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链接：https://mp.weixin.qq.com/s/uTv44vJFFJI_l6b5YKSXYQ
作者：连凌能
Android App中图片的展示是很基本也很重要的一个功能，在Android平台上有很多的图片加载解决方案，但是官方认可的是Glide。Android App的页面是有生命周期的，Glide比较好的一个功能就是具有生命周期管理功能，能够根据页面和APP的生命周期来管理图片的加载和停止，也开放接口供用户在内存紧张时手动进行内存管理。本文重点是生命周期源码的分析，不会从简单的使用着手。
这是Glide源码分析的第二篇文章，第一篇是《Glide缓存流程》，从资源的获取流程对源码进行分析。本篇会聚焦于生命周期模块的原理。开始之前先思考下面这几个问题：
Glide怎么实现页面生命周期？
Glide为什么对Fragment做缓存？
Glide如何监听网络变化？
先来看with函数的执行, 会构造glide单例，而
RequestManagerRetriever在initializeGlide中会进行构造。
// Glide.java
public static RequestManager with(@NonNull Activity activity) {
return getRetriever(activity).get(activity);
}
@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();
}
@NonNull
public static Glide get(@NonNull Context context) {
if (glide == null) {
synchronized (Glide.class) {
if (glide == null) {
checkAndInitializeGlide(context);
}
}
}
return glide;
}
private static void checkAndInitializeGlide(@NonNull Context context) {
// In the thread running initGlide(), one or more classes may call Glide.get(context).
// Without this check, those calls could trigger infinite recursion.
if (isInitializing) {
throw new IllegalStateException("You cannot call Glide.get() in registerComponents(),"
+ " use the provided Glide instance instead");
}
isInitializing = true;
initializeGlide(context);
isInitializing = false;
}构造完成RequestManagerRetriever通过get返回一个 RequestManager, 如果不在主线程，默认会传入 getApplicationContext,也就是不进行生命周期管理：
在getRequestManagerFragment中先查看当前Activity中有没有FRAGMENT_TAG这个标签对应的Fragment，如果有就直接返回
如果没有，会判断pendingRequestManagerFragments中有没有，如果有就返回
// RequestManagerRetriever.java
@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));
}
}
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;
}
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;
}
public boolean handleMessage(Message message) {
...
switch (message.what) {
case ID_REMOVE_FRAGMENT_MANAGER:
android.app.FragmentManager fm = (android.app.FragmentManager) message.obj;
key = fm;
removed = pendingRequestManagerFragments.remove(fm);
break;
...
}
...
}这里面需要注意一个问题，就是如果with()函数中传进来的不是Activity，而是Fragment，那么也会去创建一个没有界面的RequestManagerFragment,而它的父Fragment就是传进来的Fragment。
上面为什么需要pendingRequestManagerFragments先进行缓存呢？这个放到下面第二个问题中说明。先接着往下看生命周期的传递。
RequestManagerFragment是一个很重要的类，Glide就是通过它作为生命周期的分发入口，RequestManagerFragment的默认构造函数会实例化一个ActivityFragmentLifecycle，在每个生命周期onStart/onStop/onDestroy中会调用ActivityFragmentLifecycle：
// RequestManagerFragment.java
public class RequestManagerFragment extends Fragment {
private static final String TAG = "RMFragment";
private final ActivityFragmentLifecycle lifecycle;
@Nullable private RequestManager requestManager;
public RequestManagerFragment() {
this(new ActivityFragmentLifecycle());
}
RequestManagerFragment(@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();
}
...
}RequestManagerFragment里面有一个实例RequestManager,在前面的fragmentGet,RequestManagerFragment拿到以后会尝试获取它的RequestManager，第一次获取肯定是没有,就会重新构造一个, 通过RequestManagerRetriever构造时传入的RequestManagerFactory工厂类实例化一个RequestManager, 把RequestManagerFragment中的ActivityFragmentLifecycle传进去：
// RequestManagerRetriever.java
public interface RequestManagerFactory {
@NonNull
RequestManager build(
@NonNull Glide glide,
@NonNull Lifecycle lifecycle,
@NonNull RequestManagerTreeNode requestManagerTreeNode,
@NonNull Context context);
}
private static final RequestManagerFactory DEFAULT_FACTORY = new RequestManagerFactory() {
@NonNull
@Override
public RequestManager build(@NonNull Glide glide, @NonNull Lifecycle lifecycle,
@NonNull RequestManagerTreeNode requestManagerTreeNode, @NonNull Context context) {
return new RequestManager(glide, lifecycle, requestManagerTreeNode, context);
}
};很明显生命周期的关键就在ActivityFragmentLifecycle, 在RequestManagerFragment中相应生命周期中会回调它，那么猜测它肯定是在里面维护了一个观察者列表，相应事件发生的时候进行通知, 看下它的源码：
// ActivityFragmentLifecycle.java
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();
}
}
}所以RequestManagerFragment把这个传给RequestManager后，肯定会注册观察者，看一下RequestManager的相关代码,在构造函数里面lifecycle.addListener(this);,把自己注册为观察者：
// RequestManager.java
public class RequestManager implements LifecycleListener,
ModelTypes<RequestBuilder<Drawable>> {
...
protected final Glide glide;
protected final Context context;
@Synthetic final Lifecycle lifecycle;
private final RequestTracker requestTracker;
private final RequestManagerTreeNode treeNode;
private final TargetTracker targetTracker = new TargetTracker();
private final Runnable addSelfToLifecycle = new Runnable() {
@Override
public void run() {
lifecycle.addListener(RequestManager.this);
}
};
private final Handler mainHandler = new Handler(Looper.getMainLooper());
private final ConnectivityMonitor connectivityMonitor;
private RequestOptions requestOptions;
public RequestManager(
@NonNull Glide glide, @NonNull Lifecycle lifecycle,
@NonNull RequestManagerTreeNode treeNode, @NonNull Context context) {
this(
glide,
lifecycle,
treeNode,
new RequestTracker(),
glide.getConnectivityMonitorFactory(),
context);
}
// Our usage is safe here.
@SuppressWarnings("PMD.ConstructorCallsOverridableMethod")
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;
connectivityMonitor =
factory.build(
context.getApplicationContext(),
new RequestManagerConnectivityListener(requestTracker));
if (Util.isOnBackgroundThread()) {
mainHandler.post(addSelfToLifecycle);
} else {
lifecycle.addListener(this);
}
lifecycle.addListener(connectivityMonitor);
setRequestOptions(glide.getGlideContext().getDefaultRequestOptions());
glide.registerRequestManager(this);
}在看下RequestManager对应的生命周期里面, 在这里面分别启动，停止和销毁请求：
// RequestManager
@Override
public void onStart() {
resumeRequests();
targetTracker.onStart();
}
@Override
public void onStop() {
pauseRequests();
targetTracker.onStop();
}
@Override
public void onDestroy() {
targetTracker.onDestroy();
for (Target<?> target : targetTracker.getAll()) {
clear(target);
}
targetTracker.clear();
requestTracker.clearRequests();
lifecycle.removeListener(this);
lifecycle.removeListener(connectivityMonitor);
mainHandler.removeCallbacks(addSelfToLifecycle);
glide.unregisterRequestManager(this);
}再贴一次RequestManagerRetriever中获取Fragment的代码，前面留了一个疑问，为什么这里会需要一个pendingRequestManagerFragments对Fragment进行缓存。
// RequestManagerRetriever.java
/**
* Pending adds for RequestManagerFragments.
*/
@SuppressWarnings("deprecation")
@VisibleForTesting
final Map<android.app.FragmentManager, RequestManagerFragment> pendingRequestManagerFragments = new HashMap<>();
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;
}我们看一个情况：
Glide.with(Context).load(ImageUrl1).into(imageview1); // task1
Glide.with(Context).load(ImageUrl2).into(imageview2); // task2Android开发应该都知道主线程有一个Handler机制，会往消息队列中放消息，通过Looper按顺序取出来执行。那么主线程中的执行顺序和消息队列中的执行顺序关系是什么？看个栗子：
private void start() {
mHandler = new Handler(getMainLooper());
VLog.i("HandlerRunT", "=========Begin!============");
mHandler.post(new Runnable() {
@Override
public void run() {
VLog.i("HandlerRunT", "=========First!============");
}
});
VLog.i("HandlerRunT", "=========Middle!============");
mHandler.sendMessage(Message.obtain(mHandler, new Runnable() {
@Override
public void run() {
VLog.i("HandlerRunT", "=========Second!============");
}
}));
VLog.i("HandlerRunT", "=========End!============");
Next();
}
private void Next() {
VLog.i("HandlerRunT", "=========Next Begin!============");
mHandler.post(new Runnable() {
@Override
public void run() {
VLog.i("HandlerRunT", "=========Next First!============");
}
});
VLog.i("HandlerRunT", "=========Next Middle!============");
mHandler.sendMessage(Message.obtain(mHandler, new Runnable() {
@Override
public void run() {
VLog.i("HandlerRunT", "=========Next Second!============");
}
}));
VLog.i("HandlerRunT", "=========Next End!============");
}在start中打印的顺序和它里面的Handler中的信息哪个先打印？start中handler的信息和Next函数中的信息打印顺序是怎样的？看下打印结果：
HandlerRunT: =========Begin!============
HandlerRunT: =========Middle!============
HandlerRunT: =========End!============
HandlerRunT: =========Next Begin!============
HandlerRunT: =========Next Middle!============
HandlerRunT: =========Next End!============
HandlerRunT: =========First!============
HandlerRunT: =========Second!============
HandlerRunT: =========Next First!============
HandlerRunT: =========Next Second!============Handler中的顺序会在主线程之后，Handler中的消息执行顺序就是队列先进先出。
上面执行到task1的时候,在下面这两行代码，add操作会往消息队列放一个消息，这里标记为msg1:
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();// FragmentManager.java
public void enqueueAction(OpGenerator action, boolean allowStateLoss) {
if (!allowStateLoss) {
checkStateLoss();
}
synchronized (this) {
if (mDestroyed || mHost == null) {
if (allowStateLoss) {
// This FragmentManager isn't attached, so drop the entire transaction.
return;
}
throw new IllegalStateException("Activity has been destroyed");
}
if (mPendingActions == null) {
mPendingActions = new ArrayList<>();
}
mPendingActions.add(action);
scheduleCommit();
}
}
private void scheduleCommit() {
synchronized (this) {
boolean postponeReady =
mPostponedTransactions != null && !mPostponedTransactions.isEmpty();
boolean pendingReady = mPendingActions != null && mPendingActions.size() == 1;
if (postponeReady || pendingReady) {
mHost.getHandler().removeCallbacks(mExecCommit);
mHost.getHandler().post(mExecCommit);
}
}
}那么如果不把task1中构造的RequestManagerFragment放到pendingRequestManagerFragments中，那么在执行task2的时候也会再重新构造一个RequestManagerFragment,并且往主线程中放一个消息msg2,这个时候就会出现重复add的情况。
所以在前面new 出来一个RequestManagerFragment,随后就把它放到pendingRequestManagerFragments中，那么task2再进来的时候从缓存中能取到，就不会再重新new和add了。
那么下一个问题来了，为什么会出现下面这行代码，add后又需要马上发一个消息remove掉？在前面阻止掉task2重复new和add的操作后，就把这个缓存删掉，可以避免内存泄漏和内存压力：
// RequestManagerRetriever.java
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();从上面页面生命周期的分析部分知道，对于任务的控制都是通过RequestManager，还是到它里面去看，实现网络变化监听的就是ConnectivityMonitor:
// RequestManager.java
public class RequestManager implements LifecycleListener,
ModelTypes<RequestBuilder<Drawable>> {
...
protected final Glide glide;
protected final Context context;
@Synthetic final Lifecycle lifecycle;
private final RequestTracker requestTracker;
private final RequestManagerTreeNode treeNode;
private final TargetTracker targetTracker = new TargetTracker();
private final Handler mainHandler = new Handler(Looper.getMainLooper());
private final ConnectivityMonitor connectivityMonitor;
...
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;
connectivityMonitor =
factory.build(
context.getApplicationContext(),
new RequestManagerConnectivityListener(requestTracker));
if (Util.isOnBackgroundThread()) {
mainHandler.post(addSelfToLifecycle);
} else {
lifecycle.addListener(this);
}
lifecycle.addListener(connectivityMonitor);
...
}所以也是把它注册为ActivityFragmentLifecycle的观察者，ConnectivityMonitor通过ConnectivityMonitorFactory进行构造,提供了默认实现类DefaultConnectivityMonitorFactory:
// DefaultConnectivityMonitorFactory.java
public class DefaultConnectivityMonitorFactory implements ConnectivityMonitorFactory {
private static final String TAG = "ConnectivityMonitor";
private static final String NETWORK_PERMISSION = "android.permission.ACCESS_NETWORK_STATE";
@NonNull
@Override
public ConnectivityMonitor build(
@NonNull Context context,
@NonNull ConnectivityMonitor.ConnectivityListener listener) {
int permissionResult = ContextCompat.checkSelfPermission(context, NETWORK_PERMISSION);
boolean hasPermission = permissionResult == PackageManager.PERMISSION_GRANTED;
return hasPermission
? new DefaultConnectivityMonitor(context, listener) : new NullConnectivityMonitor();
}
}接着就往下看DefaultConnectivityMonitor, 在onStart中registerReceiver监听手机网络状态变化的广播,然后在connectivityReceiver中调用isConnect进行网络状态确认，根据网络状态是否变化，如果有变化就回调监听ConnectivityMonitor.ConnectivityListener:
final class DefaultConnectivityMonitor implements ConnectivityMonitor {
private static final String TAG = "ConnectivityMonitor";
private final Context context;
@SuppressWarnings("WeakerAccess") @Synthetic final ConnectivityListener listener;
@SuppressWarnings("WeakerAccess") @Synthetic boolean isConnected;
private boolean isRegistered;
private final BroadcastReceiver connectivityReceiver = new BroadcastReceiver() {
@Override
public void onReceive(@NonNull Context context, Intent intent) {
boolean wasConnected = isConnected;
isConnected = isConnected(context);
if (wasConnected != isConnected) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "connectivity changed, isConnected: " + isConnected);
}
listener.onConnectivityChanged(isConnected);
}
}
};
DefaultConnectivityMonitor(@NonNull Context context, @NonNull ConnectivityListener listener) {
this.context = context.getApplicationContext();
this.listener = listener;
}
private void register() {
if (isRegistered) {
return;
}
// Initialize isConnected.
isConnected = isConnected(context);
try {
// See #1405
context.registerReceiver(connectivityReceiver,
new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION));
isRegistered = true;
} catch (SecurityException e) {
// See #1417, registering the receiver can throw SecurityException.
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Failed to register", e);
}
}
}
private void unregister() {
if (!isRegistered) {
return;
}
context.unregisterReceiver(connectivityReceiver);
isRegistered = false;
}
@SuppressWarnings("WeakerAccess")
@Synthetic
// Permissions are checked in the factory instead.
@SuppressLint("MissingPermission")
boolean isConnected(@NonNull Context context) {
ConnectivityManager connectivityManager =
Preconditions.checkNotNull(
(ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE));
NetworkInfo networkInfo;
try {
networkInfo = connectivityManager.getActiveNetworkInfo();
} catch (RuntimeException e) {
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Failed to determine connectivity status when connectivity changed", e);
}
// Default to true;
return true;
}
return networkInfo != null && networkInfo.isConnected();
}
@Override
public void onStart() {
register();
}
@Override
public void onStop() {
unregister();
}
@Override
public void onDestroy() {
// Do nothing.
}
}ConnectivityMonitor.ConnectivityListener是在RequestManager中传入,有网络重新连接后重启请求：
// RequestManager.java
private static class RequestManagerConnectivityListener implements ConnectivityMonitor
.ConnectivityListener {
private final RequestTracker requestTracker;
RequestManagerConnectivityListener(@NonNull RequestTracker requestTracker) {
this.requestTracker = requestTracker;
}
@Override
public void onConnectivityChanged(boolean isConnected) {
if (isConnected) {
requestTracker.restartRequests();
}
}
}在Glide构造的时候会调用registerComponentCallbacks进行全局注册, 系统在内存紧张的时候回调onTrimMemory,然后根据系统内存紧张级别进行memoryCache/bitmapPool/arrayPool的回收：
// Glide.java
public static Glide get(@NonNull Context context) {
if (glide == null) {
synchronized (Glide.class) {
if (glide == null) {
checkAndInitializeGlide(context);
}
}
}
return glide;
}
private static void initializeGlide(@NonNull Context context, @NonNull GlideBuilder builder) {
Context applicationContext = context.getApplicationContext();
...
applicationContext.registerComponentCallbacks(glide);
Glide.glide = glide;
}
@Override
public void onTrimMemory(int level) {
trimMemory(level);
}
public void trimMemory(int level) {
Util.assertMainThread();
memoryCache.trimMemory(level);
bitmapPool.trimMemory(level);
arrayPool.trimMemory(level);
}再回顾前面的四个问题，我相信聪明的你已经有了答案，文章的各小节标题就是根据问题来进行分析的，这么就不再赘述了，要不有凑字数的嫌疑。Glide的源码是比较庞大而且高质量的，所以一两篇文章是说不清楚的，后面对于Glide的源码分析还会有后续的文章，欢迎关注。
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