在上篇Glide文章中，我們介紹了Glide圖片加載框架的使用，通過(guò)之前的學(xué)習(xí)，我們可能已經(jīng)能熟練的將Glide圖片加載框架運(yùn)用到我們的項(xiàng)目中，但是如果有人問(wèn)你它是如何加載，工作原理是怎樣的？為什么自定義GlideModule只需要在Manifest文件中加入meta-data即可？等等很多加載流程以及使用的注意事項(xiàng)。當(dāng)然要想搞明白這些問(wèn)題，就需要我們對(duì)Glide源碼有個(gè)大致的認(rèn)識(shí)，去剖析源碼深處的奧秘。
接下來(lái)就讓我們一起去進(jìn)入Glide的源碼世界，本篇文章分析的是Glide 3.7.0版本。特別提醒，閱讀本片文章之前要對(duì)Glide的用法要先有一個(gè)了解，可以先閱讀上篇文章，詳談高大上的圖片加載框架Glide -應(yīng)用篇。
此篇文章是自己學(xué)習(xí)的一個(gè)記錄，若對(duì)閱讀文章的你有一定幫助，很是高興，當(dāng)然文章如有不足或者錯(cuò)誤的地方，歡迎指正，避免我給其他讀者錯(cuò)誤引導(dǎo)。
如果你閱讀過(guò)上篇文章，或者你使用過(guò)Glide，就知道Glide加載圖片的最簡(jiǎn)單方式就是
Glide.with(context).load(url). placeholder(R.drawable.placeholder).into(imageView)。
那么這篇文章就以這句簡(jiǎn)單的代碼為主線，逐步深入Glide的源碼。
Glide.with(context)
//獲取RequestManager對(duì)象，該類實(shí)現(xiàn)了LifeCycleListener接口，綁定Activity/Fragment生命周期，對(duì)請(qǐng)求進(jìn)行暫停，恢復(fù)，清除操作
public static RequestManager with(Context context) {
//得到RequestManagerRetriever實(shí)例，該類注意將RequestManager和自定義Fragment（如RequestManagerFragment，SupportRequestManagerFragment）綁定，從而實(shí)現(xiàn)在生命周期管理回調(diào)
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(context);
}
Glide有四個(gè)靜態(tài)的重載方法with()，其內(nèi)部都通過(guò)RequestManagerRetriever相應(yīng)的get重載方法獲取一個(gè)RequestManager對(duì)象。RequestManagerRetriever提供各種重載方法的好處就是可以將Glide的加載請(qǐng)求與Activity/Fragment的生命周期綁定而自動(dòng)執(zhí)行請(qǐng)求，暫停操作。
接下來(lái)我們拿Activity參數(shù)分析Glide請(qǐng)求如何和綁定生命周期自動(dòng)請(qǐng)求，暫停，以及銷毀。
public RequestManager get(Activity activity) {
if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
return get(activity.getApplicationContext());
} else {
//判斷activity是否已經(jīng)是銷毀狀態(tài)
assertNotDestroyed(activity);
//獲取FragmentManager 對(duì)象
android.app.FragmentManager fm = activity.getFragmentManager();
//創(chuàng)建Fragment，RequestManager并將其綁定
return fragmentGet(activity, fm);
}
}
assertNotDestroyed主要斷言Activity是否已經(jīng)Destroyed。若是沒(méi)有銷毀，或者Activity的FragmentManager ，然后通過(guò)fragmentGet返回RequestManager。
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
RequestManager fragmentGet(Context context, android.app.FragmentManager fm) {
//*獲取RequestManagerFragment，主要利用Frament進(jìn)行請(qǐng)求的生命周期管理
RequestManagerFragment current = getRequestManagerFragment(fm);
RequestManager requestManager = current.getRequestManager();
//requestManager 為空，即首次加載初始化requestManager ，并調(diào)用setRequestManager設(shè)置到RequestManagerFragment
if (requestManager == null) {
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
//獲取Fragment對(duì)象
RequestManagerFragment getRequestManagerFragment(final android.app.FragmentManager fm) {
RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
current = pendingRequestManagerFragments.get(fm);
if (current == null) {
current = new RequestManagerFragment();
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
最終通過(guò)getRequestManagerFragment（）方法獲取一個(gè)RequestManagerFragment 對(duì)象。
public class RequestManagerFragment extends Fragment {
private final ActivityFragmentLifecycle lifecycle;
//省略部分代碼...
@Override
public void onStart() {
super.onStart();
//關(guān)聯(lián)lifecycle相應(yīng)onStart方法
lifecycle.onStart();
}
@Override
public void onStop() {
super.onStop();
//關(guān)聯(lián)lifecycle相應(yīng)onStop方法
lifecycle.onStop();
}
@Override
public void onDestroy() {
super.onDestroy();
//關(guān)聯(lián)lifecycle相應(yīng)onDestroy方法
lifecycle.onDestroy();
}
}
此時(shí)我們看到RequestManagerFragment 繼承了Fragment.并且在其生命周期onStart(),onStop(),onDestory()，調(diào)用了ActivityFragmentLifecycle 相應(yīng)的方法，ActivityFragmentLifecycle實(shí)現(xiàn)了Lifecycle 接口，在其中通過(guò)addListener（LifecycleListener listener）回調(diào)相應(yīng)（LifecycleListener的 onStart(),onStop(),onDestory()）周期方法。LifecycleListener是監(jiān)聽(tīng)生命周期時(shí)間接口。
再次回到fragmentGet方法里下面一句代碼
//創(chuàng)建RequestManager傳入Lifecycle實(shí)現(xiàn)類，如ActivityFragmentLifecycle
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
對(duì)于RequestManager類，該類實(shí)現(xiàn)了LifecycleListener，如下代碼
/**
* A class for managing and starting requests for Glide. Can use activity, fragment and connectivity lifecycle events to
* intelligently stop, start, and restart requests. Retrieve either by instantiating a new object, or to take advantage
* built in Activity and Fragment lifecycle handling, use the static Glide.load methods with your Fragment or Activity.
*/
public class RequestManager implements LifecycleListener {
//An interface for listening to Activity/Fragment lifecycle events.
private final Lifecycle lifecycle;
public RequestManager(Context context, Lifecycle lifecycle, RequestManagerTreeNode treeNode) {
this(context, lifecycle, treeNode, new RequestTracker(), new ConnectivityMonitorFactory());
}
RequestManager(Context context, final Lifecycle lifecycle, RequestManagerTreeNode treeNode,
RequestTracker requestTracker, ConnectivityMonitorFactory factory) {
this.context = context.getApplicationContext();
this.lifecycle = lifecycle;
this.treeNode = treeNode;
//A class for tracking, canceling, and restarting in progress, completed, and failed requests.
this.requestTracker = requestTracker;
//通過(guò)Glide的靜態(tài)方法獲取Glide實(shí)例。單例模式
this.glide = Glide.get(context);
this.optionsApplier = new OptionsApplier();
//通過(guò)工廠類ConnectivityMonitorFactory的build方法獲取ConnectivityMonitor （一個(gè)用于監(jiān)控網(wǎng)絡(luò)連接事件的接口）
ConnectivityMonitor connectivityMonitor = factory.build(context,
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()) {
new Handler(Looper.getMainLooper()).post(new Runnable() {
@Override
public void run() {
lifecycle.addListener(RequestManager.this);
}
});
} else {
//設(shè)置監(jiān)聽(tīng)
lifecycle.addListener(this);
}
lifecycle.addListener(connectivityMonitor);
}
/**
* Lifecycle callback that registers for connectivity events (if the android.permission.ACCESS_NETWORK_STATE
* permission is present) and restarts failed or paused requests.
*/
@Override
public void onStart() {
// onStart might not be called because this object may be created after the fragment/activity's onStart method.
resumeRequests();
}
/**
* Lifecycle callback that unregisters for connectivity events (if the android.permission.ACCESS_NETWORK_STATE
* permission is present) and pauses in progress loads.
*/
@Override
public void onStop() {
pauseRequests();
}
/**
* Lifecycle callback that cancels all in progress requests and clears and recycles resources for all completed
* requests.
*/
@Override
public void onDestroy() {
requestTracker.clearRequests();
}
}
它將剛創(chuàng)建的fragment的lifeCycle傳入，并將RequestManager這個(gè)listener添加到lifeCycle中，從而實(shí)現(xiàn)綁定。在RequestManager的構(gòu)造方法里看到了requestTracker，該對(duì)象就是跟蹤請(qǐng)求取消，重啟，完成，失敗。RequestManagerFragment 主要是用來(lái)連接生命周期方法，RequestManager用來(lái)實(shí)現(xiàn)生命周期中請(qǐng)求方法，而RequestManagerRetriever綁定了RequestManager。
GlideModule實(shí)現(xiàn)
在構(gòu)造方法中還初始化了通過(guò)Glide.get(context);初始化了Glide對(duì)象
/**
* Get the singleton.
*
* @return the singleton
*/
public static Glide get(Context context) {
if (glide == null) {
//同步Glide
synchronized (Glide.class) {
if (glide == null) {
Context applicationContext = context.getApplicationContext();
//解析清單文件配置的自定義GlideModule的metadata標(biāo)簽，返回一個(gè)GlideModule集合
List<GlideModule> modules = new ManifestParser(applicationContext).parse();
GlideBuilder builder = new GlideBuilder(applicationContext);
//循環(huán)集合，執(zhí)行GlideModule 實(shí)現(xiàn)類中的方法
for (GlideModule module : modules) {
module.applyOptions(applicationContext, builder);
}
glide = builder.createGlide();
for (GlideModule module : modules) {
//注冊(cè)組件
module.registerComponents(applicationContext, glide);
}
}
}
}
return glide;
}
通過(guò)get方法單例方式獲取實(shí)例，并在初始化時(shí)實(shí)現(xiàn)了GlideModule配置功能。具體怎么實(shí)現(xiàn)的呢？接下來(lái)具體分析一下，在初始化時(shí)new 了一個(gè)ManifestParser對(duì)象并且調(diào)用了parse（）方法返回一個(gè)GlideModule類型的List.
//解析metadata具體實(shí)現(xiàn)
public List<GlideModule> parse() {
List<GlideModule> modules = new ArrayList<GlideModule>();
try {
//通過(guò)PackageManager獲取metadata所有信息
ApplicationInfo appInfo = context.getPackageManager().getApplicationInfo(
context.getPackageName(), PackageManager.GET_META_DATA);
//清單文件含有metadata
if (appInfo.metaData != null) {
//通過(guò)key遍歷metadata（對(duì)于GlideModule，key就是GlideModule的實(shí)現(xiàn)類的全路徑類名）
for (String key : appInfo.metaData.keySet()) {
//過(guò)濾key對(duì)應(yīng)的value等于GLIDE_MODULE_VALUE（字符串GlideModule）
if (GLIDE_MODULE_VALUE.equals(appInfo.metaData.get(key))) {
//符合條件加入集合中
modules.add(parseModule(key));
}
}
}
} catch (PackageManager.NameNotFoundException e) {
throw new RuntimeException("Unable to find metadata to parse GlideModules", e);
}
return modules;
}
在parse（）方法中通過(guò)getApplicationInfo方法獲取metaData信息，若有metaData數(shù)據(jù)（appInfo.metaData != null），如果metaData的值為GlideModule則調(diào)用parseModule（key），方法返回GlideModule并add到返回的List中。
查看parseModule(String className)方法
//通過(guò)反射獲取GlideModule實(shí)例
private static GlideModule parseModule(String className) {
Class<?> clazz;
try {
clazz = Class.forName(className);
} catch (ClassNotFoundException e) {
throw new IllegalArgumentException("Unable to find GlideModule implementation", e);
}
Object module;
try {
module = clazz.newInstance();
} catch (InstantiationException e) {
throw new RuntimeException("Unable to instantiate GlideModule implementation for " + clazz, e);
} catch (IllegalAccessException e) {
throw new RuntimeException("Unable to instantiate GlideModule implementation for " + clazz, e);
}
if (!(module instanceof GlideModule)) {
throw new RuntimeException("Expected instanceof GlideModule, but found: " + module);
}
return (GlideModule) module;
}
到此我們看到通過(guò)反射的方式獲取我們?cè)谇鍐挝募新暶鞯淖远x的GlideModule對(duì)象。在獲取到
GlideModule集合之后，遍歷了集合并調(diào)用相應(yīng)的applyOptions和registerComponents方法，而Glide對(duì)象的生成是通過(guò)GlideBuilder的createGlide方法創(chuàng)建。
Glide createGlide() {
if (sourceService == null) {
final int cores = Math.max(1, Runtime.getRuntime().availableProcessors());
//初始化線程池
sourceService = new FifoPriorityThreadPoolExecutor(cores);
}
if (diskCacheService == null) {
diskCacheService = new FifoPriorityThreadPoolExecutor(1);
}
MemorySizeCalculator calculator = new MemorySizeCalculator(context);
//設(shè)置Bitmap池
if (bitmapPool == null) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
int size = calculator.getBitmapPoolSize();
bitmapPool = new LruBitmapPool(size);
} else {
bitmapPool = new BitmapPoolAdapter();
}
}
if (memoryCache == null) {
memoryCache = new LruResourceCache(calculator.getMemoryCacheSize());
}
if (diskCacheFactory == null) {
//內(nèi)部磁盤緩存
diskCacheFactory = new InternalCacheDiskCacheFactory(context);
}
if (engine == null) {
//初始化引擎類
engine = new Engine(memoryCache, diskCacheFactory, diskCacheService, sourceService);
}
if (decodeFormat == null) {
decodeFormat = DecodeFormat.DEFAULT;
}
return new Glide(engine, memoryCache, bitmapPool, context, decodeFormat);
}
看到這都是做的一些初始化操作，并將參數(shù)傳遞到Glide構(gòu)造方法。對(duì)于Glide構(gòu)造方法做的都是一些默認(rèn)的初始化操作，可以自己去查看源碼，此處不再貼出。
通過(guò)上面的分析，你就會(huì)理解，為什么之前提到配置信息只需要實(shí)現(xiàn)GlideModule接口，重寫其中的方法，并再清單文件配置metaData，并且metaData的key是自定義GlideModule的全路徑名，value值必須是GlideModule.會(huì)明白當(dāng)我們不想讓自定義的GlideModule生效時(shí)只需要?jiǎng)h除相應(yīng)的GlideModule。當(dāng)使用了混淆時(shí)為什么要配置...
-keep public class * implements com.bumptech.glide.module.GlideModule
requestManager.load##
對(duì)于load方法也是可以接收String，Url，Integer等類型的重載方法，在這里，我們拿String類型參數(shù)分析。
public DrawableTypeRequest<String> load(String string) {
return (DrawableTypeRequest<String>) fromString().load(string);
}
public DrawableTypeRequest<String> fromString() {
return loadGeneric(String.class);
}
private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
// 省略一段代碼
return optionsApplier.apply(
// 創(chuàng)建DrawableTypeRequest，它是GenericRequestBuilder的子類
new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
glide, requestTracker, lifecycle, optionsApplier));
}
@Override
public DrawableRequestBuilder<ModelType> load(ModelType model) {
//調(diào)用弗雷loadd方法
super.load(model);
return this;
}
返回的是DrawableTypeRequest對(duì)象，DrawableTypeRequest繼承關(guān)系如下
這里寫圖片描述
，而對(duì)于DrawableRequestBuilder類使用的是一個(gè)創(chuàng)建者模式，對(duì)于常用函數(shù)placeholder（），error（），transform等設(shè)置都是在此設(shè)置，
/**
* {@inheritDoc}
*/
@Override
public DrawableRequestBuilder<ModelType> placeholder(Drawable drawable) {
super.placeholder(drawable);
return this;
}
我們看到最終又調(diào)用了父類方法
/**
* Sets an Android resource id for a {@link android.graphics.drawable.Drawable} resourceto display while a resource
* is loading.
*
* @param resourceId The id of the resource to use as a placeholder
* @return This request builder.
*/
public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> placeholder(
int resourceId) {
this.placeholderId = resourceId;
return this;
}
通過(guò)查看父類（GenericRequestBuilder）源碼你會(huì)發(fā)現(xiàn)我們每次調(diào)用placeholder（），error()的等這些方法，其實(shí)都是給該類中的變量賦值。
經(jīng)過(guò)一系列操作后，最終調(diào)用into(imageView)方法來(lái)完成圖片的最終加載
創(chuàng)建請(qǐng)求
/**
* Sets the {@link ImageView} the resource will be loaded into, cancels any existing loads into the view, and frees
* any resources Glide may have previously loaded into the view so they may be reused.
*
* @see Glide#clear(android.view.View)
*
* @param view The view to cancel previous loads for and load the new resource into.
* @return The {@link com.bumptech.glide.request.target.Target} used to wrap the given {@link ImageView}.
*/
public Target<TranscodeType> into(ImageView view) {
Util.assertMainThread();
if (view == null) {
throw new IllegalArgumentException("You must pass in a non null View");
}
if (!isTransformationSet && view.getScaleType() != null) {
switch (view.getScaleType()) {
case CENTER_CROP:
applyCenterCrop();
break;
case FIT_CENTER:
case FIT_START:
case FIT_END:
applyFitCenter();
break;
//$CASES-OMITTED$
default:
// Do nothing.
}
}
return into(glide.buildImageViewTarget(view, transcodeClass));
}
由上面看到最終調(diào)用的into方法是
/**
* Set the target the resource will be loaded into.
*
* @see Glide#clear(com.bumptech.glide.request.target.Target)
*
* @param target The target to load the resource into.
* @return The given target.
*/
public <Y extends Target<TranscodeType>> Y into(Y target) {
Util.assertMainThread();
if (target == null) {
throw new IllegalArgumentException("You must pass in a non null Target");
}
if (!isModelSet) {
throw new IllegalArgumentException("You must first set a model (try #load())");
}
//獲取Request 對(duì)象
Request previous = target.getRequest();
//requestTracker是請(qǐng)求跟蹤類對(duì)象，主要管理請(qǐng)求的發(fā)起，暫停，清除
if (previous != null) {
previous.clear();
requestTracker.removeRequest(previous);
previous.recycle();
}
//創(chuàng)建請(qǐng)求對(duì)象
Request request = buildRequest(target);
target.setRequest(request);
//將target加入lifecycle
lifecycle.addListener(target);
//執(zhí)行請(qǐng)求
requestTracker.runRequest(request);
return target;
}
上面都執(zhí)行都調(diào)用了 Util.assertMainThread();判斷只能在主線程中執(zhí)行。（更新View當(dāng)然需要在主線程），在Glide中Target我們可以理解成View,只是Glide對(duì)我們的View做了一層封裝。
之后通過(guò)buildRequest創(chuàng)建請(qǐng)求對(duì)象。
//創(chuàng)建請(qǐng)求對(duì)象
private Request buildRequest(Target<TranscodeType> target) {
if (priority == null) {
//默認(rèn)加載優(yōu)先級(jí) NORMAL
priority = Priority.NORMAL;
}
//創(chuàng)建Request
return buildRequestRecursive(target, null);
}
private Request buildRequestRecursive(Target<TranscodeType> target, ThumbnailRequestCoordinator parentCoordinator) {
if (thumbnailRequestBuilder != null) {
if (isThumbnailBuilt) {
throw new IllegalStateException("You cannot use a request as both the main request and a thumbnail, "
+ "consider using clone() on the request(s) passed to thumbnail()");
}
// Recursive case: contains a potentially recursive thumbnail request builder.
if (thumbnailRequestBuilder.animationFactory.equals(NoAnimation.getFactory())) {
thumbnailRequestBuilder.animationFactory = animationFactory;
}
if (thumbnailRequestBuilder.priority == null) {
thumbnailRequestBuilder.priority = getThumbnailPriority();
}
if (Util.isValidDimensions(overrideWidth, overrideHeight)
&& !Util.isValidDimensions(thumbnailRequestBuilder.overrideWidth,
thumbnailRequestBuilder.overrideHeight)) {
thumbnailRequestBuilder.override(overrideWidth, overrideHeight);
}
ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
Request fullRequest = obtainRequest(target, sizeMultiplier, priority, coordinator);
// Guard against infinite recursion.
isThumbnailBuilt = true;
// Recursively generate thumbnail requests.
Request thumbRequest = thumbnailRequestBuilder.buildRequestRecursive(target, coordinator);
isThumbnailBuilt = false;
coordinator.setRequests(fullRequest, thumbRequest);
return coordinator;
} else if (thumbSizeMultiplier != null) {
// Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse.
ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
Request fullRequest = obtainRequest(target, sizeMultiplier, priority, coordinator);
Request thumbnailRequest = obtainRequest(target, thumbSizeMultiplier, getThumbnailPriority(), coordinator);
coordinator.setRequests(fullRequest, thumbnailRequest);
return coordinator;
} else {
// Base case: no thumbnail.
return obtainRequest(target, sizeMultiplier, priority, parentCoordinator);
}
}
最后調(diào)用obtainRequest方法
private Request obtainRequest(Target<TranscodeType> target, float sizeMultiplier, Priority priority,
RequestCoordinator requestCoordinator) {
return GenericRequest.obtain(
loadProvider,
model,
signature,
context,
priority,
target,
sizeMultiplier,
placeholderDrawable,
placeholderId,
errorPlaceholder,
errorId,
fallbackDrawable,
fallbackResource,
requestListener,
requestCoordinator,
glide.getEngine(),
transformation,
transcodeClass,
isCacheable,
animationFactory,
overrideWidth,
overrideHeight,
diskCacheStrategy);
}
最終通過(guò)GenericRequest.obtain方法創(chuàng)建了
public static <A, T, Z, R> GenericRequest<A, T, Z, R> obtain(...) {
@SuppressWarnings("unchecked")
GenericRequest<A, T, Z, R> request = (GenericRequest<A, T, Z, R>) REQUEST_POOL.poll();
if (request == null) {
request = new GenericRequest<A, T, Z, R>();
}
//利用設(shè)置的參數(shù)初始化Request對(duì)象
request.init(...);
//返回Request對(duì)象
return request;
}
至此請(qǐng)求對(duì)象創(chuàng)建成功，在通過(guò)buildRequest創(chuàng)建請(qǐng)求成功后，使用了target.setRequest(request);將請(qǐng)求設(shè)置到target,并通過(guò)addListener將target加入到lifecycle。上面執(zhí)行了那么多都只是請(qǐng)求創(chuàng)建，請(qǐng)求的執(zhí)行時(shí)通過(guò)requestTracker.runRequest(request);開(kāi)始的。
發(fā)送請(qǐng)求
/**
* Starts tracking the given request.
*/
public void runRequest(Request request) {
//添加request對(duì)象到集合中
requests.add(request);
if (!isPaused) {
//如果當(dāng)前狀態(tài)是非暫停的，調(diào)用begin方法發(fā)送請(qǐng)求
request.begin();
} else {
//將請(qǐng)求加入到掛起的請(qǐng)求集合
pendingRequests.add(request);
}
}
在上面幾句代碼，我們看到，每次提交請(qǐng)求都將請(qǐng)求加入了一個(gè)set中，用它來(lái)管理請(qǐng)求，然后通過(guò)request的實(shí)現(xiàn)類GenericRequest查看begin方法執(zhí)行的內(nèi)容
/**
* {@inheritDoc}
*/
@Override
public void begin() {
startTime = LogTime.getLogTime();
if (model == null) {
//加載錯(cuò)誤占位圖設(shè)置
onException(null);
return;
}
status = Status.WAITING_FOR_SIZE;
//驗(yàn)證寬高是否合法
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
//發(fā)送請(qǐng)求
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
//加載前默認(rèn)占位圖設(shè)置回調(diào)
target.onLoadStarted(getPlaceholderDrawable());
}
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished run method in " + LogTime.getElapsedMillis(startTime));
}
}
//獲取設(shè)置加載開(kāi)始時(shí)占位圖片的Drawable 對(duì)象
private Drawable getPlaceholderDrawable() {
if (placeholderDrawable == null && placeholderResourceId > 0) {
placeholderDrawable = context.getResources().getDrawable(placeholderResourceId);
}
return placeholderDrawable;
}
上面有一句!isComplete() && !isFailed() && canNotifyStatusChanged()判斷，如果都為真會(huì)回調(diào)target.onLoadStarted(getPlaceholderDrawable());我們可以看到Target的實(shí)現(xiàn)類ImageViewTarget中onLoadStarted的回調(diào)執(zhí)行語(yǔ)句
//給ImageView設(shè)置Drawable
@Override
public void onLoadStarted(Drawable placeholder) {
view.setImageDrawable(placeholder);
}
現(xiàn)在你是不是有一種柳暗花明又一村的感覺(jué)，終于明白為什么設(shè)置placeHolder后，會(huì)在加載前有一個(gè)占位圖，當(dāng)然設(shè)置加載錯(cuò)誤圖片占位圖的原理也是一樣的。只不過(guò)回調(diào)執(zhí)行時(shí)機(jī)不同。
/**
* A callback method that should never be invoked directly.
*/
@Override
public void onSizeReady(int width, int height) {
//省略部分代碼
status = Status.RUNNING;//將請(qǐng)求狀態(tài)更新為運(yùn)行狀態(tài)
//省略部分代碼
// 進(jìn)入Engine的入口，請(qǐng)求執(zhí)行的核心方法
loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
priority, isMemoryCacheable, diskCacheStrategy, this);
loadedFromMemoryCache = resource != null;
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
}
Engine類封裝了數(shù)據(jù)獲取的重要入口方法，向request層提供這些API，比如load(), release(), clearDiskCache()等方法
public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
//斷言是否在主線程
Util.assertMainThread();
long startTime = LogTime.getLogTime();
final String id = fetcher.getId();
//創(chuàng)建Enginekey
EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
transcoder, loadProvider.getSourceEncoder());
//從緩存加載圖片
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
// 獲取數(shù)據(jù)成功，會(huì)回調(diào)target的onResourceReady()
cb.onResourceReady(cached);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from cache", startTime, key);
}
return null;
}
// 嘗試從活動(dòng)Resources 中獲取，它表示的是當(dāng)前正在使用的Resources，與內(nèi)存緩存不同之處是clear緩存時(shí)不會(huì)clear它。
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
//獲取成功回調(diào)
cb.onResourceReady(active);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
EngineJob current = jobs.get(key);
//判斷jobs中是否已經(jīng)存在任務(wù)，如果存在說(shuō)明任務(wù)之前已經(jīng)提交了
if (current != null) {
current.addCallback(cb);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Added to existing load", startTime, key);
}
return new LoadStatus(cb, current);
}
//緩存沒(méi)有獲取到，創(chuàng)建EngineJob 對(duì)象
EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
transcoder, diskCacheProvider, diskCacheStrategy, priority);
//EngineRunnable 是任務(wù)執(zhí)行階段的入口
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
jobs.put(key, engineJob);
engineJob.addCallback(cb);
// 開(kāi)始提交job
engineJob.start(runnable);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
我們看到先根據(jù)調(diào)用loadFromCache從內(nèi)存加載，若返回值為空再次從活動(dòng)的資源中加載，若再次為空查看jobs是否提交過(guò)任務(wù)，若沒(méi)有提交則創(chuàng)建EngineRunnable，并將任務(wù)提交到engineJob中。我們先看下EngineJob中的start方法
//提交任務(wù)，將任務(wù)加入到線程池
public void start(EngineRunnable engineRunnable) {
this.engineRunnable = engineRunnable;
//提交任務(wù)到diskCacheService線程池
future = diskCacheService.submit(engineRunnable);
}
接下來(lái)看線程類EngineRunnable的run方法，它是任務(wù)執(zhí)行的入口
//任務(wù)運(yùn)行入口
@Override
public void run() {
if (isCancelled) {
return;
}
Exception exception = null;
Resource<?> resource = null;
try {
//數(shù)據(jù)的獲取，編解碼
resource = decode();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception decoding", e);
}
exception = e;
}
//如果當(dāng)前狀態(tài)是取消，則回收各種資源防止內(nèi)存泄露
if (isCancelled) {
if (resource != null) {
resource.recycle();
}
return;
}
if (resource == null) {
//加載失敗回調(diào)
onLoadFailed(exception);
} else {
//加載成功回調(diào)
onLoadComplete(resource);
}
}
private Resource<?> decode() throws Exception {
if (isDecodingFromCache()) {
//// 從DiskLruCache中獲取數(shù)據(jù)并解碼
return decodeFromCache();
} else {
// 從其他途徑獲取數(shù)據(jù)并解碼，如網(wǎng)絡(luò)，本地File，數(shù)據(jù)流等
return decodeFromSource();
}
}
DiskLruCache獲取數(shù)據(jù)
private Resource<?> decodeFromCache() throws Exception {
Resource<?> result = null;
try {
result = decodeJob.decodeResultFromCache();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Exception decoding result from cache: " + e);
}
}
if (result == null) {
result = decodeJob.decodeSourceFromCache();
}
return result;
}
之后調(diào)用decodeJob類中的decodeResultFromCache
public Resource<Z> decodeResultFromCache() throws Exception {
if (!diskCacheStrategy.cacheResult()) {
return null;
}
long startTime = LogTime.getLogTime();
//從DiskCache中獲取資源
Resource<T> transformed = loadFromCache(resultKey);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded transformed from cache", startTime);
}
startTime = LogTime.getLogTime();
Resource<Z> result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from cache", startTime);
}
return result;
}
//從DiskCache中獲取資源
private Resource<T> loadFromCache(Key key) throws IOException {
//根據(jù)key從DiskCache獲取文件
File cacheFile = diskCacheProvider.getDiskCache().get(key);
if (cacheFile == null) {
return null;
}
Resource<T> result = null;
try {
result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);
} finally {
if (result == null) {
diskCacheProvider.getDiskCache().delete(key);
}
}
return result;
}
接下來(lái)我們分析decodeFromSource方法
// 調(diào)用decodeJob來(lái)完成數(shù)據(jù)獲取和編解碼
private Resource<?> decodeFromSource() throws Exception {
return decodeJob.decodeFromSource();
}
public Resource<Z> decodeFromSource() throws Exception {
// 獲取數(shù)據(jù)，解碼
Resource<T> decoded = decodeSource();
//編碼并保存
return transformEncodeAndTranscode(decoded);
}
// 獲取數(shù)據(jù)，解碼
private Resource<T> decodeSource() throws Exception {
Resource<T> decoded = null;
try {
long startTime = LogTime.getLogTime();
//數(shù)據(jù)拉取
final A data = fetcher.loadData(priority);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Fetched data", startTime);
}
if (isCancelled) {
return null;
}
//編碼
decoded = decodeFromSourceData(data);
} finally {
fetcher.cleanup();
}
return decoded;
}
在數(shù)據(jù)獲取時(shí)先調(diào)用DataFetcher的loadData()拉取數(shù)據(jù)，對(duì)于DataFetcher的實(shí)現(xiàn)類有好幾個(gè)，我們拿從url拉取數(shù)據(jù)為例，也就是HttpUrlFetcher類
@Override
public InputStream loadData(Priority priority) throws Exception {
return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
}
//返回InputStream 對(duì)象
private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl, Map<String, String> headers)
throws IOException {
if (redirects >= MAXIMUM_REDIRECTS) {
throw new IOException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
} else {
// Comparing the URLs using .equals performs additional network I/O and is generally broken.
// See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
try {
if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
throw new IOException("In re-direct loop");
}
} catch (URISyntaxException e) {
// Do nothing, this is best effort.
}
}
// 靜態(tài)工廠模式創(chuàng)建HttpURLConnection對(duì)象
urlConnection = connectionFactory.build(url);
for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
}
//設(shè)置請(qǐng)求參數(shù)
//設(shè)置連接超時(shí)時(shí)間2500ms
urlConnection.setConnectTimeout(2500);
//設(shè)置讀取超時(shí)時(shí)間2500ms
urlConnection.setReadTimeout(2500);
//不使用http緩存
urlConnection.setUseCaches(false);
urlConnection.setDoInput(true);
// Connect explicitly to avoid errors in decoders if connection fails.
urlConnection.connect();
if (isCancelled) {
return null;
}
final int statusCode = urlConnection.getResponseCode();
if (statusCode / 100 == 2) {
//請(qǐng)求成功
return getStreamForSuccessfulRequest(urlConnection);
} else if (statusCode / 100 == 3) {
//
String redirectUrlString = urlConnection.getHeaderField("Location");
if (TextUtils.isEmpty(redirectUrlString)) {
throw new IOException("Received empty or null redirect url");
}
URL redirectUrl = new URL(url, redirectUrlString);
return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
} else {
if (statusCode == -1) {
throw new IOException("Unable to retrieve response code from HttpUrlConnection.");
}
throw new IOException("Request failed " + statusCode + ": " + urlConnection.getResponseMessage());
}
}
看到這終于看到了網(wǎng)絡(luò)加載請(qǐng)求，我們也可以自定義DataFetcher，從而使用其他網(wǎng)絡(luò)庫(kù)，如OkHttp，Volley.
最后我們?cè)倏聪聇ransformEncodeAndTranscode方法
private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
long startTime = LogTime.getLogTime();
// 根據(jù)ImageView的scaleType等參數(shù)計(jì)算真正被ImageView使用的圖片寬高，并保存真正寬高的圖片。
Resource<T> transformed = transform(decoded);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transformed resource from source", startTime);
}
// 寫入到DiskLruCache中，下次就可以直接從DiskLruCache獲取使用
writeTransformedToCache(transformed);
startTime = LogTime.getLogTime();
// 轉(zhuǎn)碼，將源圖片轉(zhuǎn)碼為ImageView所需的圖片格式
Resource<Z> result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from source", startTime);
}
return result;
}
至此，圖片加載流程已經(jīng)介紹完畢，當(dāng)然還有很多的地方?jīng)]有提到，相信如果你在閱讀本文的同時(shí)，自己跟蹤源碼會(huì)輕松很多，如果自己不跟著源碼走的話，可能這篇文章看幾遍對(duì)Glide原理理解的也是云里霧里，或者說(shuō)當(dāng)時(shí)看的懂，但是很快就不記得。所以切記自己要跟著源碼過(guò)一遍。
本片文章實(shí)在是長(zhǎng)，能讀完本文章也是需要一定毅力的...若文章有不足或者錯(cuò)誤的地方，歡迎指正，以防止給其他讀者錯(cuò)誤引導(dǎo)。
