Activity是如何創(chuàng)建的
我們都知道在手機(jī)上點(diǎn)擊應(yīng)用的圖標(biāo)蹦肴，系統(tǒng)會(huì)通過ActivityManagerService去啟動(dòng)它的主Activity，接下來我們就來一步步看看在它究竟是如何啟動(dòng)應(yīng)用的。
首先手機(jī)開機(jī)的時(shí)候我們看到的界面其實(shí)是Launcher應(yīng)用的主Activity,Launcher其實(shí)就是一個(gè)會(huì)被系統(tǒng)默認(rèn)啟動(dòng)的安卓應(yīng)用。在上面點(diǎn)擊已裝應(yīng)用的圖標(biāo)，就會(huì)調(diào)用Actvity.startActivity去啟動(dòng)其他的應(yīng)用。而Activity實(shí)際上是繼承ContextWrapper的,所以調(diào)的是ContextWrapper.startActivity方法:
public class ContextWrapper extends Context {
...
Context mBase;
...
public ContextWrapper(Context base) {
mBase = base;
}
...
@Override
public void startActivity(Intent intent) {
mBase.startActivity(intent);
}
...
}
可以看到這里使用了委托的方式馒闷，實(shí)際上是調(diào)了mBase.startActivity。那這個(gè)mBase到底是什么呢叁征？讓我們來看看Context的繼承關(guān)系圖:
Context只有兩個(gè)直接的子類,一個(gè)ContextImpl纳账，一個(gè)ContextWrapper。ContextWrapper類如其名僅僅是一個(gè)包裝的功能捺疼，它的成員變量mBase其實(shí)就是ContextImpl疏虫，所有實(shí)際的工作都是由ContextImpl去實(shí)現(xiàn)的。
于是我們就去看看ContextImpl.startActivity:
class ContextImpl extends Context {
...
@Override
public void startActivity(Intent intent) {
...
startActivity(intent, null);
}
@Override
public void startActivity(Intent intent, Bundle options) {
...
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity)null, intent, -1, options);
}
...
}
public final class ActivityThread {
...
Instrumentation mInstrumentation;
...
public Instrumentation getInstrumentation() {
return mInstrumentation;
}
...
}
好吧啤呼，這里又將鍋甩給了Instrumentation卧秘。Instrumentation其實(shí)也是一個(gè)比較偏但是很有作用的東西(通過它我們能做到很多神奇的事情，例如Hook 應(yīng)用入口 Activity)官扣，主要用于監(jiān)控Activity翅敌，這里我就不詳細(xì)講了，感興趣的同學(xué)可以直接去搜索一下惕蹄。我們直接看啟動(dòng)activity相關(guān)的代碼:
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, null, options);
...
}
這里的ActivityManagerNative最后拿到的就是ActivtyManagerService的IBinder,所以最后會(huì)使用Binder機(jī)制調(diào)用系統(tǒng)端的ActivityManagerService去創(chuàng)建Activity蚯涮。
但是我們知道ActivityManagerService和我們的應(yīng)用是在兩個(gè)進(jìn)程的，如果在ActivityManagerService中創(chuàng)建了Activity的話我們的應(yīng)用也是獲取不了的焊唬。
其實(shí)ActivityManagerService主要功能不是創(chuàng)建Activity恋昼，而是管理Activity棧看靠。它在創(chuàng)建新的Activity的時(shí)候還是會(huì)通過Binder機(jī)制調(diào)回應(yīng)用進(jìn)程的ActivityThread去處理赶促。最后ActivityManagerService只保存Activity的token。由于中間代碼過于曲折挟炬，我這里就不貼出來了.這里直接看ActivityThread的代碼吧：
public final class ActivityThread {
final ArrayMap<IBinder, ActivityClientRecord> mActivities
= new ArrayMap<IBinder, ActivityClientRecord>();
...
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
Activity a = performLaunchActivity(r, customIntent);
...
}
...
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
...
r.activity = activity;
...
mActivities.put(r.token, r);
}
}
可以看到這里也是用Instrumentation去創(chuàng)建Activity的鸥滨，創(chuàng)建完之后就將它丟到一個(gè)Map里面。而Instrumentation.newActivity則很簡單谤祖，通過反射去創(chuàng)建Activity:
public class Instrumentation {
...
public Activity newActivity(ClassLoader cl, String className, Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
return (Activity)cl.loadClass(className).newInstance();
}
...
}
總結(jié)下來婿滓，上面講的的方法的調(diào)用時(shí)序圖如下:
Activity的生命周期是如何被調(diào)用的
這里涉及到幾個(gè)類：ActivityManagerService、ActivityStackSupervisor粥喜、ActivityStack凸主、ActivityThread。
ActivityManagerService負(fù)責(zé)通過binder機(jī)制接收啟動(dòng)應(yīng)用的請求额湘，它內(nèi)部有各個(gè)ActivityStackSupervisor成員變量卿吐，用于管理Activity棧：
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
...
/** Run all ActivityStacks through this */
ActivityStackSupervisor mStackSupervisor;
...
}
ActivityStackSupervisor管理用于Activity棧列表旁舰，它負(fù)責(zé)將Activity壓入對應(yīng)的Activity棧中:
public final class ActivityStackSupervisor {
...
/** All the non-launcher stacks */
private ArrayList<ActivityStack> mStacks = new ArrayList<ActivityStack>();
...
}
ActivityStack用于管理Activity的生命周期，例如在新Activity被壓入的時(shí)候調(diào)用舊棧頂Activity的onPasuse和onStop還有新activity的onStart和onResume嗡官。
final class ActivityStack {
...
final void startPausingLocked(boolean userLeaving, boolean uiSleeping) {
...
prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing, userLeaving, prev.configChangeFlags);
...
}
...
}
ActivityStack并不會(huì)直接調(diào)用Activity的生命周期方法箭窜，而是通過ActivityThread間接調(diào)用。由于ActivityStack在系統(tǒng)進(jìn)程中,而ActivityThread在應(yīng)用進(jìn)程中衍腥，所以通過Binder機(jī)制調(diào)用之后去到ActivityThread那里不是主線程磺樱，于是ActivityThread內(nèi)部就使用了Handler機(jī)制同步到主線程中調(diào)用:
public final class ActivityThread {
...
public final void schedulePauseActivity(IBinder token, boolean finished, boolean userLeaving, int configChanges) {
queueOrSendMessage(
finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
token,
(userLeaving ? 1 : 0),
configChanges);
}
...
private void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {
synchronized (this) {
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
mH.sendMessage(msg);
}
}
...
private class H extends Handler {
...
public void handleMessage(Message msg) {
...
switch (msg.what) {
...
case PAUSE_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
case RESUME_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
handleResumeActivity((IBinder)msg.obj, true,
msg.arg1 != 0, true);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
...
}
...
}
...
private void handlePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges) {
ActivityClientRecord r = mActivities.get(token);
...
}
}
這里需要提的一點(diǎn)是AcvitiyThread里面可能有不止一個(gè)activity。所以需要傳一個(gè)token去指定調(diào)用哪個(gè)activity婆咸。handlePauseActivity方法最終會(huì)調(diào)用mInstrumentation.callActivityOnPause再調(diào)用到Activity.onPause竹捉。這里就不繼續(xù)展示代碼了。
總結(jié)一下尚骄，上面講到的這些類之間的關(guān)系是這樣的:
ActivityThread通過handler機(jī)制將activity的生命周期同步到主線程中調(diào)用:
