Glide源码之生命周期实现Glide源码之生命周期实现
Glide源码之生命周期实现Glide源码之生命周期实现
Glide这个图片加载框架有个比较好的一点就是图片加载回根据Activity或Fragment生命周期的变化而变化，假如Activity或者Fragment销毁的时候，Glide会停止加载，以节省资源防止内存泄漏。至于Glide内部是怎么实现这个功能，这就是我们这一篇的内容。 首先，我们又来看一下Glide最基础的加载语句：
Glide.with(this).load("url").into(imageview)而实现生命周期的绑定最重要的语句就是Glide.with(this)，在with()方法里会传入Activity或者Fragment的对象，以便进行绑定。with()方法代码如下(注:以Activity为例):
public static RequestManager with(@NonNull Activity activity) {
return getRetriever(activity).get(activity);
}而with()方法里，与生命周期相关的就是get()方法，源码如下:
public RequestManager get(@NonNull Activity activity) {
if (Util.isOnBackgroundThread()) {
//假如是后台线程调用
return get(activity.getApplicationContext());
} else if (activity instanceof FragmentActivity) {
return get((FragmentActivity) activity);
} else {
//上面的代码最终都会执行这个，所以这里是最重要的。
assertNotDestroyed(activity);
frameWaiter.registerSelf(activity);
android.app.FragmentManager fm = activity.getFragmentManager();
return fragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
}
}上面的代码最终都会走最后一步，即获取FragmentManager，然后传给fragmentGet()方法，这个方法才是生成管理生命周期的关键：
private RequestManager fragmentGet(@NonNull Context context,
@NonNull android.app.FragmentManager fm,
@Nullable android.app.Fragment parentHint,
boolean isParentVisible) {
//创建RequestManagerFragment对象
RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
//获取请求管理类
RequestManager requestManager = current.getRequestManager();
if (requestManager == null) {
//假如没有，就生产该对象
Glide glide = Glide.get(context);
requestManager =factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
if (isParentVisible) {
requestManager.onStart();
}
current.setRequestManager(requestManager);
}
return requestManager;
}
private RequestManagerFragment getRequestManagerFragment(
@NonNull final android.app.FragmentManager fm,
@Nullable android.app.Fragment parentHint) {
//从FragmentManager获取RequestManagerFragment看有没有
RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
//FragmentManager没有RequestManagerFragment，再从Glide缓存拿
current = pendingRequestManagerFragments.get(fm);
if (current == null) {
//假如也没有，就生产新的RequestManagerFragment对象
current = new RequestManagerFragment();
current.setParentFragmentHint(parentHint);
//将新的对象保存起来
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}上面的注释表明的基本流程，其中第一步就是获取RequestManagerFragment对象，再看看getRequestManagerFragment()里面的方法，，也没什么特别就是获取RequestManagerFragment再保存起来，那RequestManagerFragment是什么呢，其实RequestManagerFragment就是一个Fragment,
public class RequestManagerFragment extends Fragment {
//省略....
public RequestManagerFragment() {
this(new ActivityFragmentLifecycle());
}
RequestManagerFragment(@NonNull ActivityFragmentLifecycle lifecycle) {
this.lifecycle = lifecycle;
}
//省略....
}我们刚才传入的Activity，生成一个Fragment用来干嘛？其实RequestManagerFragment是一个空的fragment，没有布局和View操作，与Activity绑定，仅仅用来监听生命周期。
在RequestManagerFragment创建的时候会通过构造器创建ActivityFragmentLifecycle对象，他是生命周期的回调监听，实现了Lifecycle接口。
public interface Lifecycle {
void addListener(@NonNull LifecycleListener listener);
void removeListener(@NonNull LifecycleListener listener);
}
class ActivityFragmentLifecycle implements Lifecycle {
private final Set<LifecycleListener> lifecycleListeners =
Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
private boolean isStarted;
private boolean isDestroyed;
@Override
public void addListener(@NonNull LifecycleListener listener) {
lifecycleListeners.add(listener);
if (isDestroyed) {
listener.onDestroy();
} else if (isStarted) {
listener.onStart();
} else {
listener.onStop();
}
}
@Override
public void removeListener(@NonNull LifecycleListener listener) {
lifecycleListeners.remove(listener);
}
void onStart() {
isStarted = true;
for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
lifecycleListener.onStart();
}
}
void onStop() {
isStarted = false;
for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
lifecycleListener.onStop();
}
}
void onDestroy() {
isDestroyed = true;
for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
lifecycleListener.onDestroy();
}
}
}Lifecycle负责实现注册和移除LifecycleListener接口，而ActivityFragmentLifecycle集成Lifecycle,同时还包含还包含onStart，onStop，onDestroy这三个方法，这三个方法是对应这个RequestManagerFragment也就是Frgament的onStart，onStop，onDestroy三个回调方法。当创建RequestManagerFragment结束后，当前页面加载的时候生命周期传递到RequestManagerFragment，就会执行对象的生命周期的方法，当执行onStart时候，调用ActivityFragmentLifecycle的onStart，当执行onStop时候，调用ActivityFragmentLifecycle的onStop由此类推。 不过还有一个问题，就是ActivityFragmentLifecycle的addListener是在哪里赋值的，当创建完RequestManagerFragment后，还会调
requestManager =factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);生产RequestManager，监听就在这个时候，源码如下：
private static final RequestManagerRetriever.RequestManagerFactory DEFAULT_FACTORY = new RequestManagerRetriever.RequestManagerFactory() {
//对应这个上面的build方法
@NonNull
public RequestManager build(@NonNull Glide glide, @NonNull Lifecycle lifecycle, @NonNull RequestManagerTreeNode requestManagerTreeNode, @NonNull Context context) {
return new RequestManager(glide, lifecycle, requestManagerTreeNode, context);
}
};
//生产RequestManager对象
public RequestManager(
@NonNull Glide glide,
@NonNull Lifecycle lifecycle,
@NonNull RequestManagerTreeNode treeNode,
@NonNull Context context) {
this(
glide,
lifecycle,
treeNode,
new RequestTracker(),
glide.getConnectivityMonitorFactory(),
context);
}
RequestManager(
Glide glide,
Lifecycle lifecycle,
RequestManagerTreeNode treeNode,
RequestTracker requestTracker,
ConnectivityMonitorFactory factory,
Context context) {
this.glide = glide;
this.lifecycle = lifecycle;
this.treeNode = treeNode;
this.requestTracker = requestTracker;
this.context = context;
//省略代码...
//设置生命周期监听事件
if (Util.isOnBackgroundThread()) {
Util.postOnUiThread(addSelfToLifecycle);
} else {
lifecycle.addListener(this);
}
lifecycle.addListener(connectivityMonitor);
//省略代码...
} 上面的current.getGlideLifecycle()参数，该参数就是ActivityFragmentLifecycle，然后调用ActivityFragmentLifecycle的addListener方法进行注册，这样ActivityFragmentLifecycle生命周期管理类执行对应生命周期的时候就会调用RequestManager中的对应方法。对应的onStart就会开启网络请求，和设置ImageView监听到ImageView的onStart方法，onStart方法就这样监听到了。
当调ActivityFragmentLifecycle的onStart，onStop，onDestroy方法就会回调RequestManager的onStart，onStop，onDestroy方法，如下代码：
public class RequestManager implements LifecycleListener, ……{
@Override
public synchronized void onStart() {
//发起请求
resumeRequests();
//View的Target回调
targetTracker.onStart();
}
@Override
public synchronized void onStop() {
//暂停请求
pauseRequests();
//View的Target回调
targetTracker.onStop();
}
@Override
public synchronized void onDestroy() {
销毁请求
targetTracker.onDestroy();
//View的Target回调
for (Target<?> target : targetTracker.getAll()) {
clear(target);
}
//清空回调信息
targetTracker.clear();
requestTracker.clearRequests();
lifecycle.removeListener(this);
lifecycle.removeListener(connectivityMonitor);
mainHandler.removeCallbacks(addSelfToLifecycle);
glide.unregisterRequestManager(this);
}
}这样就实现了Glide的生命周期管理，管理了请求开发，暂停，销毁这三个状态，使得更加节省资源，防止内存泄漏。也就是这个的基本流程