前言:开发的过程中一定少不了图片加载库，对于Android图片加载库，我们在最常听到的有老牌图片加载框架UniversalImageLoader，Glide和Picasso，还有Facebook的Fresco。关于老牌框架UIL在这里先不谈了，我们的目的也是选择一款适合自己的框架，关于Glide和Picasso，可以说这两个框架就像龙凤胎，有太多的相似点，它们的比较看这篇文章，里面详细的比较了这两款框架，关于Fresco,可以说是结合了许多图片库的优点，基本满足了所有的网络图片展示需求，拥有三级缓存，内存管理是它的特色，但是api没有其他的使用起来简便，也不能加载gif图，并且体积几乎有5M, 对比glide只有几百k，所以一般使用这个框架的都是对图片处理需求大要求高那种。所以根据自己的偏好选择一款就好了（没有最好的框架，只有最适合自己的框架）。本文是偏爱Glide，并对加载流程进行分析。
本文章是基于Glide V4版本, 和V3会有差异
关于V4和V3版本的差异，参考官网说明，当你读完源码，可能就更能理解所做的改变了。Glide从v3迁移到v4
全篇篇幅较长 编辑的时候出现了好多次如下图片，所以分了两篇写加载的过程。感觉能把代码撸到最后的都是战士。。
关于Glide加载图片最常使用的一句，也是最简单的语句就是：：
Glide.with(this).load(url).into(imageView);
三步走：先with()，再load()，最后into()
看似简单的一句话，却想不到在这句话下面做了多少的处理，看到最后，会惊异于Glide库的强大。
话不多少，我就是从头一点一点的过到最后，耗时也是蛮久，中间几度迷路在代码中，看到眼睛都要花了，
先放一张总结的思维脑图，帮助大家理清
1.Glide.with()
with()方法是Glide类中的一组静态方法，它有好几个方法重载,正是通过这几个重载,with()方法才可以接收Context、Activity或者Fragment类型的参数。也就是说我们选择的范围非常广，不管是在Activity还是Fragment中调用with()方法，都可以直接传this。那如果调用的地方既不在Activity中也不在Fragment中呢？也没关系，我们可以获取当前应用程序的ApplicationContext，传入到with()方法当中。
public class Glide {
...省略
/*
* Begin a load with Glide by passing in a context.
* @param context Any context, will not be retained.
* @return A RequestManager for the top level application that can be used to start a load.
* @see #with(android.app.Activity)
* @see #with(android.app.Fragment)
* @see #with(android.support.v4.app.Fragment)
* @see #with(android.support.v4.app.FragmentActivity)
*/
public static RequestManager with(Context context) {
return getRetriever(context).get(context);
}
public static RequestManager with(Activity activity) {
return getRetriever(activity).get(activity);
}
public static RequestManager with(FragmentActivity activity) {
return getRetriever(activity).get(activity);
}
public static RequestManager with(android.app.Fragment fragment) {
return getRetriever(fragment.getActivity()).get(fragment);
}
public static RequestManager with(Fragment fragment) {
return getRetriever(fragment.getActivity()).get(fragment);
}
public static RequestManager with(View view) {
return getRetriever(view.getContext()).get(view);
}
}
然后在重载的方法中通过getRetriever(传入context)
public class Glide {
...省略
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();
}
}
通过静态get()方法得到一个Glide对象，单例实现，保证一个应用程序只有一个实例。在 checkAndInitializeGlide(context);这个方法中做一些实例重要对象的操作。（自行查看。比如缓存对象、注解、管理请求的工厂等）然后通过getRequestManagerRetriever()返回一个RequestManagerRetriever对象
public class Glide {
...省略
/**
* Get the singleton.
*
* @return the singleton
*/
public static Glide get(Context context) {
if (glide == null) {
synchronized (Glide.class) {
if (glide == null) {
checkAndInitializeGlide(context);
}
}
}
return glide;
}
public RequestManagerRetriever getRequestManagerRetriever() {
return requestManagerRetriever;
}
}
最后调用RequestManagerRetriever的实例get()方法的一系列重载，去获取RequestManager对象。（Glide.with()的产物）
RequestManagerRetriever类
public class RequestManagerRetriever implements Handler.Callback {
...省略
public RequestManager get(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);
}
public RequestManager get(FragmentActivity activity) {
if (Util.isOnBackgroundThread()) {
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
FragmentManager fm = activity.getSupportFragmentManager();
return supportFragmentGet(activity, fm, null /*parentHint*/);
}
}
public RequestManager get(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);
}
}
public RequestManager get(Activity activity) {
if (Util.isOnBackgroundThread()) {
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
android.app.FragmentManager fm = activity.getFragmentManager();
return fragmentGet(activity, fm, null /*parentHint*/);
}
}
public RequestManager get(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());
}
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);
}
}
从这些get重载方法里我们可以了解到：
-
对于Application类型参数：
-
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。
-
如果传入的就是Application类型参数(Glide.with(getApplicationContext()))，那么会通过getApplicationManager()方法返回RequestManager。Glide加载图片的生命周期是和with里的参数保持一致，而对于Application类型，只有当应用程序被杀掉的时候，图片加载才会停止。
-
RequestManagerRetriever类
...
private final RequestManagerFactory factory;
private volatile RequestManager applicationManager;
...
private RequestManager getApplicationManager(Context context) {
if (applicationManager == null) {
synchronized (this) {
if (applicationManager == null) {
Glide glide = Glide.get(context.getApplicationContext());
applicationManager =
factory.build(
glide,
new ApplicationLifecycle(),
new EmptyRequestManagerTreeNode(),
context.getApplicationContext());
}
}
}
return applicationManager;
}
/**
* Used internally to create {@link RequestManager}s.
*/
public interface RequestManagerFactory {
RequestManager build(
Glide glide,
Lifecycle lifecycle,
RequestManagerTreeNode requestManagerTreeNode,
Context context);
}
-
对于非Application类型：
最终的流程是一样的：
首先是获取 自己的FragmentManager，然后
-
Fragment类型调用supportFragmentGet()方法，在这个方法里调用getSupportRequestManagerFragment()
-
Activity类型调用fragmentGet()方法，在这个方法里调用getRequestManagerFragment()
可以看到在这两个方法做了相同的事：添加一个新的Fragment---RequestManagerFragment到当前页面;而这个Fragment的作用就是为了方便Glide控制生命周期,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。
RequestManagerRetriever类
private RequestManager supportFragmentGet(Context context, FragmentManager fm,
Fragment parentHint) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
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;
}
private RequestManager fragmentGet(Context context, android.app.FragmentManager fm,
android.app.Fragment parentHint) {
RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
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;
}
SupportRequestManagerFragment getSupportRequestManagerFragment(
final FragmentManager fm, Fragment parentHint) {
SupportRequestManagerFragment current =
(SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
current = pendingSupportRequestManagerFragments.get(fm);
if (current == null) {
current = new SupportRequestManagerFragment();
current.setParentFragmentHint(parentHint);
pendingSupportRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
RequestManagerFragment getRequestManagerFragment(
final android.app.FragmentManager fm, android.app.Fragment parentHint) {
RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
current = pendingRequestManagerFragments.get(fm);
if (current == null) {
current = new RequestManagerFragment();
current.setParentFragmentHint(parentHint);
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
RequestManagerFragment类
public RequestManagerFragment() {
this(new ActivityFragmentLifecycle());
}
// For testing only.
@SuppressLint("ValidFragment")
RequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
this.lifecycle = lifecycle;
}
至此 我们了解了Glide.with()方法背后的操作。一系列操作最终得到的是RequestManager
2.load()
load()方法明显就在RequestManager类，那我们接下来看一下这个load方法做了什么
public class RequestManager implements LifecycleListener {
...省略
/**
* A helper method equivalent to calling {@link #asDrawable()} and then {@link
* RequestBuilder#load(Object)} with the given model.
*
* @return A new request builder for loading a {@link Drawable} using the given model.
*/
@CheckResult
public RequestBuilder load(@Nullable Object model) {
return asDrawable().load(model);
}
@CheckResult
public RequestBuilder asDrawable() {
return as(Drawable.class);
}
@CheckResult
public RequestBuilder as(Class resourceClass) {
return new RequestBuilder<>(glide, this, resourceClass, context);
}
}
能调用RequestManager类的load方法，说明还没有指定类型，此时load方法默认置为asDrawable()，然后通过返回的RequestBuilder类中的load()加载图片，最终返回RequestBuilder。
而我们知道实际使用的时候会有加载为asBitmap、asFile()等操作，此时我们看这几个方法
RequestManager类
private static final RequestOptions DECODE_TYPE_BITMAP = decodeTypeOf(Bitmap.class).lock();
private static final RequestOptions DECODE_TYPE_GIF = decodeTypeOf(GifDrawable.class).lock();
@CheckResult
public RequestBuilder asBitmap() {
return as(Bitmap.class).apply(DECODE_TYPE_BITMAP);
}
@CheckResult
public RequestBuilder asGif() {
return as(GifDrawable.class).apply(DECODE_TYPE_GIF);
}
@CheckResult
public RequestBuilder asDrawable() {
return as(Drawable.class);
}
@CheckResult
public RequestBuilder asFile() {
return as(File.class).apply(skipMemoryCacheOf(true));
}
@CheckResult
public RequestBuilder as(Class resourceClass) {
return new RequestBuilder<>(glide, this, resourceClass, context);
}
然后调用RequestManager类的apply方法返回RequestBuilder
RequestManager类
@CheckResult
public RequestBuilder apply(@NonNull RequestOptions requestOptions) {
Preconditions.checkNotNull(requestOptions);
this.requestOptions = getMutableOptions().apply(requestOptions);
return this;
}
此时load还是由RequestBuilder类调用，所以就是先确定类型，如果不指定 默认为asDrawable
而对于RequestBuilder类的load方法也有多个重载，这个方法用于指定待加载的图片资源，所以也就说明Glide支持加载各种各样的图片资源，包括网络图片、本地图片、应用资源、二进制流、Uri对象，Url对象，String字符串
RequestBuilder类
public class RequestBuilder implements Cloneable {
public RequestBuilder load(@Nullable Object model) {
return loadGeneric(model);
}
@CheckResult
public RequestBuilder load(@Nullable Bitmap bitmap) {
return loadGeneric(bitmap)
.apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}
@CheckResult
public RequestBuilder load(@Nullable Drawable drawable) {
return loadGeneric(drawable)
.apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}
@CheckResult
public RequestBuilder load(@Nullable String string) {
return loadGeneric(string);
}
public RequestBuilder load(@Nullable Uri uri) {
return loadGeneric(uri);
}
@CheckResult
public RequestBuilder load(@Nullable File file) {
return loadGeneric(file);
}
@CheckResult
public RequestBuilder load(@RawRes @DrawableRes @Nullable Integer resourceId) {
return loadGeneric(resourceId).apply(signatureOf(ApplicationVersionSignature.obtain(context)));
}
public RequestBuilder load(@Nullable URL url) {
return loadGeneric(url);
}
@CheckResult
public RequestBuilder load(@Nullable byte[] model) {
return loadGeneric(model).apply(signatureOf(new ObjectKey(UUID.randomUUID().toString()))
.diskCacheStrategy(DiskCacheStrategy.NONE).skipMemoryCache(true /*skipMemoryCache*/));
}
private RequestBuilder loadGeneric(@Nullable Object model) {
this.model = model;
isModelSet = true;
return this;
}
}
所以实际使用时,我们还可以根据需要这样使用load()方法：
// 加载本地图片
File file = new File(getExternalCacheDir() + "/image.jpg");
Glide.with(this).load(file).into(imageView);
// 加载应用资源
int resource = R.drawable.image;
Glide.with(this).load(resource).into(imageView);
// 加载二进制流
byte[] image = getImageBytes();
Glide.with(this).load(image).into(imageView);
// 加载Uri对象
Uri imageUri = getImageUri();
Glide.with(this).load(imageUri).into(imageView);