一、Glide基本用法

 Glide
  .with(this)
  .load(path)
  .into(ivBg);

二、Glide 源码讲解
Glide源码分为三部分，with() , load(), into(),本文现讲述with部分。

  public static RequestManager with(@NonNull Activity activity) {
    return getRetriever(activity).get(activity);
  }
  @NonNull
  private static RequestManagerRetriever getRetriever(@Nullable Context context) {
    return Glide.get(context).getRequestManagerRetriever();
  }
  @NonNull
  public static Glide get(@NonNull Context context) {
    if (glide == null) {
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context);
        }
      }
    }

    return glide;
  }

看下GlideBuilder中初始化的一些类

 Glide build(@NonNull Context context) {
    //创建执行器，用于从数据源获取数据，例如网络请求
    if (sourceExecutor == null) {
      sourceExecutor = GlideExecutor.newSourceExecutor();
    }
    //创建执行器，用于从本地缓存获取数据
    if (diskCacheExecutor == null) {
      diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();
    }
    
    if (animationExecutor == null) {
      animationExecutor = GlideExecutor.newAnimationExecutor();
    }
    //根据当前机器参数计算需要的缓存大小
    if (memorySizeCalculator == null) {
      memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();
    }
  
    if (connectivityMonitorFactory == null) {
      connectivityMonitorFactory = new DefaultConnectivityMonitorFactory();
    }
    //创建bitmap池，用于回收LruCache缓存的图片，把图片会受到BitmapPool中，这样下次再创建图片的时候，可以复用，避免连续创建回收内存，造成内存抖动
    if (bitmapPool == null) {
      int size = memorySizeCalculator.getBitmapPoolSize();
      if (size > 0) {
        bitmapPool = new LruBitmapPool(size);
      } else {
        bitmapPool = new BitmapPoolAdapter();
      }
    }

    if (arrayPool == null) {
      arrayPool = new LruArrayPool(memorySizeCalculator.getArrayPoolSizeInBytes());
    }
    //创建内存缓存
    if (memoryCache == null) {
      memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
    }
  //创建磁盘缓存
    if (diskCacheFactory == null) {
      diskCacheFactory = new InternalCacheDiskCacheFactory(context);
    }
   //创建engin用于真正的处理request请求，如发起网络图片，从磁盘读取图片等
    if (engine == null) {
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(),
              GlideExecutor.newAnimationExecutor(),
              isActiveResourceRetentionAllowed);
    }

    if (defaultRequestListeners == null) {
      defaultRequestListeners = Collections.emptyList();
    } else {
      defaultRequestListeners = Collections.unmodifiableList(defaultRequestListeners);
    }
  //创建RequestManagerRetriever
    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptions.lock(),
        defaultTransitionOptions,
        defaultRequestListeners,
        isLoggingRequestOriginsEnabled);
  }

Glide创建

Glide() {
    this.engine = engine;
    this.bitmapPool = bitmapPool;
    this.arrayPool = arrayPool;
    this.memoryCache = memoryCache;
    this.requestManagerRetriever = requestManagerRetriever;
    this.connectivityMonitorFactory = connectivityMonitorFactory;

    DecodeFormat decodeFormat = defaultRequestOptions.getOptions().get(Downsampler.DECODE_FORMAT);
    bitmapPreFiller = new BitmapPreFiller(memoryCache, bitmapPool, decodeFormat);

    final Resources resources = context.getResources();

    registry = new Registry();
    registry.register(new DefaultImageHeaderParser());
 
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {
      registry.register(new ExifInterfaceImageHeaderParser());
    }

    List<ImageHeaderParser> imageHeaderParsers = registry.getImageHeaderParsers();
    Downsampler downsampler =
        new Downsampler(
            imageHeaderParsers,
            resources.getDisplayMetrics(),
            bitmapPool,
            arrayPool);
    ByteBufferGifDecoder byteBufferGifDecoder =
        new ByteBufferGifDecoder(context, imageHeaderParsers, bitmapPool, arrayPool);
    ResourceDecoder<ParcelFileDescriptor, Bitmap> parcelFileDescriptorVideoDecoder =
        VideoDecoder.parcel(bitmapPool);
    ByteBufferBitmapDecoder byteBufferBitmapDecoder = new ByteBufferBitmapDecoder(downsampler);
    StreamBitmapDecoder streamBitmapDecoder = new StreamBitmapDecoder(downsampler, arrayPool);
    ResourceDrawableDecoder resourceDrawableDecoder =
        new ResourceDrawableDecoder(context);
    ResourceLoader.StreamFactory resourceLoaderStreamFactory =
        new ResourceLoader.StreamFactory(resources);
    ResourceLoader.UriFactory resourceLoaderUriFactory =
        new ResourceLoader.UriFactory(resources);
    ResourceLoader.FileDescriptorFactory resourceLoaderFileDescriptorFactory =
        new ResourceLoader.FileDescriptorFactory(resources);
    ResourceLoader.AssetFileDescriptorFactory resourceLoaderAssetFileDescriptorFactory =
        new ResourceLoader.AssetFileDescriptorFactory(resources);
    BitmapEncoder bitmapEncoder = new BitmapEncoder(arrayPool);

    BitmapBytesTranscoder bitmapBytesTranscoder = new BitmapBytesTranscoder();
    GifDrawableBytesTranscoder gifDrawableBytesTranscoder = new GifDrawableBytesTranscoder();

    ContentResolver contentResolver = context.getContentResolver();

    registry
        .append(ByteBuffer.class, new ByteBufferEncoder())
        .append(InputStream.class, new StreamEncoder(arrayPool))
      ...
        .register(
            Drawable.class,
            byte[].class,
            new DrawableBytesTranscoder(
                bitmapPool, bitmapBytesTranscoder, gifDrawableBytesTranscoder))
        .register(GifDrawable.class, byte[].class, gifDrawableBytesTranscoder);

    ImageViewTargetFactory imageViewTargetFactory = new ImageViewTargetFactory();
    glideContext =
        new GlideContext(
            context,
            arrayPool,
            registry,
            imageViewTargetFactory,
            defaultRequestOptions,
            defaultTransitionOptions,
            defaultRequestListeners,
            engine,
            isLoggingRequestOriginsEnabled,
            logLevel);
  }

Glide创建代码，主要是对register对象调用append,register函数，设置一些参数，这些参数主要是对后边图片解析，加载
在Glide创建过程中，可以了解到在GlideBuilder中创建了RequestManagerRetriever，获取RequestManger对象需要调用RequestMangerRetriever.get方法

  @NonNull
  public RequestManager get(@NonNull Activity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      android.app.FragmentManager fm = activity.getFragmentManager();
      return fragmentGet(
          activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }
  @Deprecated
  @NonNull
  private RequestManager fragmentGet(@NonNull Context context,
      @NonNull android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint, isParentVisible);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }
@NonNull
  private RequestManagerFragment getRequestManagerFragment(
      @NonNull final android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      current = pendingRequestManagerFragments.get(fm);
      if (current == null) {
        current = new RequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingRequestManagerFragments.put(fm, current);
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

这几个方法主要是获取到RequestManagerFragment，再获取RequestManager. getRequestManagerFragment()方法中，获取或者创建RequestManagerFragment，也就是在 Activity中添加无视图的Fragment。获取到Fragment后，会获取Fragment中的RequestManager，如果RequestManager为NULL，就会通过factory进行创建。

创建方法中会将fragment中lifecycle传入到RequestManager中， 这里涉及到一个观察者模式。Fragment中有ActivityFragmentLifecycle对象，RequestManger实现LifecycleListener,在RequestManger中将自己添加到lifecycle的lifecycleListeners集合中，从而完成了图片加载框架与Activity 生命周期的绑定。，当lifecycle生命周期方法被执行人，他的所有listener也会执行对应的方法，也就是RequestManager对应的方法就会被执行

 @Override
  public synchronized void onStart() {
    resumeRequests();
    targetTracker.onStart();
  }

  @Override
  public synchronized void onStop() {
    pauseRequests();
    targetTracker.onStop();
  }

  @Override
  public synchronized void onDestroy() {
    targetTracker.onDestroy();
    for (Target<?> target : targetTracker.getAll()) {
      clear(target);
    }
    targetTracker.clear();
    requestTracker.clearRequests();
    lifecycle.removeListener(this);
    lifecycle.removeListener(connectivityMonitor);
    mainHandler.removeCallbacks(addSelfToLifecycle);
    glide.unregisterRequestManager(this);
  }

  /**
   * Attempts to always load the resource as a {@link android.graphics.Bitmap}, even if it could
   * actually be animated.
   *
   * @return A new request builder

最终会有这样的RequestTracker来对Request请求进行操作，了解一下RequestTracker

public class RequestTracker {
  private static final String TAG = "RequestTracker";
//保存当前所有Request请求集合
  private final Set<Request> requests =
      Collections.newSetFromMap(new WeakHashMap<Request, Boolean>());

  @SuppressWarnings("MismatchedQueryAndUpdateOfCollection")
  private final List<Request> pendingRequests = new ArrayList<>();
  private boolean isPaused;

  /**
   * Starts tracking the given request.
   */
  public void runRequest(@NonNull Request request) {
//添加到集合并执行请求
    requests.add(request);
    if (!isPaused) {
      request.begin();
    } else {
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Paused, delaying request");
      }
      pendingRequests.add(request);
    }
  }

  @VisibleForTesting
  void addRequest(Request request) {
    requests.add(request);
  }

  public boolean clearRemoveAndRecycle(@Nullable Request request) {
    return clearRemoveAndMaybeRecycle(request, /*isSafeToRecycle=*/ true);
  }

  private boolean clearRemoveAndMaybeRecycle(@Nullable Request request, boolean isSafeToRecycle) {
     if (request == null) {
      return true;
    }
    boolean isOwnedByUs = requests.remove(request);
    // Avoid short circuiting.
    isOwnedByUs = pendingRequests.remove(request) || isOwnedByUs;
    if (isOwnedByUs) {
      request.clear();
      if (isSafeToRecycle) {
        request.recycle();
      }
    }
    return isOwnedByUs;
  }

  /**
   * Returns {@code true} if requests are currently paused, and {@code false} otherwise.
   */
  public boolean isPaused() {
    return isPaused;
  }

  /**
   * Stops any in progress requests.
   */
  public void pauseRequests() {
    isPaused = true;
    for (Request request : Util.getSnapshot(requests)) {
      if (request.isRunning()) {
        request.clear();
        pendingRequests.add(request);
      }
    }
  }

  /** Stops any in progress requests and releases bitmaps associated with completed requests. */
  public void pauseAllRequests() {
    isPaused = true;
    for (Request request : Util.getSnapshot(requests)) {
      if (request.isRunning() || request.isComplete()) {
        request.clear();
        pendingRequests.add(request);
      }
    }
  }

  /**
   * Starts any not yet completed or failed requests.
   */
  public void resumeRequests() {
    isPaused = false;
    for (Request request : Util.getSnapshot(requests)) {
      if (!request.isComplete() && !request.isRunning()) {
        request.begin();
      }
    }
    pendingRequests.clear();
  }

  public void clearRequests() {
    for (Request request : Util.getSnapshot(requests)) {
      clearRemoveAndMaybeRecycle(request, /*isSafeToRecycle=*/ false);
    }
    pendingRequests.clear();
  }

  public void restartRequests() {
    for (Request request : Util.getSnapshot(requests)) {
      if (!request.isComplete() && !request.isCleared()) {
        request.clear();
        if (!isPaused) {
          request.begin();
        } else {
          // Ensure the request will be restarted in onResume.
          pendingRequests.add(request);
        }
      }
    }
  }

  @Override
  public String toString() {
    return super.toString() + "{numRequests=" + requests.size() + ", isPaused=" + isPaused + "}";
  }
}

以上代码可以了解到，RequestTracker对所有Request请求进行了保存，并且提供相应操作方法，如begin();
以上基本就是获取RequestManager整个流程。用一张图总结一下

YFEI9K386GEOJ09R3EUXAVV.png