前置准备条件：

implementation 'com.github.bumptech.glide:glide:4.4.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.4.0'
implementation 'jp.wasabeef:glide-transformations:2.0.2'//图形变换依赖库

初步使用

RequestOptions options = new RequestOptions()
        .placeholder(R.drawable.ic_launcher_background)
        .error(R.drawable.error)
        .diskCacheStrategy(DiskCacheStrategy.NONE);
        .override(200, 100);
Glide.with(this)
     .load(url)
     .apply(options)
     .into(imageView);

使用方式很简单，接下来我们来一步步分析它的加载流程。

(一) Glide.with

  @NonNull
  public static RequestManager with(@NonNull Context context) {
    return getRetriever(context).get(context);
  }
  @NonNull
  public static RequestManager with(@NonNull Activity activity) {
    return getRetriever(activity).get(activity);
  }
  @NonNull
  public static RequestManager with(@NonNull FragmentActivity activity) {
    return getRetriever(activity).get(activity);
  }
  @NonNull
  public static RequestManager with(@NonNull Fragment fragment) {
    return getRetriever(fragment.getActivity()).get(fragment);
  }
 @NonNull
  public static RequestManager with(@NonNull android.app.Fragment fragment) {
    return getRetriever(fragment.getActivity()).get(fragment);
  }
  @NonNull
  public static RequestManager with(@NonNull View view) {
    return getRetriever(view.getContext()).get(view);
  }

可以看出Glide.with有六个实现的方法，分别对应不同的使用场景，这里就不多说了，主要看getRetriever()方法。

@NonNull
  private static RequestManagerRetriever getRetriever(@Nullable Context context) {
    // Context could be null for other reasons (ie the user passes in null), but in practice it will
    // only occur due to errors with the Fragment lifecycle.
    Preconditions.checkNotNull(
        context,
        "You cannot start a load on a not yet attached View or a Fragment where getActivity() "
            + "returns null (which usually occurs when getActivity() is called before the Fragment "
            + "is attached or after the Fragment is destroyed).");
    return Glide.get(context).getRequestManagerRetriever();
  }

其实内部调用了getRequestManagerRetriever()这个方法，获取一个requestManagerRetriever的对象。在这个方法里面，Glide调用了get方法完成了初始化的操作。

  @NonNull
  public static Glide get(@NonNull Context context) {
    if (glide == null) {
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context);
        }
      }
    }

    return glide;
  }

 private static void initializeGlide(@NonNull Context context, @NonNull GlideBuilder builder) {
    Context applicationContext = context.getApplicationContext();
    ***
    在此处根据annotationGeneratedModule获取 RequestManagerFactory 

    RequestManagerRetriever.RequestManagerFactory factory =
        annotationGeneratedModule != null
            ? annotationGeneratedModule.getRequestManagerFactory() : null;
    builder.setRequestManagerFactory(factory);
    
    if (annotationGeneratedModule != null) {
      annotationGeneratedModule.applyOptions(applicationContext, builder);
    }
    Glide glide = builder.build(applicationContext);
    ***
    Glide.glide = glide;
  }

那么Glide又是通过GlideBuilder生成的，我们来看一下内部是如何生成的。

@NonNull
  Glide build(@NonNull Context context) {
    // 1. 线程池
    if (sourceExecutor == null) {
      sourceExecutor = GlideExecutor.newSourceExecutor();
    }
    if (diskCacheExecutor == null) {
      diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();
    }
    if (animationExecutor == null) {
      animationExecutor = GlideExecutor.newAnimationExecutor();
    }
    // 2. 内存的缓存策略
    if (memorySizeCalculator == null) {
      memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();
    }
    //3.监听网络变化，里面注册监听事件
    if (connectivityMonitorFactory == null) {
      connectivityMonitorFactory = new DefaultConnectivityMonitorFactory();
    }
    // 4.Bitmap 复用池
    if (bitmapPool == null) {
      int size = memorySizeCalculator.getBitmapPoolSize();
      if (size > 0) {
        bitmapPool = new LruBitmapPool(size);
      } else {
        bitmapPool = new BitmapPoolAdapter();
      }
    }
    // 5. 数组 复用池
    if (arrayPool == null) {
      arrayPool = new LruArrayPool(memorySizeCalculator.getArrayPoolSizeInBytes());
    }
   // 6. 内存计算，磁盘缓存的工厂
    if (memoryCache == null) {
      memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
    }
    if (diskCacheFactory == null) {
      diskCacheFactory = new InternalCacheDiskCacheFactory(context);
    }
   // 7. 构建了一个负责管理线程池与缓存的执行引擎
    if (engine == null) {
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(),
              GlideExecutor.newAnimationExecutor(),
              isActiveResourceRetentionAllowed);
    }

   ***
     // 8. 创建RequestManagerRetriever 
    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory);
     // 9. 创建了 Glide 对象
    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptions.lock(),
        defaultTransitionOptions,
        defaultRequestListeners,
        isLoggingRequestOriginsEnabled);
  }
}