前言：
今天在看Glide的源码的时候发现其在初始化的时候，注册了一个cb（ComponentCallbacks），想到Activity 等四大组件都实现了该cb接口，所以就看了源码其调用的逻辑和时间，下面简单分析
glide = builder.createGlide(applicationContext);
for (GlideModule module : manifestModules) {
module.registerComponents(applicationContext, glide.registry);
}
if (annotationGeneratedModule != null) {
annotationGeneratedModule.registerComponents(applicationContext, glide.registry);
}
其中讲glide对象注册到了applicationcontext里面
public void registerComponentCallbacks(ComponentCallbacks callback) {
getApplicationContext().registerComponentCallbacks(callback);
}
其中getApplicationContext()返回的是application对象
public void registerComponentCallbacks(ComponentCallbacks callback) {
synchronized (mComponentCallbacks) {
mComponentCallbacks.add(callback);
}
}
从上面的代码可以知道将glide（实现了cb接口）添加到了当前的application对象里面。
那cb里面的接口是什么时候被触发的呢？
由于fragmenwork的代码比较复杂，本人只找到在ams里面发起的调用：
ActivityManagerService:
case GC_BACKGROUND_PROCESSES_MSG: {
synchronized (ActivityManagerService.this) {
performAppGcsIfAppropriateLocked();
当系统发生gc后台进程的时候，会发这样一个消息，接下来就系代码调用堆栈了：
*/
final void performAppGcsLocked() {
final int N = mProcessesToGc.size();
if (N <= 0) {
return;
}
if (canGcNowLocked()) {
while (mProcessesToGc.size() > 0) {
ProcessRecord proc = mProcessesToGc.remove(0);
if (proc.curRawAdj > ProcessList.PERCEPTIBLE_APP_ADJ || proc.reportLowMemory) {
if ((proc.lastRequestedGc+GC_MIN_INTERVAL)
<= SystemClock.uptimeMillis()) {
// To avoid spamming the system, we will GC processes one
// at a time, waiting a few seconds between each.
performAppGcLocked(proc);
scheduleAppGcsLocked();
return;
} else {
// It hasn't been long enough since we last GCed this
// process... put it in the list to wait for its time.
addProcessToGcListLocked(proc);
break;
}
}
}
scheduleAppGcsLocked();
}
}
final void performAppGcLocked(ProcessRecord app) {
try {
app.lastRequestedGc = SystemClock.uptimeMillis();
if (app.thread != null) {
if (app.reportLowMemory) {
app.reportLowMemory = false;
app.thread.scheduleLowMemory();
} else {
app.thread.processInBackground();
}
}
} catch (Exception e) {
// whatever.
}
}
其中app.thread.scheduleLowMemory(),thread就是ActivityThread
@Override
public void scheduleLowMemory() {
sendMessage(H.LOW_MEMORY, null);
}
case LOW_MEMORY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "lowMemory");
handleLowMemory();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
final void handleLowMemory() {
ArrayList<ComponentCallbacks2> callbacks = collectComponentCallbacks(true, null);
final int N = callbacks.size();
for (int i=0; i<N; i++) {
callbacks.get(i).onLowMemory();
}
// Ask SQLite to free up as much memory as it can, mostly from its page caches.
if (Process.myUid() != Process.SYSTEM_UID) {
int sqliteReleased = SQLiteDatabase.releaseMemory();
EventLog.writeEvent(SQLITE_MEM_RELEASED_EVENT_LOG_TAG, sqliteReleased);
}
// Ask graphics to free up as much as possible (font/image caches)
Canvas.freeCaches();
// Ask text layout engine to free also as much as possible
Canvas.freeTextLayoutCaches();
BinderInternal.forceGc("mem");
}'
###这个方法就是收集当前实现cb的接口，然后给上面的方法调用
ArrayList<ComponentCallbacks2> collectComponentCallbacks(
boolean allActivities, Configuration newConfig) {
ArrayList<ComponentCallbacks2> callbacks
= new ArrayList<ComponentCallbacks2>();
synchronized (mResourcesManager) {
final int NAPP = mAllApplications.size();
for (int i=0; i<NAPP; i++) {
callbacks.add(mAllApplications.get(i));
}
final int NACT = mActivities.size();
for (int i=0; i<NACT; i++) {
ActivityClientRecord ar = mActivities.valueAt(i);
Activity a = ar.activity;
if (a != null) {
Configuration thisConfig = applyConfigCompatMainThread(
mCurDefaultDisplayDpi, newConfig,
ar.packageInfo.getCompatibilityInfo());
if (!ar.activity.mFinished && (allActivities || !ar.paused)) {
// If the activity is currently resumed, its configuration
// needs to change right now.
callbacks.add(a);
} else if (thisConfig != null) {
// Otherwise, we will tell it about the change
// the next time it is resumed or shown. Note that
// the activity manager may, before then, decide the
// activity needs to be destroyed to handle its new
// configuration.
if (DEBUG_CONFIGURATION) {
Slog.v(TAG, "Setting activity "
+ ar.activityInfo.name + " newConfig=" + thisConfig);
}
ar.newConfig = thisConfig;
}
}
}
final int NSVC = mServices.size();
for (int i=0; i<NSVC; i++) {
callbacks.add(mServices.valueAt(i));
}
}
synchronized (mProviderMap) {
final int NPRV = mLocalProviders.size();
for (int i=0; i<NPRV; i++) {
callbacks.add(mLocalProviders.valueAt(i).mLocalProvider);
}
}
return callbacks;
}
由于四大组件分别是在这里被collect，所以不需要特定注册，从上面可以知道接下来就是调用四大组件的onlowMemory
Application：
@CallSuper
public void onLowMemory() {
Object[] callbacks = collectComponentCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ComponentCallbacks)callbacks[i]).onLowMemory();
}
}
}
