前言

關(guān)于Glide的優(yōu)缺點(diǎn)和使用劳闹，這里不在闡述碾牌，具體請(qǐng)看GitHub上官方介紹曹阔。這里主要結(jié)合別人的文章和自己看源碼的一些見(jiàn)解茉盏，分享一下自己的理解鉴未。Glide龐大的圖片庫(kù)做的封裝性極好支持的格式也很多，開(kāi)發(fā)者的代碼設(shè)計(jì)嚴(yán)格遵守了軟件設(shè)計(jì)的六大原則鸠姨，一開(kāi)始看的時(shí)候感覺(jué)結(jié)構(gòu)好麻煩铜秆，好多接口，好多回調(diào)讶迁，后來(lái)細(xì)想了一下這些都是軟件設(shè)計(jì)的六大原則約束连茧，帶來(lái)的好處就是低耦合，易維護(hù)巍糯，等等很多優(yōu)點(diǎn)啸驯，我會(huì)在分析的時(shí)候講一下里邊設(shè)計(jì)的設(shè)計(jì)模式。

整個(gè)Glide的流程圖

如下是一張大概的流程圖

Glide時(shí)序圖.png

發(fā)現(xiàn)的問(wèn)題

自定義AppGlideModule中applyOptions方法設(shè)置默認(rèn)圖片格式PREFER_RGB_565無(wú)效祟峦，查看源碼會(huì)發(fā)現(xiàn)在glide在解碼圖片的時(shí)候罚斗，會(huì)根據(jù)解析圖片頭得到的圖片格式設(shè)置解碼格式：Png格式圖片設(shè)置為PREFER_ARGB_8888，jpg格式的圖片設(shè)置為PREFER_RGB_565宅楞。
-驗(yàn)證方法

@GlideModule
public class CustomGlideModule extends AppGlideModule {
    @Override
    public void applyOptions(@NonNull Context context, @NonNull GlideBuilder builder) {
          //設(shè)置格式為565  
          builder.setDefaultRequestOptions(RequestOptions.formatOf(DecodeFormat.PREFER_RGB_565));
    }
}

在MainActivity中通過(guò)如下方式打印一下圖片的格式

 Glide.with(this).asBitmap().addListener(new RequestListener<Bitmap>() {
            @Override
            public boolean onLoadFailed(@Nullable GlideException e, Object model, Target<Bitmap> target, boolean isFirstResource) {
                return false;
            }

            @Override
            public boolean onResourceReady(Bitmap resource, Object model, Target<Bitmap> target, DataSource dataSource, boolean isFirstResource) {
                Bitmap.Config config = resource.getConfig();
                Log.i("jawe", "onResourceReady: config="+config);
                return false;
            }
        }).load("http://d.lanrentuku.com/down/png/1904/international_food/fried_rice.png").into(imageView);

這里打印出來(lái)的config就是ARGB_8888
-查看源碼
具體在Downsample.java的calculateConfig方法內(nèi)

private void calculateConfig(
....){
....
...
   boolean hasAlpha = false;
    try {
      hasAlpha = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool).hasAlpha();
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(
            TAG,
            "Cannot determine whether the image has alpha or not from header"
                + ", format "
                + format,
            e);
      }
    }
    //這里根據(jù)是否有透明度設(shè)置解碼格式
    optionsWithScaling.inPreferredConfig =
        hasAlpha ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565;
    if (optionsWithScaling.inPreferredConfig == Config.RGB_565) {
      optionsWithScaling.inDither = true;
    }
}

主流程分析

一般的使用如下：
Glide.with(this).load("http://d.lanrentuku.com/down/png/1904/international_food/fried_rice.png") .apply(new RequestOptions().error(R.drawable.ic_launcher_background).placeholder (R.mipmap.ic_launcher).override(500, 500)).into(iv);
越是看起來(lái)很簡(jiǎn)單的代碼针姿，背后的封裝性很復(fù)雜，下面我們就一步步分析一下

Glide.with做的事情

 @NonNull
  public static RequestManager with(@NonNull FragmentActivity activity) {
    //1.getRetriever(activity)得到RequestManager檢索器
    //2.通過(guò)檢索器得到一個(gè)RequestManager
    return getRetriever(activity).get(activity);
  }
  @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.
    return Glide.get(context).getRequestManagerRetriever();
  }

其中Glide.get(context)得到的是Glide單例對(duì)象厌衙，然后getRequestManagerRetriever();得到檢索器

private static volatile Glide glide;
  public static Glide get(@NonNull Context context) {
    if (glide == null) {
      GeneratedAppGlideModule annotationGeneratedModule =
          getAnnotationGeneratedGlideModules(context.getApplicationContext());
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context, annotationGeneratedModule);
        }
      }
    }

    return glide;
  }

這里采用的雙重校驗(yàn)加鎖(DCL)的方式實(shí)現(xiàn)單例模式,其中g(shù)lide使用volatile修飾加入內(nèi)存屏障距淫，禁止指令重排序，使得其他線程的修改能被直接讀取最新值婶希，即高并發(fā)時(shí)候線程的可見(jiàn)性溉愁。
Glide在V4中推薦使用注解的方式配置Glide選項(xiàng)，Glide在創(chuàng)建Glide對(duì)象的時(shí)候會(huì)根據(jù)編譯期生成的RequestManagerRetriever.RequestManagerFactory設(shè)置Glide然后調(diào)用Glide glide = builder.build(applicationContext);創(chuàng)建Glide對(duì)象。

private static void initializeGlide(
      @NonNull Context context, @Nullable GeneratedAppGlideModule generatedAppGlideModule) {
    initializeGlide(context, new GlideBuilder(), generatedAppGlideModule);
  }
private static void initializeGlide(
      @NonNull Context context,
      @NonNull GlideBuilder builder,
      @Nullable GeneratedAppGlideModule annotationGeneratedModule) {
...
    RequestManagerRetriever.RequestManagerFactory factory =
        annotationGeneratedModule != null
            ? annotationGeneratedModule.getRequestManagerFactory()
            : null;
    builder.setRequestManagerFactory(factory);
...
    Glide glide = builder.build(applicationContext);
...
    Glide.glide = glide;

這里邊其中GlideBuilder內(nèi)部實(shí)現(xiàn)了很多默認(rèn)的Glide配置拐揭，我們一起看看

/** A builder class for setting default structural classes for Glide to use. */
@SuppressWarnings("PMD.ImmutableField")
public final class GlideBuilder {
  private final Map<Class<?>, TransitionOptions<?, ?>> defaultTransitionOptions = new ArrayMap<>();
  private Engine engine;//加載引擎
  private BitmapPool bitmapPool;//bitmap復(fù)用池
  private ArrayPool arrayPool;//array復(fù)用池
  private MemoryCache memoryCache;//內(nèi)存緩存
  private GlideExecutor sourceExecutor;//source線程池
  private GlideExecutor diskCacheExecutor;//磁盤緩存線程池
  private DiskCache.Factory diskCacheFactory;//磁盤緩存工廠
  private MemorySizeCalculator memorySizeCalculator;//緩存大小計(jì)算器
  private ConnectivityMonitorFactory connectivityMonitorFactory;
  private int logLevel = Log.INFO;
  private RequestOptionsFactory defaultRequestOptionsFactory =
      new RequestOptionsFactory() {
        @NonNull
        @Override
        public RequestOptions build() {
          return new RequestOptions();
        }
      };
  @Nullable private RequestManagerFactory requestManagerFactory;//requestManager工廠
  private GlideExecutor animationExecutor;
...
}

通過(guò)注釋可以很清楚的知道這個(gè)類的作用就是用來(lái)構(gòu)建Glide撤蟆，這里也可以看出使用了構(gòu)建者模式(builder),將復(fù)雜對(duì)象的表示和它的實(shí)現(xiàn)分離。
我們看一下build方法

 Glide build(@NonNull Context context) {
    //GlideExecutor創(chuàng)建線程池,這里我把GlideExecutor理解是一個(gè)工具類
    if (sourceExecutor == null) {
      sourceExecutor = GlideExecutor.newSourceExecutor();
    }
    ...
    ...
    //根據(jù)設(shè)備內(nèi)存大小計(jì)算最優(yōu)的內(nèi)存堂污，復(fù)用池家肯，磁盤緩存大小等
    if (memorySizeCalculator == null) {
      memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();
    }

   ......
    if (engine == null) {
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(),
              animationExecutor,
              isActiveResourceRetentionAllowed);
    }

    if (defaultRequestListeners == null) {
      defaultRequestListeners = Collections.emptyList();
    } else {
      defaultRequestListeners = Collections.unmodifiableList(defaultRequestListeners);
    }

    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptionsFactory,
        defaultTransitionOptions,
        defaultRequestListeners,
        isLoggingRequestOriginsEnabled,
        isImageDecoderEnabledForBitmaps,
        hardwareBitmapFdLimit,
        minHardwareDimension);
  }

這里體現(xiàn)了設(shè)計(jì)模式種的單一職責(zé)模式，builder里邊主要做的事情：
1.根據(jù)不同的任務(wù)特性結(jié)合設(shè)備的cpu等特征設(shè)置不同的線程池
2.根據(jù)設(shè)備的內(nèi)存和磁盤大小設(shè)置最優(yōu)的內(nèi)存和磁盤大小盟猖，以及構(gòu)建復(fù)用池
3.構(gòu)建加載引擎（Engine）
4.構(gòu)建Glide對(duì)象
我們接下來(lái)看一下Glide的構(gòu)造

Glide(
      @NonNull Context context,
      @NonNull Engine engine,
      @NonNull MemoryCache memoryCache,
      @NonNull BitmapPool bitmapPool,
      @NonNull ArrayPool arrayPool,
      @NonNull RequestManagerRetriever requestManagerRetriever,
      @NonNull ConnectivityMonitorFactory connectivityMonitorFactory,
      int logLevel,
      @NonNull RequestOptionsFactory defaultRequestOptionsFactory,
      @NonNull Map<Class<?>, TransitionOptions<?, ?>> defaultTransitionOptions,
      @NonNull List<RequestListener<Object>> defaultRequestListeners,
      boolean isLoggingRequestOriginsEnabled,
      boolean isImageDecoderEnabledForBitmaps,
      int hardwareBitmapFdLimit,
      int minHardwareDimension) {
...
 registry
        /** 編碼器 磁盤緩存使用的*/
        .append(ByteBuffer.class, new ByteBufferEncoder())
        .append(InputStream.class, new StreamEncoder(arrayPool))
        /* Bitmaps 注冊(cè)解碼器 使用byteBufferBitmapDecoder從ByteBuffer 解碼成bitmap */
        .append(Registry.BUCKET_BITMAP, ByteBuffer.class, Bitmap.class, byteBufferBitmapDecoder)
        .append(Registry.BUCKET_BITMAP, InputStream.class, Bitmap.class, streamBitmapDecoder)
        .append(
            Registry.BUCKET_BITMAP,
            ParcelFileDescriptor.class,
            Bitmap.class,
            parcelFileDescriptorVideoDecoder)
        .append(
            Registry.BUCKET_BITMAP,
            AssetFileDescriptor.class,
            Bitmap.class,
            VideoDecoder.asset(bitmapPool))
        .append(Bitmap.class, Bitmap.class, UnitModelLoader.Factory.<Bitmap>getInstance())
        .append(Registry.BUCKET_BITMAP, Bitmap.class, Bitmap.class, new UnitBitmapDecoder())
        /**注冊(cè) bitmap的編碼器 用于磁盤緩存*/
        .append(Bitmap.class, bitmapEncoder)
        /* BitmapDrawables */
        .append(
            Registry.BUCKET_BITMAP_DRAWABLE,
            ByteBuffer.class,
            BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, byteBufferBitmapDecoder))
        .append(
            Registry.BUCKET_BITMAP_DRAWABLE,
            InputStream.class,
            BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, streamBitmapDecoder))
        .append(
            Registry.BUCKET_BITMAP_DRAWABLE,
            ParcelFileDescriptor.class,
            BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, parcelFileDescriptorVideoDecoder))
        .append(BitmapDrawable.class, new BitmapDrawableEncoder(bitmapPool, bitmapEncoder))
        /* GIFs */
        .append(
            Registry.BUCKET_GIF,
            InputStream.class,
            GifDrawable.class,
            new StreamGifDecoder(imageHeaderParsers, byteBufferGifDecoder, arrayPool))
        .append(Registry.BUCKET_GIF, ByteBuffer.class, GifDrawable.class, byteBufferGifDecoder)
        .append(GifDrawable.class, new GifDrawableEncoder())
        /* GIF Frames */
        // Compilation with Gradle requires the type to be specified for UnitModelLoader here.
        .append(
            GifDecoder.class, GifDecoder.class, UnitModelLoader.Factory.<GifDecoder>getInstance())
        .append(
            Registry.BUCKET_BITMAP,
            GifDecoder.class,
            Bitmap.class,
            new GifFrameResourceDecoder(bitmapPool))
        /* Drawables */
        .append(Uri.class, Drawable.class, resourceDrawableDecoder)
        .append(
            Uri.class, Bitmap.class, new ResourceBitmapDecoder(resourceDrawableDecoder, bitmapPool))
        /* Files 文件解碼器 */
        .register(new ByteBufferRewinder.Factory())
        .append(File.class, ByteBuffer.class, new ByteBufferFileLoader.Factory())
        .append(File.class, InputStream.class, new FileLoader.StreamFactory())
        .append(File.class, File.class, new FileDecoder())
        .append(File.class, ParcelFileDescriptor.class, new FileLoader.FileDescriptorFactory())
        // Compilation with Gradle requires the type to be specified for UnitModelLoader here.
        .append(File.class, File.class, UnitModelLoader.Factory.<File>getInstance())
        /* Models 網(wǎng)絡(luò) */
        .register(new InputStreamRewinder.Factory(arrayPool))
        .append(int.class, InputStream.class, resourceLoaderStreamFactory)
        .append(int.class, ParcelFileDescriptor.class, resourceLoaderFileDescriptorFactory)
        .append(Integer.class, InputStream.class, resourceLoaderStreamFactory)
        .append(Integer.class, ParcelFileDescriptor.class, resourceLoaderFileDescriptorFactory)
        .append(Integer.class, Uri.class, resourceLoaderUriFactory)
        .append(int.class, AssetFileDescriptor.class, resourceLoaderAssetFileDescriptorFactory)
        .append(Integer.class, AssetFileDescriptor.class, resourceLoaderAssetFileDescriptorFactory)
        .append(int.class, Uri.class, resourceLoaderUriFactory)
        .append(String.class, InputStream.class, new DataUrlLoader.StreamFactory<String>())
        .append(Uri.class, InputStream.class, new DataUrlLoader.StreamFactory<Uri>())
        .append(String.class, InputStream.class, new StringLoader.StreamFactory())
        .append(String.class, ParcelFileDescriptor.class, new StringLoader.FileDescriptorFactory())
        .append(
            String.class, AssetFileDescriptor.class, new StringLoader.AssetFileDescriptorFactory())
        .append(Uri.class, InputStream.class, new HttpUriLoader.Factory())
        .append(Uri.class, InputStream.class, new AssetUriLoader.StreamFactory(context.getAssets()))
        .append(
            Uri.class,
            ParcelFileDescriptor.class,
            new AssetUriLoader.FileDescriptorFactory(context.getAssets()))
        .append(Uri.class, InputStream.class, new MediaStoreImageThumbLoader.Factory(context))
        .append(Uri.class, InputStream.class, new MediaStoreVideoThumbLoader.Factory(context))
        .append(Uri.class, InputStream.class, new UriLoader.StreamFactory(contentResolver))
        .append(
            Uri.class,
            ParcelFileDescriptor.class,
            new UriLoader.FileDescriptorFactory(contentResolver))
        .append(
            Uri.class,
            AssetFileDescriptor.class,
            new UriLoader.AssetFileDescriptorFactory(contentResolver))
        .append(Uri.class, InputStream.class, new UrlUriLoader.StreamFactory())
        .append(URL.class, InputStream.class, new UrlLoader.StreamFactory())
        .append(Uri.class, File.class, new MediaStoreFileLoader.Factory(context))
        .append(GlideUrl.class, InputStream.class, new HttpGlideUrlLoader.Factory())
        .append(byte[].class, ByteBuffer.class, new ByteArrayLoader.ByteBufferFactory())
        .append(byte[].class, InputStream.class, new ByteArrayLoader.StreamFactory())
        .append(Uri.class, Uri.class, UnitModelLoader.Factory.<Uri>getInstance())
        .append(Drawable.class, Drawable.class, UnitModelLoader.Factory.<Drawable>getInstance())
        .append(Drawable.class, Drawable.class, new UnitDrawableDecoder())
        /* Transcoders 轉(zhuǎn)碼器 */
        .register(Bitmap.class, BitmapDrawable.class, new BitmapDrawableTranscoder(resources))
        .register(Bitmap.class, byte[].class, bitmapBytesTranscoder)
        .register(
            Drawable.class,
            byte[].class,
            new DrawableBytesTranscoder(
                bitmapPool, bitmapBytesTranscoder, gifDrawableBytesTranscoder))
        .register(GifDrawable.class, byte[].class, gifDrawableBytesTranscoder);
...
 glideContext =
        new GlideContext(
            context,
            arrayPool,
            registry,
            imageViewTargetFactory,
            defaultRequestOptionsFactory,
            defaultTransitionOptions,
            defaultRequestListeners,
            engine,
            isLoggingRequestOriginsEnabled,
            logLevel);

構(gòu)造方法內(nèi)部核心就是把所需要的編碼器讨衣，解碼器，加載模型式镐，轉(zhuǎn)碼器都注冊(cè)到注冊(cè)表中反镇，以供后邊加載圖片 緩存圖片的時(shí)候使用。這里解釋一下Glide中的組件以及作用：
1.ModelLoader, 將自定義的 Model(Url,File, Uri,任意的 POJO )加載成 Data(InputStreams, FileDescriptors)娘汞。
2.ResourceDecoder, 將 Data 類型(InputStreams, FileDescriptors)進(jìn)行解碼成新的 Resources(Drawables, Bitmaps)
3.Encoder, 用于向 Glide 的磁盤緩存寫 Data (InputStreams, FileDesciptors)歹茶。
4.ResourceTranscoder，用于在不同的資源類型之間做轉(zhuǎn)換你弦，例如惊豺，從 BitmapResource 轉(zhuǎn)換為 DrawableResource 。
5.ResourceEncoder禽作，用于向 Glide 的磁盤緩存寫 Resources(BitmapResource, DrawableResource)尸昧。

RequestManagerRetriever#get(activity)

構(gòu)建RequestManger對(duì)象

 @NonNull
  public RequestManager get(@NonNull FragmentActivity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      FragmentManager fm = activity.getSupportFragmentManager();
      return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }

這里是在主線程中執(zhí)行的，所以執(zhí)行else條件

  @NonNull
    private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    SupportRequestManagerFragment current =
        getSupportRequestManagerFragment(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;
  }

這里可以看出RequestManager 綁定一個(gè)SupportRequestManagerFragment

  @NonNull
  private SupportRequestManagerFragment getSupportRequestManagerFragment(
      @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) {
    SupportRequestManagerFragment current =
        (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      current = pendingSupportRequestManagerFragments.get(fm);
      if (current == null) {
        current = new SupportRequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingSupportRequestManagerFragments.put(fm, current);
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

當(dāng)前的頁(yè)面綁定一個(gè)隱藏?zé)o界面的Fragment旷偿，來(lái)感知組件的生命周期烹俗，這樣做的目的是避免內(nèi)存泄漏，如果activity頁(yè)面中使用Glide加載一張圖片萍程，activity被用戶關(guān)掉了衷蜓，但是Glide單例中的ViewTarget還在引用ImageView對(duì)象，而ImageView對(duì)象又引用activity對(duì)象尘喝，那么這個(gè)activity對(duì)象就不會(huì)被回收掉，也就是內(nèi)存泄漏了斋陪。這里可以看出一個(gè)fragmentManager只有一個(gè)fragment朽褪。

注意這里有一個(gè)疑問(wèn)，為什么要用handler發(fā)送消息的形式移除fm无虚，這里看起來(lái)沒(méi)有線程切換缔赠，完全可以直接移除。關(guān)于這個(gè)問(wèn)題我問(wèn)了我的同事友题，同事的解釋是正確的嗤堰，因?yàn)閏ommitAllowingStateLoss 這個(gè)事務(wù)是異步執(zhí)行的，為了保證一個(gè)fragmentManager只有一個(gè)綁定的SupportRequestManagerFragment度宦，所以添加一個(gè)集合控制邏輯踢匣，當(dāng)?shù)谝淮翁峤皇聞?wù) 異步執(zhí)行的時(shí)候告匠，如果用戶接著第二次get的時(shí)候，findFragmentByTag是null（前邊的fragment還沒(méi)有掛載到activity上）這時(shí)候從集合中查到了离唬，就返回去后专。但是如果不從集合中移出去就會(huì)內(nèi)存泄漏，為了保證是在掛載成功后输莺，從集合中移掉fm戚哎，所以使用handler發(fā)送消息，利用handler的消息機(jī)制嫂用，消息隊(duì)列排隊(duì)取出消息處理的型凳，所以移除的動(dòng)作是在commit后執(zhí)行的。

RequestManager

• 介紹：RequestManager是Glide中管理和啟動(dòng)Request的一個(gè)類嘱函，可以感知組件生命周期甘畅，不同的周期采用不同的管理策略。
• 分析：前邊的分析看到RequestManager是通過(guò)工廠構(gòu)建產(chǎn)生的实夹，注解生成默認(rèn)的工廠是new方式創(chuàng)建對(duì)象

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

    connectivityMonitor =
        factory.build(
            context.getApplicationContext(),
            new RequestManagerConnectivityListener(requestTracker));

    // If we're the application level request manager, we may be created on a background thread.
    // In that case we cannot risk synchronously pausing or resuming requests, so we hack around the
    // issue by delaying adding ourselves as a lifecycle listener by posting to the main thread.
    // This should be entirely safe.
    if (Util.isOnBackgroundThread()) {
      mainHandler.post(addSelfToLifecycle);
    } else {
      lifecycle.addListener(this);//1
    }
    lifecycle.addListener(connectivityMonitor);

    defaultRequestListeners =
        new CopyOnWriteArrayList<>(glide.getGlideContext().getDefaultRequestListeners());
    setRequestOptions(glide.getGlideContext().getDefaultRequestOptions());

    glide.registerRequestManager(this);
  }

這里最重要的就是注釋1處橄浓，把requestManger當(dāng)前對(duì)象添加到lifecycle的監(jiān)聽(tīng)接口中，這個(gè)lifecycle就是前邊在構(gòu)建SupportRequestManagerFragment對(duì)象的時(shí)候構(gòu)建出來(lái)的ActivityFragmentLifecycle亮航，而ActivityFragmentLifecycle 就是一個(gè)觀察者荸实，觀察Fragment的生命周期，然后通知所有的訂閱者(LifecycleListener)缴淋，訂閱者其中就有RequestManager准给，看一下ActivityFragmentLifecycle

class ActivityFragmentLifecycle implements Lifecycle {
  private final Set<LifecycleListener> lifecycleListeners =
      Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
  private boolean isStarted;
  private boolean isDestroyed;
...
  @Override
  public void addListener(@NonNull LifecycleListener listener) {
    lifecycleListeners.add(listener);

    if (isDestroyed) {
      listener.onDestroy();
    } else if (isStarted) {
      listener.onStart();
    } else {
      listener.onStop();
    }
  }
...
}

其中addListener內(nèi)部就像一個(gè)粘性事件一樣，添加訂閱的時(shí)候重抖，能夠訂閱到訂閱前的信息露氮。

內(nèi)存管理

Android是一個(gè)內(nèi)存很敏感的應(yīng)用，當(dāng)系統(tǒng)內(nèi)存不足的時(shí)候钟沛，就會(huì)殺死一些進(jìn)程回收內(nèi)存畔规。為了讓應(yīng)用存活更久，Glide做了內(nèi)存優(yōu)化恨统，其中Glide 實(shí)現(xiàn)了ComponentCallbacks2接口并且在onTrimMemory(int level)中根據(jù)不同的級(jí)別做出相應(yīng)的內(nèi)存管理叁扫。

public void trimMemory(int level) {
    // Engine asserts this anyway when removing resources, fail faster and consistently
    Util.assertMainThread();
    // Request managers need to be trimmed before the caches and pools, in order for the latter to
    // have the most benefit.
    for (RequestManager manager : managers) {
      manager.onTrimMemory(level);
    }
    // memory cache needs to be trimmed before bitmap pool to trim re-pooled Bitmaps too. See #687.
    memoryCache.trimMemory(level);
    bitmapPool.trimMemory(level);
    arrayPool.trimMemory(level);
  }

總結(jié)

Glide.with(activity)采用的外觀設(shè)計(jì)模式，創(chuàng)建Request Manger對(duì)象 能夠根據(jù)組件生命周期的不同階段畜埋，來(lái)暫停莫绣，啟動(dòng)，停止請(qǐng)求悠鞍，請(qǐng)求的處理流程requestTrack中管理的对室。Glide.with的流程圖如下：

Glide.with.png