参考系列文章:
蛮久之前囫囵吞枣地瞄过Glide部分源码，最近由于某个契机又心血来潮比较系统的过了一遍它的源码，发现它的蛮多设计还是比较有意思的(该系列笔记基于Glide 4.12.0这个版本进行分析)。
首先Glide的使用十分简单，只需要三行代码就能完成图片的下载、缓存和显示:
Glide.with(this)
.load(url)
.into(img)
但为了我们可以方便的使用，实际上每一行代码的背后都帮我们做了不少脏活累活。
Glide.with
我们一行行来看，首先看Glide.with方法，它其实有一系列的重载:
public static RequestManager with(@NonNull Context context) {
return getRetriever(context).get(context);
}
public static RequestManager with(@NonNull Activity activity) {
return getRetriever(activity).get(activity);
}
public static RequestManager with(@NonNull FragmentActivity activity) {
return getRetriever(activity).get(activity);
}
public static RequestManager with(@NonNull Fragment fragment) {
return getRetriever(fragment.getContext()).get(fragment);
}
public static RequestManager with(@NonNull android.app.Fragment fragment) {
return getRetriever(fragment.getActivity()).get(fragment);
}
public static RequestManager with(@NonNull View view) {
return getRetriever(view.getContext()).get(view);
}
内容大同小异，都是用getRetriever方法获取RequestManagerRetriever，然后从里面get出RequestManager。
Glide 单例模式实现
追踪getRetriever方法，可以比较容易看出来实际上是从Glide单例里面获取RequestManagerRetriever，而且我们还能看到Glide使用的是单例模式的双重校验锁方式:
private static RequestManagerRetriever getRetriever(@Nullable Context context) {
...
return Glide.get(context).getRequestManagerRetriever();
}
public static Glide get(@NonNull Context context) {
if (glide == null) {
...
synchronized (Glide.class) {
if (glide == null) {
checkAndInitializeGlide(context, annotationGeneratedModule);
}
}
}
return glide;
}
Glide生命周期绑定
glide其实已经帮我们对activty生命周期进行了绑定：在onStop的时候停止加载，在onStart的时候继续加载，在onDestory清除任务和进行缓存的回收。它的原理是通过在Activity里面添加一个不可见的fragment，在里面监听生命周期。
我们可以看到在RequestManagerRetriever.get方法最终会add一个SupportRequestManagerFragment到Activity里面:
public RequestManager get(@NonNull FragmentActivity activity) {
...
FragmentManager fm = activity.getSupportFragmentManager();
return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
...
}
@NonNull
private RequestManager supportFragmentGet(Context context, FragmentManager fm, Fragment parentHint, boolean isParentVisible) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
RequestManager requestManager = current.getRequestManager();
...
return requestManager;
}
@NonNull
private SupportRequestManagerFragment getSupportRequestManagerFragment(final FragmentManager fm, Fragment parentHint) {
SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
...
current = new SupportRequestManagerFragment();
...
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
...
}
return current;
}
SupportRequestManagerFragment里面有个lifecycle成员，我们可以向它注册监听，RequestManager也正是这么干的:
public class SupportRequestManagerFragment extends Fragment {
...
private final ActivityFragmentLifecycle lifecycle;
...
public SupportRequestManagerFragment(@NonNull ActivityFragmentLifecycle lifecycle) {
this.lifecycle = 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();
}
...
}
//RequestManager
RequestManager(
Glide glide,
Lifecycle lifecycle,
RequestManagerTreeNode treeNode,
RequestTracker requestTracker,
ConnectivityMonitorFactory factory,
Context context) {
...
lifecycle.addListener(connectivityMonitor);
...
}
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();
...
}
这种添加不可见Fragment监听生命周期的技巧还是挺实用的。RxPermissions里面也用了这种技巧去监听onActivityResult回调
RequestManager.load
第二行的load方法，虽然只有一行但是实际上它做了两件事情:
public RequestBuilder<Drawable> load(@Nullable String string) {
return asDrawable().load(string);
}
- new了一个请求Drawable的RequestBuilder:
public RequestBuilder<Drawable> asDrawable() {
return as(Drawable.class);
}
public <ResourceType> RequestBuilder<ResourceType> as(
@NonNull Class<ResourceType> resourceClass) {
return new RequestBuilder<>(glide, this, resourceClass, context);
}
- 往builder里设置了model:
public RequestBuilder<TranscodeType> load(@Nullable String string) {
return loadGeneric(string);
}
private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
...
this.model = model;
...
}
没错这里使用的是Builder模式。
RequestBuilder.into
into实际上也是干了两件事情，build了一个Request出来，然后runRequest执行它:
// RequestBuilder
public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
...
return into(
glideContext.buildImageViewTarget(view, transcodeClass),
/*targetListener=*/ null,
requestOptions,
Executors.mainThreadExecutor());
}
private <Y extends Target<TranscodeType>> Y into(
@NonNull Y target,
@Nullable RequestListener<TranscodeType> targetListener,
BaseRequestOptions<?> options,
Executor callbackExecutor) {
...
Request request = buildRequest(target, targetListener, options, callbackExecutor);
...
target.setRequest(request);
requestManager.track(target, request);
return target;
}
// RequestManager
synchronized void track(@NonNull Target<?> target, @NonNull Request request) {
targetTracker.track(target);
requestTracker.runRequest(request);
}
这里其实还有个细节，我们展开讲讲。如果我们中途想取消对该view的图片加载，可以用下面两行代码进行取消:
Glide.with(this)
.clear(img)
这样就要求我们要记录view和request的关联，可能有些同学会想到用map<View,Request>这样的形式去保存，但是这样的话需要我们额外做出一些手段去防止view的内存泄露(如弱引用等)。其实安卓里比较常用的手段是将需要关联的东西用setTag方法保存到view里面:
// ViewTarget
public void setRequest(@Nullable Request request) {
setTag(request);
}
private void setTag(@Nullable Object tag) {
isTagUsedAtLeastOnce = true;
view.setTag(tagId, tag);
}
于是clear的时候只需要用View.getTag把request再拿出来就好:
public void clear(@NonNull View view) {
clear(new ClearTarget(view));
}
public void clear(@Nullable final Target<?> target) {
if (target == null) {
return;
}
untrackOrDelegate(target);
}
private void untrackOrDelegate(@NonNull Target<?> target) {
boolean isOwnedByUs = untrack(target);
Request request = target.getRequest();
if (!isOwnedByUs && !glide.removeFromManagers(target) && request != null) {
target.setRequest(null);
request.clear();
}
}
request的执行
runRequest的逻辑其实也比较简单，如果没有pause就begin request，如果不是就放到pendingRequests列表里面等待后面再begin:
public void runRequest(@NonNull Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
request.clear();
...
pendingRequests.add(request);
}
}
而request的begin方法的核心流程就是先获取图片的尺寸，然后使用engine.load去启动真正的加载逻辑:
public void begin() {
...
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
...
}
public void onSizeReady(int width, int height) {
...
loadStatus =
engine.load(
glideContext,
model,
requestOptions.getSignature(),
this.width,
this.height,
requestOptions.getResourceClass(),
transcodeClass,
priority,
requestOptions.getDiskCacheStrategy(),
requestOptions.getTransformations(),
requestOptions.isTransformationRequired(),
requestOptions.isScaleOnlyOrNoTransform(),
requestOptions.getOptions(),
requestOptions.isMemoryCacheable(),
requestOptions.getUseUnlimitedSourceGeneratorsPool(),
requestOptions.getUseAnimationPool(),
requestOptions.getOnlyRetrieveFromCache(),
this,
callbackExecutor);
...
}
到这里其实一切都比较好理解，下一步就到了Engine这个Glide的核心模块，Glide的实际加载和多级缓存都是由它去调度的，由于该模块比较复杂，我们留到下一篇单独来聊。
作者：嘉伟咯
链接：https://www.jianshu.com/p/85da220d8442
来源：简书
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