url, int redirects, URL lastUrl, Map<String, String> headers) throws HttpException {
    ...

    urlConnection = buildAndConfigureConnection(url, headers);

    try {
      // Connect explicitly to avoid errors in decoders if connection fails.
      urlConnection.connect();
      // Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
      stream = urlConnection.getInputStream();
    } catch (IOException e) {
      ...
    }

    if (isCancelled) {
      return null;
    }

    final int statusCode = getHttpStatusCodeOrInvalid(urlConnection);
    if (isHttpOk(statusCode)) {
      return getStreamForSuccessfulRequest(urlConnection);
    } else if (isHttpRedirect(statusCode)) {
      String redirectUrlString = urlConnection.getHeaderField(REDIRECT_HEADER_FIELD);
      ...
      cleanup();
      return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
    } else if (statusCode == INVALID_STATUS_CODE) {
      throw new HttpException(statusCode);
    } else {
      ...
    }
}

由於cancel方法一般是在主線程調用的，所以這裏採用的是volatile關鍵字這種方式，在正式獲取網絡數據時會進行攔截；

• 如果攔截到了，那直接返回null；如果沒攔截到，就獲取到數據
• 以上最終都會回調到DecodeJob的onDataFetcherReady方法

onDataFetcherReady ---> decodeFromRetrievedData ---> decodeFromData

private void decodeFromRetrievedData() {
     ...
     Resource<R> resource = null;
     try {
       resource = decodeFromData(currentFetcher, currentData, currentDataSource);
     } catch (GlideException e) {
       ...
     }
     if (resource != null) {
       notifyEncodeAndRelease(resource, currentDataSource, isLoadingFromAlternateCacheKey);
     } else {
       //任務被攔截，嘗試其他的加載方式
       runGenerators();
     }
}

decodeFromData中會進行判斷，如果data為Null就直接返回Null（被攔截時會是null）,這個時候會執行runGenerators方法

runGenerators方法實際上就是加載流程的流轉，比如先從文件中加載，文件中沒有，就去網絡加載，一個個去試這樣。當然這裏面肯定也有cancel攔截的

private void runGenerators() {
    currentThread = Thread.currentThread();
    startFetchTime = LogTime.getLogTime();
    boolean isStarted = false;
    
    //被cancel攔截就不會嘗試其他加載方式，直接任務取消
    
    while (!isCancelled
        && currentGenerator != null
        && !(isStarted = currentGenerator.startNext())) {
      stage = getNextStage(stage);
      currentGenerator = getNextGenerator();

      if (stage == Stage.SOURCE) {
        reschedule();
        return;
      }
    }
    // We've run out of stages and generators, give up.
    if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
      notifyFailed();
    }
}

所以當加載數據的流程被攔截了，比如網絡請求返回null，那到這裏這個流程就攔截下來了，直接

notifyFailed任務失敗了。

那如果網絡加載的時候沒有攔截住呢？data就不會為null，就會走notifyEncodeAndRelease方法，最終會一直走到EngineJob的notifyCallbacksOfResult方法，這個方法裏面又會有cancel攔截。這樣也就是數據加載前加載後都被攔截下來了

Glide中的線程池

自定義ThreadFactory，主要的一個功能是實現限制部分線程網絡訪問，利用的是嚴苛模式

@Override
public Thread newThread(@NonNull final Runnable runnable) {
   Thread newThread =
       delegate.newThread(
           new Runnable() {
             @Override
             public void run() {
               if (preventNetworkOperations) {
                 StrictMode.setThreadPolicy(
                     new ThreadPolicy.Builder().detectNetwork().penaltyDeath().build());
               }
               try {
                 runnable.run();
               } catch (Throwable t) {
                 uncaughtThrowableStrategy.handle(t);
               }
             }
           });
   newThread.setName("glide-" + name + "-thread-" + threadNum.getAndIncrement());
   return newThread;
}

線程池主要有這幾種：

• AnimationExecutor：加載動畫相關，禁止訪問網絡；如果CPU核心數大於4，就是2個線程，否則是一個線程，核心線程數和最大線程數相同
• diskCacheExecutor：從磁盤加載圖片，禁止訪問網絡；線程數為1，核心線程數和最大線程數相同
• sourceExecutor：可以訪問網絡，線程數跟CPU核心數有關，不大於4，核心線程數和最大線程數相同
• newUnlimitedSourceExecutor：用於網絡請求圖片，沒有核心線程，只有非核心線程，類似CacheThreadPoll

Glide做了線程池優化，核心線程也會遵循超時策略

public GlideExecutor build() {
  if (TextUtils.isEmpty(name)) {
    throw new IllegalArgumentException(
        "Name must be non-null and non-empty, but given: " + name);
  }
  ThreadPoolExecutor executor =
      new ThreadPoolExecutor(
          corePoolSize,
          maximumPoolSize,
          /*keepAliveTime=*/ threadTimeoutMillis,
          TimeUnit.MILLISECONDS,
          new PriorityBlockingQueue<Runnable>(),
          new DefaultThreadFactory(
              threadFactory, name, uncaughtThrowableStrategy, preventNetworkOperations));

  if (threadTimeoutMillis != NO_THREAD_TIMEOUT) {
    executor.allowCoreThreadTimeOut(true);
  }

  return new GlideExecutor(executor);
}

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