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鏈接：https://mp.weixin.qq.com/s/uTv44vJFFJI_l6b5YKSXYQ
作者：連凌能
Android App中圖片的展示是很基本也很重要的一個功能，在Android平臺上有很多的圖片加載解決方案，但是官方認可的是Glide。Android App的頁面是有生命周期的，Glide比較好的一個功能就是具有生命周期管理功能，能夠根據頁面和APP的生命周期來管理圖片的加載和停止，也開放接口供用戶在內存緊張時手動進行內存管理。本文重點是生命周期源碼的分析，不會從簡單的使用著手。
這是Glide源碼分析的第二篇文章，第一篇是《Glide緩存流程》，從資源的獲取流程對源碼進行分析。本篇會聚焦于生命周期模塊的原理。開始之前先思考下面這幾個問題：
Glide怎么實現頁面生命周期？
Glide為什么對Fragment做緩存？
Glide如何監聽網絡變化？
先來看with函數的執行, 會構造glide單例，而?
RequestManagerRetriever在initializeGlide中會進行構造。
// 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;
}構造完成RequestManagerRetriever通過get返回一個 RequestManager, 如果不在主線程，默認會傳入 getApplicationContext,也就是不進行生命周期管理：
在getRequestManagerFragment中先查看當前Activity中有沒有FRAGMENT_TAG這個標簽對應的Fragment，如果有就直接返回
如果沒有，會判斷pendingRequestManagerFragments中有沒有，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;
...
}
...
}這里面需要注意一個問題，就是如果with()函數中傳進來的不是Activity，而是Fragment，那么也會去創建一個沒有界面的RequestManagerFragment,而它的父Fragment就是傳進來的Fragment。
上面為什么需要pendingRequestManagerFragments先進行緩存呢？這個放到下面第二個問題中說明。先接著往下看生命周期的傳遞。
RequestManagerFragment是一個很重要的類，Glide就是通過它作為生命周期的分發入口，RequestManagerFragment的默認構造函數會實例化一個ActivityFragmentLifecycle，在每個生命周期onStart/onStop/onDestroy中會調用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里面有一個實例RequestManager,在前面的fragmentGet,RequestManagerFragment拿到以后會嘗試獲取它的RequestManager，第一次獲取肯定是沒有,就會重新構造一個, 通過RequestManagerRetriever構造時傳入的RequestManagerFactory工廠類實例化一個RequestManager, 把RequestManagerFragment中的ActivityFragmentLifecycle傳進去：
// 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);
}
};很明顯生命周期的關鍵就在ActivityFragmentLifecycle,?在RequestManagerFragment中相應生命周期中會回調它，那么猜測它肯定是在里面維護了一個觀察者列表，相應事件發生的時候進行通知, 看下它的源碼：
// 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把這個傳給RequestManager后，肯定會注冊觀察者，看一下RequestManager的相關代碼,在構造函數里面lifecycle.addListener(this);,把自己注冊為觀察者：
// 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對應的生命周期里面, 在這里面分別啟動，停止和銷毀請求：
// 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);
}再貼一次RequestManagerRetriever中獲取Fragment的代碼，前面留了一個疑問，為什么這里會需要一個pendingRequestManagerFragments對Fragment進行緩存。
// 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;
}我們看一個情況：
Glide.with(Context).load(ImageUrl1).into(imageview1); // task1
Glide.with(Context).load(ImageUrl2).into(imageview2); // task2Android開發應該都知道主線程有一個Handler機制，會往消息隊列中放消息，通過Looper按順序取出來執行。那么主線程中的執行順序和消息隊列中的執行順序關系是什么？看個栗子：
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中打印的順序和它里面的Handler中的信息哪個先打印？start中handler的信息和Next函數中的信息打印順序是怎樣的？看下打印結果：
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中的順序會在主線程之后，Handler中的消息執行順序就是隊列先進先出。
上面執行到task1的時候,在下面這兩行代碼，add操作會往消息隊列放一個消息，這里標記為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中構造的RequestManagerFragment放到pendingRequestManagerFragments中，那么在執行task2的時候也會再重新構造一個RequestManagerFragment,并且往主線程中放一個消息msg2,這個時候就會出現重復add的情況。
所以在前面new 出來一個RequestManagerFragment,隨后就把它放到pendingRequestManagerFragments中，那么task2再進來的時候從緩存中能取到，就不會再重新new和add了。
那么下一個問題來了，為什么會出現下面這行代碼，add后又需要馬上發一個消息remove掉？在前面阻止掉task2重復new和add的操作后，就把這個緩存刪掉，可以避免內存泄漏和內存壓力：
// RequestManagerRetriever.java
pendingRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();從上面頁面生命周期的分析部分知道，對于任務的控制都是通過RequestManager，還是到它里面去看，實現網絡變化監聽的就是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);
...
}所以也是把它注冊為ActivityFragmentLifecycle的觀察者，ConnectivityMonitor通過ConnectivityMonitorFactory進行構造,提供了默認實現類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();
}
}接著就往下看DefaultConnectivityMonitor, 在onStart中registerReceiver監聽手機網絡狀態變化的廣播,然后在connectivityReceiver中調用isConnect進行網絡狀態確認，根據網絡狀態是否變化，如果有變化就回調監聽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中傳入,有網絡重新連接后重啟請求：
// 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構造的時候會調用registerComponentCallbacks進行全局注冊, 系統在內存緊張的時候回調onTrimMemory,然后根據系統內存緊張級別進行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);
}再回顧前面的四個問題，我相信聰明的你已經有了答案，文章的各小節標題就是根據問題來進行分析的，這么就不再贅述了，要不有湊字數的嫌疑。Glide的源碼是比較龐大而且高質量的，所以一兩篇文章是說不清楚的，后面對于Glide的源碼分析還會有后續的文章，歡迎關注。
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