Android Glide图片加载框架--源码解析
Android Glide图片加载框架系列文章
一、简介
在本系列的上一篇文章中,我们学习了Glide的基本用法,体验了这个图片加载框架的强大功能,以及它非常简便的API。还没有看过上一篇文章的朋友,建议先去阅读 Android Glide图片加载框架(一)基本用法。
在多数情况下,我们想要在界面上加载并展示一张图片只需要一行代码就能实现,如下所示:
Glide.with(this).load(url).into(img);
虽说只有这简简单单的一行代码,但大家可能不知道的是,Glide在背后帮我们默默执行了成吨的工作。这个形容词我想了很久,因为我觉得用非常多这个形容词不足以描述Glide背后的工作量,我查到的英文资料是用tons of work来进行形容的,因此我觉得这里使用成吨来形容更加贴切一些。
虽说我们在平时使用Glide的时候格外地简单和方便,但是知其然也要知其所以然。那么今天我们就来解析一下Glide的源码,看看它在这些简单用法的背后,到底执行了多么复杂的工作。
二、源码解析
1、如何阅读源码
在开始解析Glide源码之前,我想先和大家谈一下该如何阅读源码,这个问题也是我平时被问得比较多的,因为很多人都觉得阅读源码是一件比较困难的事情。
那么阅读源码到底困难吗?
这个当然主要还是要视具体的源码而定。比如同样是图片加载框架,我读Volley的源码时就感觉酣畅淋漓,并且对Volley的架构设计和代码质量深感佩服。读Glide的源码时却让我相当痛苦,代码极其难懂。当然这里我并不是说Glide的代码写得不好,只是因为Glide和复杂程度和Volley完全不是在一个量级上的。
那么,虽然源码的复杂程度是外在的不可变条件,但我们却可以通过一些技巧来提升自己阅读源码的能力。这里我和大家分享一下我平时阅读源码时所使用的技巧,简单概括就是八个字:抽丝剥茧、点到即止
。
应该认准一个功能点,然后去分析这个功能点是如何实现的。但只要去追寻主体的实现逻辑即可,千万不要试图去搞懂每一行代码都是什么意思,那样很容易会陷入到思维黑洞当中,而且越陷越深。因为这些庞大的系统都不是由一个人写出来的,每一行代码都想搞明白,就会感觉自己是在盲人摸象,永远也研究不透。如果只是去分析主体的实现逻辑,那么就有比较明确的目的性,这样阅读源码会更加轻松,也更加有成效。
而今天带大家阅读的Glide源码就非常适合使用这个技巧,因为Glide的源码太复杂了,千万不要试图去搞明白它每行代码的作用,而是应该只分析它的主体实现逻辑。
那么我们本篇文章就先确立好一个目标,就是要通过阅读源码搞明白下面这行代码:
Glide.with(this).load(url).into(img);
到底是如何实现将一张网络图片展示到ImageView上面的。先将Glide的一整套图片加载机制的基本流程梳理清楚,然后我们再通过后面的几篇文章具体去了解Glide源码方方面面的细节。
准备好了吗?那么我们现在开始。
既然是要阅读Glide的源码,那么我们自然需要先将Glide的源码下载下来。其实如果你是使用在build.gradle中添加依赖的方式将Glide引入到项目中的,那么源码自动就已经下载下来了,在Android Studio中就可以直接进行查看。
不过,使用添加依赖的方式引入的Glide,我们只能看到它的源码,但不能做任何的修改,如果你还需要修改它的源码的话,可以到GitHub上面将它的完整源码下载下来。
Glide的GitHub主页的地址是:https://github.com/bumptech/glide
不过在这个地址下载到的永远都是最新的源码,有可能还正在处于开发当中。而我们整个系列都是使用Glide 4.8.0这个版本来进行讲解的,因此如果你需要专门去下载4.8.0版本的源码,可以到这个地址进行下载:https://github.com/bumptech/glide/tree/v4.8.0
2、with()
with()方法是Glide类中的一组静态方法,它有好几个方法重载,我们来看一下Glide类中所有with()方法的方法重载:
public class Glide{
...
@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);
}
@SuppressWarnings("deprecation")
@Deprecated
@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);
}
@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();
}
...
}
可以看到,with()方法的重载种类非常多,既可以传入Activity,也可以传入Fragment或者是Context。每一个with()方法重载的代码都非常简单,都是先调用getRetriever()方法获取RequestManagerRetriever对象,然后再调用RequestManagerRetriever的实例get()方法,去获取RequestManager对象。
而RequestManagerRetriever的实例get()方法中的逻辑是什么样的呢?我们一起来看一看:
public class RequestManagerRetriever implements Handler.Callback {
/**
* The top application level RequestManager.
*/
private volatile RequestManager applicationManager;
@NonNull
private RequestManager getApplicationManager(@NonNull Context context) {
// Either an application context or we're on a background thread.
if (applicationManager == null) {
synchronized (this) {
if (applicationManager == null) {
// Normally pause/resume is taken care of by the fragment we add to the fragment or
// activity. However, in this case since the manager attached to the application will not
// receive lifecycle events, we must force the manager to start resumed using
// ApplicationLifecycle.
// TODO(b/27524013): Factor out this Glide.get() call.
Glide glide = Glide.get(context.getApplicationContext());
applicationManager =
factory.build(
glide,
new ApplicationLifecycle(),
new EmptyRequestManagerTreeNode(),
context.getApplicationContext());
}
}
}
return applicationManager;
}
@NonNull
public RequestManager get(@NonNull Context context) {
if (context == null) {
throw new IllegalArgumentException("You cannot start a load on a null Context");
} else if (Util.isOnMainThread() && !(context instanceof Application)) {
if (context instanceof FragmentActivity) {
return get((FragmentActivity) context);
} else if (context instanceof Activity) {
return get((Activity) context);
} else if (context instanceof ContextWrapper) {
return get(((ContextWrapper) context).getBaseContext());
}
}
return getApplicationManager(context);
}
@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));
}
}
@NonNull
public RequestManager get(@NonNull Fragment fragment) {
Preconditions.checkNotNull(fragment.getActivity(),
"You cannot start a load on a fragment before it is attached or after it is destroyed");
if (Util.isOnBackgroundThread()) {
return get(fragment.getActivity().getApplicationContext());
} else {
FragmentManager fm = fragment.getChildFragmentManager();
return supportFragmentGet(fragment.getActivity(), fm, fragment, fragment.isVisible());
}
}
@SuppressWarnings("deprecation")
@NonNull
public RequestManager get(@NonNull Activity activity) {
if (Util.isOnBackgroundThread()) {
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
android.app.FragmentManager fm = activity.getFragmentManager();
return fragmentGet(
activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
}
}
@SuppressWarnings("deprecation")
@NonNull
public RequestManager get(@NonNull View view) {
if (Util.isOnBackgroundThread()) {
return get(view.getContext().getApplicationContext());
}
Preconditions.checkNotNull(view);
Preconditions.checkNotNull(view.getContext(),
"Unable to obtain a request manager for a view without a Context");
Activity activity = findActivity(view.getContext());
// The view might be somewhere else, like a service.
if (activity == null) {
return get(view.getContext().getApplicationContext());
}
// Support Fragments.
// Although the user might have non-support Fragments attached to FragmentActivity, searching
// for non-support Fragments is so expensive pre O and that should be rare enough that we
// prefer to just fall back to the Activity directly.
if (activity instanceof FragmentActivity) {
Fragment fragment = findSupportFragment(view, (FragmentActivity) activity);
return fragment != null ? get(fragment) : get(activity);
}
// Standard Fragments.
android.app.Fragment fragment = findFragment(view, activity);
if (fragment == null) {
return get(activity);
}
return get(fragment);
}
@SuppressWarnings("deprecation")
@Deprecated
@NonNull
@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
public RequestManager get(@NonNull android.app.Fragment fragment) {
if (fragment.getActivity() == null) {
throw new IllegalArgumentException(
"You cannot start a load on a fragment before it is attached");
}
if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
return get(fragment.getActivity().getApplicationContext());
} else {
android.app.FragmentManager fm = fragment.getChildFragmentManager();
return fragmentGet(fragment.getActivity(), fm, fragment, fragment.isVisible());
}
}
@SuppressWarnings("deprecation")
@Deprecated
@NonNull
RequestManagerFragment getRequestManagerFragment(Activity activity) {
return getRequestManagerFragment(
activity.getFragmentManager(), /*parentHint=*/ null, isActivityVisible(activity));
}
@SuppressWarnings("deprecation")
@NonNull
private RequestManagerFragment getRequestManagerFragment(
@NonNull final android.app.FragmentManager fm,
@Nullable android.app.Fragment parentHint,
boolean isParentVisible) {
RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
current = pendingRequestManagerFragments.get(fm);
if (current == null) {
current = new RequestManagerFragment();
current.setParentFragmentHint(parentHint);
if (isParentVisible) {
current.getGlideLifecycle().onStart();
}
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
@SuppressWarnings({"deprecation", "DeprecatedIsStillUsed"})
@Deprecated
@NonNull
private RequestManager fragmentGet(@NonNull Context context,
@NonNull android.app.FragmentManager fm,
@Nullable android.app.Fragment parentHint,
boolean isParentVisible) {
RequestManagerFragment current = getRequestManagerFragment(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;
}
@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;
}
@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;
}
}
上述代码虽然看上去逻辑有点复杂,但是将它们梳理清楚后还是很简单的。RequestManagerRetriever类中看似有很多个get()方法的重载,什么Context参数,Activity参数,Fragment参数等等,实际上只有两种情况而已,即传入Application类型的参数,和传入非Application类型的参数
。
Application类型参数
如果在Glide.with()方法中传入的是一个Application对象,那么这里就会调用带有Context参数的get()方法重载,然后会在第48行调用getApplicationManager()方法来获取一个RequestManager对象。其实这是最简单的一种情况,因为Application对象的生命周期即应用程序的生命周期,因此Glide并不需要做什么特殊的处理,它自动就是和应用程序的生命周期是同步的,如果应用程序关闭的话,Glide的加载也会同时终止。
非Application类型参数
不管你在Glide.with()方法中传入的是Activity、FragmentActivity、v4包下的Fragment、还是app包下的Fragment,最终的流程都是一样的,那就是会向当前的Activity当中添加一个隐藏的Fragment。具体添加的逻辑是在上述代码的第176行和第215行,分别对应的app包和v4包下的两种Fragment的情况。
这里为什么要添加一个隐藏的Fragment呢?
因为Glide需要知道加载的生命周期。很简单的一个道理,如果你在某个Activity上正在加载着一张图片,结果图片还没加载出来,Activity就被用户关掉了,那么图片还应该继续加载吗?当然不应该。可是 Glide并没有办法知道Activity的生命周期,于是Glide就使用了添加隐藏Fragment的这种小技巧,因为Fragment的生命周期和Activity是同步的,如果Activity被销毁了,Fragment是可以监听到的,这样Glide就可以捕获这个事件并停止图片加载了
。
这里额外再提一句,从第54行代码可以看出,如果我们是在非主线程当中使用的Glide,那么不管你是传入的Activity还是Fragment,都会被强制当成Application来处理
。不过其实这就属于是在分析代码的细节了,本篇文章我们将会把目光主要放在Glide的主线工作流程上面,后面不会过多去分析这些细节方面的内容。
总体来说,第一个with()方法的源码还是比较好理解的。其实就是为了得到一个RequestManager对象而已,然后 Glide会根据我们传入with()方法的参数来确定图片加载的生命周期
,并没有什么特别复杂的逻辑。不过复杂的逻辑还在后面等着我们呢,接下来我们开始分析第二步,load()方法。
3、load()
由于with()方法返回的是一个RequestManager对象,那么很容易就能想到,load()方法是在RequestManager类当中的,所以说我们首先要看的就是RequestManager这个类。不过在上一篇文章中我们学过,Glide是支持图片URL字符串、图片本地路径等等加载形式的,因此RequestManager中也有很多个load()方法的重载。但是这里我们不可能把每个load()方法的重载都看一遍,因此我们就只选其中一个加载图片URL字符串的load()方法来进行研究吧。
RequestManager类的简化代码如下所示:
public class RequestManager implements LifecycleListener,
ModelTypes<RequestBuilder<Drawable>> {
...
@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable String string) {
return asDrawable().load(string);
}
@NonNull
@CheckResult
public RequestBuilder<Drawable> asDrawable() {
return as(Drawable.class);
}
@NonNull
@CheckResult
public <ResourceType> RequestBuilder<ResourceType> as(
@NonNull Class<ResourceType> resourceClass) {
return new RequestBuilder<>(glide, this, resourceClass, context);
}
...
}
RequestManager类的代码是非常多的,但是经过我这样简化之后,看上去就比较清爽了。在我们只探究加载图片URL字符串这一个load()方法的情况下,那么比较重要的方法就只剩下上述代码中的这三个方法。
那么我们先来看load()方法,这个方法中的逻辑是非常简单的,只有一行代码,就是先调用了asDrawable()方法,而asDrawable()方法也极为简单,就是调用了as()方法,并且指定参数为Drawable.class,as()方法中初始化了一个RequestBuilder对象,然后调用RequestBuilder对象的load()方法。
RequestBuilder类的简化代码如下所示:
public class RequestBuilder<TranscodeType> implements Cloneable,
ModelTypes<RequestBuilder<TranscodeType>> {
protected RequestBuilder(Glide glide, RequestManager requestManager,
Class<TranscodeType> transcodeClass, Context context) {
this.glide = glide;
this.requestManager = requestManager;
this.transcodeClass = transcodeClass;
this.defaultRequestOptions = requestManager.getDefaultRequestOptions();
this.context = context;
this.transitionOptions = requestManager.getDefaultTransitionOptions(transcodeClass);
this.requestOptions = defaultRequestOptions;
this.glideContext = glide.getGlideContext();
}
@NonNull
@Override
@CheckResult
public RequestBuilder<TranscodeType> load(@Nullable String string) {
return loadGeneric(string);
}
@NonNull
private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
this.model = model;
isModelSet = true;
return this;
}
}
load()方法调用了loadGeneric()方法返回当前RequestBuilder对象,至此load()方法也就分析结束了。
3、into()
如果说前面两步都是在准备开胃小菜的话,那么现在终于要进入主菜了,因为into()方法也是整个Glide图片加载流程中逻辑最复杂的地方。
into()方法也在RequestBuilder类中,下面我们就看下RequestBuilder类的into()方法,代码如下:
public class RequestBuilder<TranscodeType> implements Cloneable,
ModelTypes<RequestBuilder<TranscodeType>> {
@NonNull
public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
Util.assertMainThread();
Preconditions.checkNotNull(view);
RequestOptions requestOptions = this.requestOptions;
if (!requestOptions.isTransformationSet()
&& requestOptions.isTransformationAllowed()
&& view.getScaleType() != null) {
// Clone in this method so that if we use this RequestBuilder to load into a View and then
// into a different target, we don't retain the transformation applied based on the previous
// View's scale type.
switch (view.getScaleType()) {
case CENTER_CROP:
requestOptions = requestOptions.clone().optionalCenterCrop();
break;
case CENTER_INSIDE:
requestOptions = requestOptions.clone().optionalCenterInside();
break;
case FIT_CENTER:
case FIT_START:
case FIT_END:
requestOptions = requestOptions.clone().optionalFitCenter();
break;
case FIT_XY:
requestOptions = requestOptions.clone().optionalCenterInside();
break;
case CENTER:
case MATRIX:
default:
// Do nothing.
}
}
return into(
glideContext.buildImageViewTarget(view, transcodeClass),
/*targetListener=*/ null,
requestOptions);
}
}
这里前面一大堆的判断逻辑我们都可以先不用管,等到后面文章讲transform的时候会再进行解释,现在我们只需要关注最后一行代码。最后一行代码先是调用了glideContext.buildImageViewTarget()方法,这个方法会构建出一个Target对象,Target对象则是用来最终展示图片用的,如果我们跟进去的话会看到如下代码:
public class GlideContext extends ContextWrapper {
@NonNull
public <X> ViewTarget<ImageView, X> buildImageViewTarget(
@NonNull ImageView imageView, @NonNull Class<X> transcodeClass) {
return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
}
}
这里其实又是调用了ImageViewTargetFactory的buildTarget()方法,我们继续跟进去,代码如下所示:
public class ImageViewTargetFactory {
@NonNull
@SuppressWarnings("unchecked")
public <Z> ViewTarget<ImageView, Z> buildTarget(@NonNull ImageView view,
@NonNull Class<Z> clazz) {
if (Bitmap.class.equals(clazz)) {
return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view);
} else if (Drawable.class.isAssignableFrom(clazz)) {
return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view);
} else {
throw new IllegalArgumentException(
"Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
}
}
}
可以看到,在buildTarget()方法中会根据传入的class参数来构建不同的Target对象。那如果你要分析这个class参数是从哪儿传过来的,这可有得你分析了,简单起见我直接帮大家梳理清楚。这个class参数其实基本上只有两种情况,如果你在使用Glide加载图片的时候调用了asBitmap()方法,那么这里就会构建出BitmapImageViewTarget对象,否则的话构建的都是DrawableImageViewTarget对象。
也就是说,通过glideContext.buildImageViewTarget()方法,我们构建出了一个DrawableImageViewTarget对象。那现在回到刚才into()方法的最后一行,可以看到,这里又将这个参数传入到了RequestBuilder另一个接收Target对象的into()方法当中了。我们来看一下这个into()方法的源码:
public class RequestBuilder<TranscodeType> implements Cloneable,
ModelTypes<RequestBuilder<TranscodeType>> {
private <Y extends Target<TranscodeType>> Y into(
@NonNull Y target,
@Nullable RequestListener<TranscodeType> targetListener,
@NonNull RequestOptions options) {
Util.assertMainThread();
Preconditions.checkNotNull(target);
if (!isModelSet) {
throw new IllegalArgumentException("You must call #load() before calling #into()");
}
options = options.autoClone();
Request request = buildRequest(target, targetListener, options);
Request previous = target.getRequest();
if (request.isEquivalentTo(previous)
&& !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
request.recycle();
// If the request is completed, beginning again will ensure the result is re-delivered,
// triggering RequestListeners and Targets. If the request is failed, beginning again will
// restart the request, giving it another chance to complete. If the request is already
// running, we can let it continue running without interruption.
if (!Preconditions.checkNotNull(previous).isRunning()) {
// Use the previous request rather than the new one to allow for optimizations like skipping
// setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions
// that are done in the individual Request.
previous.begin();
}
return target;
}
requestManager.clear(target);
target.setRequest(request);
requestManager.track(target, request);
return target;
}
}
这里我们还是只抓核心代码,其实只有两行是最关键的,第14行调用buildRequest()方法构建出了一个Request对象,还有第35行来执行这个Request。
Request是用来发出加载图片请求的,它是Glide中非常关键的一个组件。我们先来看buildRequest()方法是如何构建Request对象的:
public class RequestBuilder<TranscodeType> implements Cloneable,
ModelTypes<RequestBuilder<TranscodeType>> {
private Request buildRequest(
Target<TranscodeType> target,
@Nullable RequestListener<TranscodeType> targetListener,
RequestOptions requestOptions) {
return buildRequestRecursive(
target,
targetListener,
/*parentCoordinator=*/ null,
transitionOptions,
requestOptions.getPriority(),
requestOptions.getOverrideWidth(),
requestOptions.getOverrideHeight(),
requestOptions);
}
private Request buildRequestRecursive(
Target<TranscodeType> target,
@Nullable RequestListener<TranscodeType> targetListener,
@Nullable RequestCoordinator parentCoordinator,
TransitionOptions<?, ? super TranscodeType> transitionOptions,
Priority priority,
int overrideWidth,
int overrideHeight,
RequestOptions requestOptions) {
// Build the ErrorRequestCoordinator first if necessary so we can update parentCoordinator.
ErrorRequestCoordinator errorRequestCoordinator = null;
if (errorBuilder != null) {
errorRequestCoordinator = new ErrorRequestCoordinator(parentCoordinator);
parentCoordinator = errorRequestCoordinator;
}
Request mainRequest =
buildThumbnailRequestRecursive(
target,
targetListener,
parentCoordinator,
transitionOptions,
priority,
overrideWidth,
overrideHeight,
requestOptions);
if (errorRequestCoordinator == null) {
return mainRequest;
}
int errorOverrideWidth = errorBuilder.requestOptions.getOverrideWidth();
int errorOverrideHeight = errorBuilder.requestOptions.getOverrideHeight();
if (Util.isValidDimensions(overrideWidth, overrideHeight)
&& !errorBuilder.requestOptions.isValidOverride()) {
errorOverrideWidth = requestOptions.getOverrideWidth();
errorOverrideHeight = requestOptions.getOverrideHeight();
}
Request errorRequest = errorBuilder.buildRequestRecursive(
target,
targetListener,
errorRequestCoordinator,
errorBuilder.transitionOptions,
errorBuilder.requestOptions.getPriority(),
errorOverrideWidth,
errorOverrideHeight,
errorBuilder.requestOptions);
errorRequestCoordinator.setRequests(mainRequest, errorRequest);
return errorRequestCoordinator;
}
private Request buildThumbnailRequestRecursive(
Target<TranscodeType> target,
RequestListener<TranscodeType> targetListener,
@Nullable RequestCoordinator parentCoordinator,
TransitionOptions<?, ? super TranscodeType> transitionOptions,
Priority priority,
int overrideWidth,
int overrideHeight,
RequestOptions requestOptions) {
if (thumbnailBuilder != null) {
// Recursive case: contains a potentially recursive thumbnail request builder.
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()");
}
TransitionOptions<?, ? super TranscodeType> thumbTransitionOptions =
thumbnailBuilder.transitionOptions;
// Apply our transition by default to thumbnail requests but avoid overriding custom options
// that may have been applied on the thumbnail request explicitly.
if (thumbnailBuilder.isDefaultTransitionOptionsSet) {
thumbTransitionOptions = transitionOptions;
}
Priority thumbPriority = thumbnailBuilder.requestOptions.isPrioritySet()
? thumbnailBuilder.requestOptions.getPriority() : getThumbnailPriority(priority);
int thumbOverrideWidth = thumbnailBuilder.requestOptions.getOverrideWidth();
int thumbOverrideHeight = thumbnailBuilder.requestOptions.getOverrideHeight();
if (Util.isValidDimensions(overrideWidth, overrideHeight)
&& !thumbnailBuilder.requestOptions.isValidOverride()) {
thumbOverrideWidth = requestOptions.getOverrideWidth();
thumbOverrideHeight = requestOptions.getOverrideHeight();
}
ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
Request fullRequest =
obtainRequest(
target,
targetListener,
requestOptions,
coordinator,
transitionOptions,
priority,
overrideWidth,
overrideHeight);
isThumbnailBuilt = true;
// Recursively generate thumbnail requests.
Request thumbRequest =
thumbnailBuilder.buildRequestRecursive(
target,
targetListener,
coordinator,
thumbTransitionOptions,
thumbPriority,
thumbOverrideWidth,
thumbOverrideHeight,
thumbnailBuilder.requestOptions);
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,
targetListener,
requestOptions,
coordinator,
transitionOptions,
priority,
overrideWidth,
overrideHeight);
RequestOptions thumbnailOptions = requestOptions.clone()
.sizeMultiplier(thumbSizeMultiplier);
Request thumbnailRequest =
obtainRequest(
target,
targetListener,
thumbnailOptions,
coordinator,
transitionOptions,
getThumbnailPriority(priority),
overrideWidth,
overrideHeight);
coordinator.setRequests(fullRequest, thumbnailRequest);
return coordinator;
} else {
// Base case: no thumbnail.
return obtainRequest(
target,
targetListener,
requestOptions,
parentCoordinator,
transitionOptions,
priority,
overrideWidth,
overrideHeight);
}
}
private Request obtainRequest(
Target<TranscodeType> target,
RequestListener<TranscodeType> targetListener,
RequestOptions requestOptions,
RequestCoordinator requestCoordinator,
TransitionOptions<?, ? super TranscodeType> transitionOptions,
Priority priority,
int overrideWidth,
int overrideHeight) {
return SingleRequest.obtain(
context,
glideContext,
model,
transcodeClass,
requestOptions,
overrideWidth,
overrideHeight,
priority,
target,
targetListener,
requestListeners,
requestCoordinator,
glideContext.getEngine(),
transitionOptions.getTransitionFactory());
}
}
可以看到,buildRequest()方法的内部其实又调用了buildRequestRecursive()方法,而buildRequestRecursive()方法中调用了buildThumbnailRequestRecursive()方法,该方法里调用了obtainRequest()方法来获取一个Request对象,而obtainRequest()方法中又去调用了SingleRequest.obtain()方法。那么我们进入到这个SingleRequest的obtain()方法瞧一瞧:
public final class SingleRequest<R> implements Request,
SizeReadyCallback,
ResourceCallback,
FactoryPools.Poolable {
public static <R> SingleRequest<R> obtain(
Context context,
GlideContext glideContext,
Object model,
Class<R> transcodeClass,
RequestOptions requestOptions,
int overrideWidth,
int overrideHeight,
Priority priority,
Target<R> target,
RequestListener<R> targetListener,
@Nullable List<RequestListener<R>> requestListeners,
RequestCoordinator requestCoordinator,
Engine engine,
TransitionFactory<? super R> animationFactory) {
@SuppressWarnings("unchecked") SingleRequest<R> request =
(SingleRequest<R>) POOL.acquire();
if (request == null) {
request = new SingleRequest<>();
}
request.init(
context,
glideContext,
model,
transcodeClass,
requestOptions,
overrideWidth,
overrideHeight,
priority,
target,
targetListener,
requestListeners,
requestCoordinator,
engine,
animationFactory);
return request;
}
}
可以看到,这里在第23行去new了一个SingleRequest对象,并在最后一行返回,也就是说,obtain()方法实际上获得的就是一个SingleRequest对象。另外这里又在第25行调用了SingleRequest的init(),里面主要就是一些赋值的代码,将传入的这些参数赋值到SingleRequest的成员变量当中,我们就不再跟进去看了。
好,那现在解决了构建Request对象的问题,接下来我们看一下这个Request对象又是怎么执行的。回到刚才的into()方法,你会发现在第18行调用了requestManager.track()方法来去执行这个Request,那么我们跟进去瞧一瞧,如下所示:
public class RequestManager implements LifecycleListener,
ModelTypes<RequestBuilder<Drawable>> {
...
void track(@NonNull Target<?> target, @NonNull Request request) {
targetTracker.track(target);
requestTracker.runRequest(request);
}
...
}
track()方法里调用了requestTracker.runRequest()方法,那么我们继续跟进去瞧一瞧,如下所示:
public class RequestTracker {
...
public void runRequest(@NonNull Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
request.clear();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Paused, delaying request");
}
pendingRequests.add(request);
}
}
...
}
这里有一个简单的逻辑判断,就是先判断Glide当前是不是处理暂停状态,如果不是暂停状态就调用Request的begin()方法来执行Request,否则的话就先将Request添加到待执行队列里面,等暂停状态解除了之后再执行。
暂停请求的功能仍然不是这篇文章所关心的,这里就直接忽略了,我们重点来看这个begin()方法。由于当前的Request对象是一个SingleRequest,因此这里就需要看SingleRequest中的begin()方法了,如下所示:
public final class SingleRequest<R> implements Request,
SizeReadyCallback,
ResourceCallback,
FactoryPools.Poolable {
...
@Override
public void begin() {
assertNotCallingCallbacks();
stateVerifier.throwIfRecycled();
startTime = LogTime.getLogTime();
if (model == null) {
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
width = overrideWidth;
height = overrideHeight;
}
// Only log at more verbose log levels if the user has set a fallback drawable, because
// fallback Drawables indicate the user expects null models occasionally.
int logLevel = getFallbackDrawable() == null ? Log.WARN : Log.DEBUG;
onLoadFailed(new GlideException("Received null model"), logLevel);
return;
}
if (status == Status.RUNNING) {
throw new IllegalArgumentException("Cannot restart a running request");
}
// If we're restarted after we're complete (usually via something like a notifyDataSetChanged
// that starts an identical request into the same Target or View), we can simply use the
// resource and size we retrieved the last time around and skip obtaining a new size, starting a
// new load etc. This does mean that users who want to restart a load because they expect that
// the view size has changed will need to explicitly clear the View or Target before starting
// the new load.
if (status == Status.COMPLETE) {
onResourceReady(resource, DataSource.MEMORY_CACHE);
return;
}
// Restarts for requests that are neither complete nor running can be treated as new requests
// and can run again from the beginning.
status = Status.WAITING_FOR_SIZE;
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
&& canNotifyStatusChanged()) {
target.onLoadStarted(getPlaceholderDrawable());
}
if (IS_VERBOSE_LOGGABLE) {
logV("finished run method in " + LogTime.getElapsedMillis(startTime));
}
}
...
}
这里我们来注意几个细节,首先如果model等于null,model也就是我们在第二步load()方法中传入的图片URL地址,这个时候会调用onLoadFailed()方法,代码如下:
public final class SingleRequest<R> implements Request,
SizeReadyCallback,
ResourceCallback,
FactoryPools.Poolable {
...
private void onLoadFailed(GlideException e, int maxLogLevel) {
stateVerifier.throwIfRecycled();
int logLevel = glideContext.getLogLevel();
if (logLevel <= maxLogLevel) {
Log.w(GLIDE_TAG, "Load failed for " + model + " with size [" + width + "x" + height + "]", e);
if (logLevel <= Log.INFO) {
e.logRootCauses(GLIDE_TAG);
}
}
loadStatus = null;
status = Status.FAILED;
isCallingCallbacks = true;
try {
//TODO: what if this is a thumbnail request?
boolean anyListenerHandledUpdatingTarget = false;
if (requestListeners != null) {
for (RequestListener<R> listener : requestListeners) {
anyListenerHandledUpdatingTarget |=
listener.onLoadFailed(e, model, target, isFirstReadyResource());
}
}
anyListenerHandledUpdatingTarget |=
targetListener != null
&& targetListener.onLoadFailed(e, model, target, isFirstReadyResource());
if (!anyListenerHandledUpdatingTarget) {
setErrorPlaceholder();
}
} finally {
isCallingCallbacks = false;
}
notifyLoadFailed();
}
...
}
我们跟到onLoadFailed()方法里面去看看,你会发现它最终会调用到一个setErrorPlaceholder()当中,如下所示:
public final class SingleRequest<R> implements Request,
SizeReadyCallback,
ResourceCallback,
FactoryPools.Poolable {
...
private void setErrorPlaceholder() {
if (!canNotifyStatusChanged()) {
return;
}
Drawable error = null;
if (model == null) {
error = getFallbackDrawable();
}
// Either the model isn't null, or there was no fallback drawable set.
if (error == null) {
error = getErrorDrawable();
}
// The model isn't null, no fallback drawable was set or no error drawable was set.
if (error == null) {
error = getPlaceholderDrawable();
}
target.onLoadFailed(error);
}
...
}
这个方法中会先去获取一个error的占位图,如果获取不到的话会再去获取一个loading占位图,然后调用target.onLoadFailed()方法并将占位图传入。那么onLoadFailed()方法中做了什么呢?我们看一下:
public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z>
implements Transition.ViewAdapter {
...
@Override
public void setDrawable(Drawable drawable) {
view.setImageDrawable(drawable);
}
/**
* Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
* android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
*
* @param placeholder {@inheritDoc}
*/
@Override
public void onLoadStarted(@Nullable Drawable placeholder) {
super.onLoadStarted(placeholder);
setResourceInternal(null);
setDrawable(placeholder);
}
/**
* Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
* android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
*
* @param errorDrawable {@inheritDoc}
*/
@Override
public void onLoadFailed(@Nullable Drawable errorDrawable) {
super.onLoadFailed(errorDrawable);
setResourceInternal(null);
setDrawable(errorDrawable);
}
...
}
很简单,其实就是将这张error占位图显示到ImageView上而已,因为现在出现了异常,没办法展示正常的图片了。而如果你仔细看下刚才begin()方法的第52行,你会发现它又调用了一个target.onLoadStarted()方法,并传入了一个loading占位图,在也就说,在图片请求开始之前,会先使用这张占位图代替最终的图片显示。这也是我们在上一篇文章中学过的RequestOptions中的placeholder()和error()这两个占位图API底层的实现原理。
好,那么我们继续回到begin()方法。刚才讲了占位图的实现,那么具体的图片加载又是从哪里开始的呢?是在begin()方法的第45行和第47行。这里要分两种情况,一种是你在RequestOptions中使用了override() API为图片指定了一个固定的宽高,一种是没有指定。如果指定了的话,就会执行第45行代码,调用onSizeReady()方法。如果没指定的话,就会执行第47行代码,调用target.getSize()方法。这个target.getSize()方法的内部会根据ImageView的layout_width和layout_height值做一系列的计算,来算出图片应该的宽高。具体的计算细节我就不带着大家分析了,总之在计算完之后,它也会调用onSizeReady()方法。也就是说,不管是哪种情况,最终都会调用到onSizeReady()方法,在这里进行下一步操作。那么我们跟到这个方法里面来:
public final class SingleRequest<R> implements Request,
SizeReadyCallback,
ResourceCallback,
FactoryPools.Poolable {
...
@Override
public void onSizeReady(int width, int height) {
stateVerifier.throwIfRecycled();
if (IS_VERBOSE_LOGGABLE) {
logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
if (status != Status.WAITING_FOR_SIZE) {
return;
}
status = Status.RUNNING;
float sizeMultiplier = requestOptions.getSizeMultiplier();
this.width = maybeApplySizeMultiplier(width, sizeMultiplier);
this.height = maybeApplySizeMultiplier(height, sizeMultiplier);
if (IS_VERBOSE_LOGGABLE) {
logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
}
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);
// This is a hack that's only useful for testing right now where loads complete synchronously
// even though under any executor running on any thread but the main thread, the load would
// have completed asynchronously.
if (status != Status.RUNNING) {
loadStatus = null;
}
if (IS_VERBOSE_LOGGABLE) {
logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
}
...
}
在onSizeReady()方法的第26行,调用了Engine的load()方法。接下来我们就要看一看,这个Engine的load()方法当中,到底做了什么?代码如下所示:
public class Engine implements EngineJobListener,
MemoryCache.ResourceRemovedListener,
EngineResource.ResourceListener {
...
public <R> LoadStatus load(
GlideContext glideContext,
Object model,
Key signature,
int width,
int height,
Class<?> resourceClass,
Class<R> transcodeClass,
Priority priority,
DiskCacheStrategy diskCacheStrategy,
Map<Class<?>, Transformation<?>> transformations,
boolean isTransformationRequired,
boolean isScaleOnlyOrNoTransform,
Options options,
boolean isMemoryCacheable,
boolean useUnlimitedSourceExecutorPool,
boolean useAnimationPool,
boolean onlyRetrieveFromCache,
ResourceCallback cb) {
Util.assertMainThread();
long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;
EngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,
resourceClass, transcodeClass, options);
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
cb.onResourceReady(active, DataSource.MEMORY_CACHE);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
cb.onResourceReady(cached, DataSource.MEMORY_CACHE);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Loaded resource from cache", startTime, key);
}
return null;
}
EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);
if (current != null) {
current.addCallback(cb);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Added to existing load", startTime, key);
}
return new LoadStatus(cb, current);
}
EngineJob<R> engineJob =
engineJobFactory.build(
key,
isMemoryCacheable,
useUnlimitedSourceExecutorPool,
useAnimationPool,
onlyRetrieveFromCache);
DecodeJob<R> decodeJob =
decodeJobFactory.build(
glideContext,
model,
key,
signature,
width,
height,
resourceClass,
transcodeClass,
priority,
diskCacheStrategy,
transformations,
isTransformationRequired,
isScaleOnlyOrNoTransform,
onlyRetrieveFromCache,
options,
engineJob);
jobs.put(key, engineJob);
engineJob.addCallback(cb);
engineJob.start(decodeJob);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
...
}
load()方法中的代码虽然有点长,但大多数的代码都是在处理缓存的。关于缓存的内容我们会在下一篇文章当中学习,现在只需要从第59行看起就行。这里构建了一个EngineJob,它的主要作用就是用来开启线程的,为后面的异步加载图片做准备。接下来第67行创建了一个DecodeJob对象,从名字上来看,它好像是用来对图片进行解码的,但实际上它的任务十分繁重,待会我们就知道了。继续往下看,在89行调用了EngineJob的start()方法来运行DecodeJob,进入DecodeJob类,我们发现该类继承Runnable接口,也就是说真正执行线程任务的是DecodeJob类,那我们就看看DecodeJob类中run()方法做了啥,如下所示:
class DecodeJob<R> implements DataFetcherGenerator.FetcherReadyCallback,
Runnable,
Comparable<DecodeJob<?>>,
Poolable {
...
@Override
public void run() {
// This should be much more fine grained, but since Java's thread pool implementation silently
// swallows all otherwise fatal exceptions, this will at least make it obvious to developers
// that something is failing.
GlideTrace.beginSectionFormat("DecodeJob#run(model=%s)", model);
// Methods in the try statement can invalidate currentFetcher, so set a local variable here to
// ensure that the fetcher is cleaned up either way.
DataFetcher<?> localFetcher = currentFetcher;
try {
if (isCancelled) {
notifyFailed();
return;
}
runWrapped();
} catch (Throwable t) {
// Catch Throwable and not Exception to handle OOMs. Throwables are swallowed by our
// usage of .submit() in GlideExecutor so we're not silently hiding crashes by doing this. We
// are however ensuring that our callbacks are always notified when a load fails. Without this
// notification, uncaught throwables never notify the corresponding callbacks, which can cause
// loads to silently hang forever, a case that's especially bad for users using Futures on
// background threads.
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "DecodeJob threw unexpectedly"
+ ", isCancelled: " + isCancelled
+ ", stage: " + stage, t);
}
// When we're encoding we've already notified our callback and it isn't safe to do so again.
if (stage != Stage.ENCODE) {
throwables.add(t);
notifyFailed();
}
if (!isCancelled) {
throw t;
}
} finally {
// Keeping track of the fetcher here and calling cleanup is excessively paranoid, we call
// close in all cases anyway.
if (localFetcher != null) {
localFetcher.cleanup();
}
GlideTrace.endSection();
}
}
}
这个方法中的代码并不多,但我们仍然还是要抓重点。在第22行,这里调用了一个runWrapped()方法,看上去所有的逻辑应该都在这个runWrapped()方法执行的了,那我们跟进去瞧一瞧:
本文地址:https://blog.csdn.net/wangzhongshun/article/details/109361085
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