版本4.9.0

问题

• Glide如何实现与生命周期的绑定？
• Glide如何实现缓存？
• Glide如何实现图片压缩?

Glide如何实现与生命周期的绑定?

创建RequestManger,将其与with()传入 Activity, Fragment的生命周期绑定,
这样做的好处是当Activity/Fragment stop/destroy时，RequestManager也会做相应操作，如停掉图片加载

• 绑定Application Context

  首先无论with()传入的是什么，只要是在子线程中调用,创建的RequestManger与 Application Context绑定，这样创建的RequestMangager的生命周期与

  if (Util.isOnBackgroundThread()) {
          return get(fragment.getActivity().getApplicationContext());
       } else {
        ... ...
      }

  这样做的目的是防止Activity,Fragment内存泄漏

• Activity与FramgentActivity

  class RequestManagerFragment {
        ... ...
        private final ActivityFragmentLifecycle lifecycle;
       
        @Override
        public void onStart() {
          super.onStart();
          lifecycle.onStart();
        }
        @Override
        public void onStop() {
          super.onStop();
          lifecycle.onStop();
        }
  
        @Override
        public void onDestroy() {
          super.onDestroy();
          lifecycle.onDestroy();
          unregisterFragmentWithRoot();
        }
        ... ...
      }

  class ActivityFragmentLifecycle implements Lifecycle {
        @Override
        public void addListener(@NonNull LifecycleListener listener) {
          lifecycleListeners.add(listener);
  
          if (isDestroyed) {
            listener.onDestroy();
          } else if (isStarted) {
            listener.onStart();
          } else {
            listener.onStop();
          }
        }
        void onStart() {
          isStarted = true;
          for (LifecycleListener lifecycleListener :             Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStart();
          }
        }
  
        void onStop() {
          isStarted = false;
          for (LifecycleListener lifecycleListener :     Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStop();
          }
        }
  
        void onDestroy() {
          isDestroyed = true;
          for (LifecycleListener lifecycleListener :     Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onDestroy();
          }
        }
      }

  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);
      }

  RequestManagerFragment中创建了并对外提供ActivityFragmentLifecycle对象,
  创建RequestManager时，传入ActivityFragmentLifecycle对象

  RequestManager(
        Glide glide,
        Lifecycle lifecycle,
        RequestManagerTreeNode treeNode,
        RequestTracker requestTracker,
        ConnectivityMonitorFactory factory,
        Context context) {
  
          ... ...      
          lifecycle.addListener(this); 
          ... ...
      }
  
      @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);
      }

  这样RequestManger.onStart(),onStop(),onDestroy()与Activity的生命周期通过Activity绑定的空Fragment实现了绑定

• Fragment
  与Activity的绑定方式类似，

  FragmentManager fm = fragment.getChildFragmentManager();

  将RequestManger.onStart(),onStop(),onDestroy()与Fragment的生命周期通过Fragment绑定的空Fragment实现的绑定

• View
  通过View可以获取它所在的Activity 或 Fragment

  Activity activity = findActivity(view.getContext());
  
    @Nullable
    private Activity findActivity(@NonNull Context context) {
      if (context instanceof Activity) {
        return (Activity) context;
      } else if (context instanceof ContextWrapper) {
        return findActivity(((ContextWrapper) context).getBaseContext());
      } else {
        return null;
      }
    }
    
    @Nullable
    private Fragment findSupportFragment(@NonNull View target, @NonNull   
          FragmentActivity activity) {
          
      tempViewToSupportFragment.clear();
      findAllSupportFragmentsWithViews(
          activity.getSupportFragmentManager().getFragments(), tempViewToSupportFragment);
      Fragment result = null;
      View activityRoot = activity.findViewById(android.R.id.content);
      View current = target;
      while (!current.equals(activityRoot)) {
        result = tempViewToSupportFragment.get(current);
        if (result != null) {
          break;
        }
        if (current.getParent() instanceof View) {
          current = (View) current.getParent();
        } else {
          break;
        }
      }
  
      tempViewToSupportFragment.clear();
      return result;
    }

• Context
  通过Context获取Activity or FragmentActivity or Application Context

  if (context == null) {
        throw new IllegalArgumentException("You cannot start a load on a   nullContext");
      } else if (Util.isOnMainThread() && !(context instanceof Application)){
        if (context instanceof FragmentActivity) {
          return get((FragmentActivity) context);
        } else if (context instanceof Activity) {
          return get((Activity) context);
        } else if (context instanceof ContextWrapper) {
          return get(((ContextWrapper) context).getBaseContext());
        }
      }
      return getApplicationManager(context);

Glide如何实现缓存？

• 提供了两个内存缓存,分别存储强弱引用
  弱引用的缓存: 存放正在使用的
  强引用的缓存: 存放没有使用的

  Map<Key, ResourceWeakReference> activeEngineResources//在
      private final Map<T, Y> cache = new LinkedHashMap<>(100, 0.75f, true);//在LruCache类中

  查找内存缓存，

  1.先从弱引用的缓存查，
  2.没有再从强引用的缓存查，查到后从强引用缓存中移除，加入到弱引用的缓存

  Engine.load()方法

  EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
      if (active != null) {
        cb.onResourceReady(active, DataSource.MEMORY_CACHE);
        if (VERBOSE_IS_LOGGABLE) {
          logWithTimeAndKey("Loaded resource from active resources", startTime, key);
        }
        return null;
      }
      EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);

  Engine.loadFromCache()方法

  private EngineResource<?> loadFromCache(Key key, boolean isMemoryCacheable) {
      if (!isMemoryCacheable) {
        return null;
      }
  
      EngineResource<?> cached = getEngineResourceFromCache(key); //
      if (cached != null) {
        cached.acquire();
        activeResources.activate(key, cached);
      }
      return cached;
    }
    
    private EngineResource<?> getEngineResourceFromCache(Key key) {
      Resource<?> cached = cache.remove(key);
      final EngineResource<?> result;
      if (cached == null) {
        result = null;
      } else if (cached instanceof EngineResource) {
        // Save an object allocation if we've cached an EngineResource (the typical case).
        result = (EngineResource<?>) cached;
      } else {
        result = new EngineResource<>(cached, true /*isMemoryCacheable*/, true /*isRecyclable*/);
      }
      return result;
    }

Glide如何实现图片压缩?

Glide实现的等比压缩，保持原图长宽比例，主要是通过原图宽高和预设的宽高设置 BitmapFactory.Options.inSampleSize

float widthPercentage = requestedWidth / (float) sourceWidth;
    float heightPercentage = requestedHeight / (float) sourceHeight;
    exactScaleFactor = Math.min(widthPercentage, heightPercentage);
    int outWidth = round(exactScaleFactor * sourceWidth);
    int outHeight = round(exactScaleFactor * sourceHeight);

    int widthScaleFactor = sourceWidth / outWidth;
    int heightScaleFactor = sourceHeight / outHeight;

    int scaleFactor = rounding == SampleSizeRounding.MEMORY
        ? Math.max(widthScaleFactor, heightScaleFactor)
        : Math.min(widthScaleFactor, heightScaleFactor);
    
    int powerOfTwoSampleSize;
    // BitmapFactory does not support downsampling wbmp files on platforms <= M. See b/27305903.
    if (Build.VERSION.SDK_INT <= 23
        && NO_DOWNSAMPLE_PRE_N_MIME_TYPES.contains(options.outMimeType)) {
      powerOfTwoSampleSize = 1;
    } else {
      powerOfTwoSampleSize = Math.max(1, Integer.highestOneBit(scaleFactor));
      if (rounding == SampleSizeRounding.MEMORY
          && powerOfTwoSampleSize < (1.f / exactScaleFactor)) {
        powerOfTwoSampleSize = powerOfTwoSampleSize << 1;
      }
    }
    options.inSampleSize = powerOfTwoSampleSize;