Android | 《看完不忘系列》之Glide

時間 2020-07-20

原文原文鏈接

《看完不忘系列》將以從樹幹到細枝的思路來分析一些技術框架，本文是開篇文章，將對開源項目Glide圖片加載庫進行介紹。若是老鐵們看完仍是忘了，就 ~~回來揍我一頓~~ 點贊收藏加關注，多看兩遍~java

概覽

基於Glide最新版本4.11.0，未遷AndroidX的項目只能使用4.9.0，簡單使用：android

引入依賴，app/build.gradle：git

implementation 'com.github.bumptech.glide:glide:4.11.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.11.0' 複製代碼

一句代碼，完成圖片加載：github

Glide.with(this) //指定上下文，能夠是app、activity、fragment
 .load(url) //網絡圖片地址  .into(img); //用於展現的imageView 複製代碼

用起來簡潔優雅，而後咱們先大體預覽下Glide的一些職能，web

樹幹：核心流程

以Glide.with(this).load(url).into(img)爲起點，拆成with購車、load上牌、into發車三個環節來分析。面試

with：購車

簡單來講，with的功能就是根據傳入的上下文context來獲取圖片請求管理器RequestManager，他用來管理和啓動圖片請求，緩存

context能夠傳入app、activity、fragment，這決定了圖片請求的生命週期。一般是使用粒度較細的context，即便用當前頁面的context而不是全局的app。這樣作的好處是，打開一個頁面開啓圖片加載，而後退出頁面，圖片請求就會跟隨頁面銷燬而被取消，而不是繼續加載而浪費資源。網絡

當context是app時，經過RequestManagerRetriever得到的RequestManager是一個全局單例，這類圖片請求的生命週期將會跟隨整個app，app

class RequestManagerRetriever implements Handler.Callback {
 volatile RequestManager applicationManager;   RequestManager getApplicationManager(Context context) {  //雙重檢查鎖，獲取單例的RequestManager  if (applicationManager == null) {  synchronized (this) {  if (applicationManager == null) {  Glide glide = Glide.get(context.getApplicationContext());  applicationManager = factory.build(glide,new ApplicationLifecycle(),  new EmptyRequestManagerTreeNode(),context.getApplicationContext());  }  }  }  //返回應用級別的RequestManager單例  return applicationManager;  } } 複製代碼

當context是Activity時，建立一個SupportRequestManagerFragment（無界面的空fragment）添加到Activity，從而感知Activity的生命週期，同時建立RequestManager給空fragment持有，框架

class RequestManagerRetriever implements Handler.Callback {
 RequestManager supportFragmentGet(Context context,FragmentManager fm,  Fragment parentHint,boolean isParentVisible) {  //獲取空fragment，無則建立  SupportRequestManagerFragment current =  getSupportRequestManagerFragment(fm, parentHint, isParentVisible);  RequestManager requestManager = current.getRequestManager();  if (requestManager == null) {  Glide glide = Glide.get(context);  //若是空fragment沒有RequestManager，就建立一個  requestManager = factory.build(glide, current.getGlideLifecycle(),  current.getRequestManagerTreeNode(), context);  //讓空fragment持有RequestManager  current.setRequestManager(requestManager);  }  //返回頁面級別的RequestManager  return requestManager;  } } 複製代碼

而後看到getSupportRequestManagerFragment方法，

class RequestManagerRetriever implements Handler.Callback {
 Map<FragmentManager, SupportRequestManagerFragment> pendingSupportRequestManagerFragments =  new HashMap<>();   SupportRequestManagerFragment getSupportRequestManagerFragment(  final FragmentManager fm, Fragment parentHint, boolean isParentVisible) {  //經過tag找到Activity中的空fragment  SupportRequestManagerFragment current =  (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);  if (current == null) {  //findFragmentByTag沒找到空fragment，有多是延遲問題？再從Map中找一下  current = pendingSupportRequestManagerFragments.get(fm);  if (current == null) {  //確實沒有空fragment，就建立一個  current = new SupportRequestManagerFragment();  //...  //緩存進Map  pendingSupportRequestManagerFragments.put(fm, current);  //空fragment添加到Activity，使其能感知Activity的生命週期  fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();  //...  }  }  return current;  } } 複製代碼

綜上，經過with操做，

當context是app時，獲得應用級別RequestManager全局單例；

當context是Activity時，每一個頁面都會被添加一個空fragment，由空fragment持有頁面級別RequestManager。

注意：若是with發生在子線程，無論context是誰，都返回應用級別RequestManager單例。

發散：添加空fragment來感知頁面生命週期的思想，在Lifecycle的實現中也能夠看到，見ReportFragment的injectIfNeededIn方法。（不過這個方法在Lifecycle的2.2.0版本中有所改動，Android 10開始的設備改爲了使用Application.ActivityLifecycleCallbacks來感知，感興趣能夠康康）

至此，咱們根據存款context買到了心儀的車RequestManager，下面開始上牌~

load：上牌

load方法獲得了一個RequestBuilder圖片請求構建器，見名知意猜一下，是用來建立圖片請求的，

class RequestManager  implements ComponentCallbacks2, LifecycleListener, ModelTypes<RequestBuilder<Drawable>> {   RequestBuilder<Drawable> load(String string) {  return asDrawable().load(string);  }   RequestBuilder<Drawable> asDrawable() {  //須要加載的類型爲Drawable  return as(Drawable.class);  }   <ResourceType> RequestBuilder<ResourceType> as(Class<ResourceType> resourceClass) {  //建立一個請求構建器  return new RequestBuilder<>(glide, this, resourceClass, context);  } } 複製代碼

而後跟進asDrawable().load(string)，

class RequestBuilder<TranscodeType> extends BaseRequestOptions<RequestBuilder<TranscodeType>>  implements Cloneable, ModelTypes<RequestBuilder<TranscodeType>> {   RequestBuilder<TranscodeType> load(String string) {  return loadGeneric(string);  }   RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {  //只是簡單地賦值  this.model = model;  isModelSet = true;  return this;  } } 複製代碼

到這裏，咱們就完成了上牌，獲得了一個RequestBuilder圖片請求構建器。沒錯，上牌就這麼簡單，畢竟搖號已經夠艱難了對吧？

into：發車

階段一

來到into了，

class RequestBuilder<TranscodeType> extends BaseRequestOptions<RequestBuilder<TranscodeType>>  implements Cloneable, ModelTypes<RequestBuilder<TranscodeType>> {   ViewTarget<ImageView, TranscodeType> into(ImageView view) {  //...  BaseRequestOptions<?> requestOptions = this;  if (!requestOptions.isTransformationSet()  && requestOptions.isTransformationAllowed()  && view.getScaleType() != null) {  //根據ImageView的ScaleType，來配置參數  switch (view.getScaleType()) {  case CENTER_CROP:  requestOptions = requestOptions.clone().optionalCenterCrop();  break;  case CENTER_INSIDE:  requestOptions = requestOptions.clone().optionalCenterInside();  break;  //...  }  }  return into(  //前面提到，Target是展現圖片的載體，這裏他封裝了ImageView  glideContext.buildImageViewTarget(view, transcodeClass),null,  requestOptions,Executors.mainThreadExecutor());  } } 複製代碼

繼續跟進into重載方法，

class RequestBuilder<TranscodeType> extends BaseRequestOptions<RequestBuilder<TranscodeType>>  implements Cloneable, ModelTypes<RequestBuilder<TranscodeType>> {   <Y extends Target<TranscodeType>> Y into(Y target,RequestListener<TranscodeType> targetListener,  BaseRequestOptions<?> options,Executor callbackExecutor) {  //...  //建立圖片請求  Request request = buildRequest(target, targetListener, options, callbackExecutor);  //獲取Target載體已有的請求  Request previous = target.getRequest();  //若是兩個請求等效，而且xxx（先忽略）  if (request.isEquivalentTo(previous)  && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {  if (!Preconditions.checkNotNull(previous).isRunning()) {  //啓動異步請求  previous.begin();  }  return target;  }  requestManager.clear(target);  //圖片載體綁定圖片請求，即imageView setTag爲request  target.setRequest(request);  //啓動異步請求  requestManager.track(target, request);  return target;  } } 複製代碼

跟進requestManager.track(target, request)，

//RequestManager.java
void track(Target<?> target,Request request) {  targetTracker.track(target);  requestTracker.runRequest(request); }  //RequestTracker.java void runRequest(Request request) {  requests.add(request);  if (!isPaused) {  //開啓圖片請求  request.begin();  } else {  request.clear();  //若是處於暫停狀態，就把請求存起來，晚些處理  pendingRequests.add(request);  } } 複製代碼

到這裏，就啓動了圖片請求，

階段二

那麼，接下來重點關注的就是request.begin()了，

class SingleRequest<R> implements Request, SizeReadyCallback, ResourceCallback {
  void begin() {  synchronized (requestLock) {  //...  if (Util.isValidDimensions(overrideWidth, overrideHeight)) {  //若是已經有了明確的尺寸，開始加載  onSizeReady(overrideWidth, overrideHeight);  } else {  //沒有的話先去獲取尺寸，最終仍是走onSizeReady  target.getSize(this);  }  //...  }  }   void onSizeReady(int width, int height) {  synchronized (requestLock) {  //...  //engine.load，傳了不少參數  loadStatus =  engine.load(glideContext,model,  requestOptions.getSignature(),  this.width,this.height,  requestOptions.getResourceClass(),  transcodeClass,priority,  requestOptions.getDiskCacheStrategy(),  //...  this,callbackExecutor);  //...  }  } } 複製代碼

跟進engine.load，

class Engine implements EngineJobListener,MemoryCache.ResourceRemovedListener, EngineResource.ResourceListener {   <R> LoadStatus load(  GlideContext glideContext,  //...  Executor callbackExecutor) {  //...  EngineResource<?> memoryResource;  synchronized (this) {  //從內存加載  memoryResource = loadFromMemory(key, isMemoryCacheable, startTime);  if (memoryResource == null) {  //若是內存裏沒有緩存，則加載  return waitForExistingOrStartNewJob(  glideContext,  model,  //...  key,  startTime);  }  }  cb.onResourceReady(memoryResource, DataSource.MEMORY_CACHE);  return null;  }   <R> LoadStatus waitForExistingOrStartNewJob(...) {  //...  EngineJob<R> engineJob =engineJobFactory.build(...);  DecodeJob<R> decodeJob =decodeJobFactory.build(...);  jobs.put(key, engineJob);  //添加回調，這個cb就是SingleRequest本身，todo1  engineJob.addCallback(cb, callbackExecutor);  //engineJob開啓decodeJob  engineJob.start(decodeJob);  return new LoadStatus(cb, engineJob);  } } 複製代碼

DecodeJob是一個Runable，看看他的run方法，調用鏈以下：

DecodeJob.run -> DecodeJob.runWrapped -> DecodeJob.runGenerators ->

SourceGenerator.startNext -> SourceGenerator.startNextLoad ->

MultiModelLoader#MultiFetcher.loadData ->

HttpUrlFetcher.loadData

來到HttpUrlFetcher，

class HttpUrlFetcher implements DataFetcher<InputStream> {
  void loadData(Priority priority, DataCallback<? super InputStream> callback) {  try {  //獲取輸入流，沒有引入okhttp，則使用HttpURLConnection  InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());  //回調出去  callback.onDataReady(result);  } catch (IOException e) {  callback.onLoadFailed(e);  } finally {  }  } } 複製代碼

到這裏，網絡請求就完成了，下面看看圖片是怎麼設置上去的，前邊留了個todo1，

//添加回調，這個cb就是SingleRequest本身，todo1

callback就是SingleRequest，被回調前有一些對象包裝、解碼操做，暫不深究，來到SingleRequest，

class SingleRequest<R> implements Request, SizeReadyCallback, ResourceCallback {
  void onResourceReady(Resource<?> resource, DataSource dataSource) {  Resource<?> toRelease = null;  try {  synchronized (requestLock) {  //...  Object received = resource.get();  //...  //這裏  onResourceReady((Resource<R>) resource, (R) received, dataSource);  }  } finally {  if (toRelease != null) {  engine.release(toRelease);  }  }  }   void onResourceReady(Resource<R> resource, R result, DataSource dataSource) {  //...  try {  //...  //這裏，回調給Target載體  target.onResourceReady(result, animation);  } finally {  }  notifyLoadSuccess();  } } 複製代碼

跟進target.onResourceReady，最終來到DrawableImageViewTarget，

class DrawableImageViewTarget extends ImageViewTarget<Drawable> {
 void setResource(Drawable resource) {  //ImageView設置圖片  view.setImageDrawable(resource);  } } 複製代碼

結合階段一和二，

終於，汽車發動起來了~

把with購車、load上牌、into發車三個環節彙總起來，

能夠看到，整個圖片加載過程，就是with獲得RequestManager，load獲得RequestBuilder，而後into開啓加載：

建立Request、開啓Engine、運行DecodeJob線程、HttpUrlFetcher加載網絡數據、回調給載體Target、載體爲ImageView設置展現圖片。

細枝：補充

線程切換

在子線程下載完圖片後，如何回調主線程設置圖片？在Executors類裏，

private static final Executor MAIN_THREAD_EXECUTOR = new Executor() {
 //主線程Handler  private final Handler handler = new Handler(Looper.getMainLooper());   @Override  public void execute(Runnable command) {  //切回主線程  handler.post(command);  } }; 複製代碼

調用棧以下：

線程池

GlideBuilder類裏會初始化一些線程池：

Glide build(Context context) {
 private GlideExecutor sourceExecutor; //加載圖片  private GlideExecutor diskCacheExecutor; //管理磁盤緩存  private GlideExecutor animationExecutor; //管理動畫 } 複製代碼