activity的启动过程分析

 android的activity有多重的启动途径，这里我们将从桌面点击应用图标时的启动过程。

1、packageManager查询AndroidManifest.xml.,过滤以下intent过滤条件的Activity

<intent-filter>

<action android:name="android.intent.action.Main"/>

<category android:name="android.intent.category.LAUNCHEr">

android桌面的代码位于packges/apps/Launcher2,Launcher捕获用户在home页的点击事件并调用startActivitySafely方法。

进而调用startActivity。startActivity位于android的frameWork的activity中，其实该方法只是对startActivtyForResult的封装。下面给出startActivityForResult的代码。

public void More ...startActivityForResult(Intent intent, int requestCode, Bundle options) {
3423        if (mParent == null) {//非子activty，本例情形
3424            Instrumentation.ActivityResult ar =
3425                mInstrumentation.execStartActivity(
3426                    this, mMainThread.getApplicationThread(), mToken, this,
3427                    intent, requestCode, options);
3428            if (ar != null) {
3429                mMainThread.sendActivityResult(
3430                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
3431                    ar.getResultData());
3432            }
3433            if (requestCode >= 0) {//本例中requstCode = -1

3434                // If this start is requesting a result, we can avoid making
3435                // the activity visible until the result is received.  Setting
3436                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
3437                // activity hidden during this time, to avoid flickering.
3438                // This can only be done when a result is requested because
3439                // that guarantees we will get information back when the
3440                // activity is finished, no matter what happens to it.
3441                mStartedActivity = true;
3442            }
3443
3444            final View decor = mWindow != null ? mWindow.peekDecorView() : null;
3445            if (decor != null) {
3446                decor.cancelPendingInputEvents();
3447            }
3448            // TODO Consider clearing/flushing other event sources and events for child windows.
3449        } else {
3450            if (options != null) {
3451                mParent.startActivityFromChild(this, intent, requestCode, options);
3452            } else {
3453                // Note we want to go through this method for compatibility with
3454                // existing applications that may have overridden it.
3455                mParent.startActivityFromChild(this, intent, requestCode);
3456            }
3457        }
3458    }

这里着重要看

execStartActivity(this, mMainThread.getApplicationThread(), mToken, this,intent, requestCode, options)

这里实现了跨进程的Activity调度。该方法位于framworks/base/core/java/android/app/Instrumentation.java,此方法通过ActivityManager的startActivity唤起新的Activity，下面给出代码：

public ActivityResult More ...execStartActivity(
1398 Context who, IBinder contextThread, IBinder token, Activity target,
1399 Intent intent, int requestCode, Bundle options) {
1400 IApplicationThread whoThread = (IApplicationThread) contextThread;
1401 if (mActivityMonitors != null) {//
1402 synchronized (mSync) {
1403 final int N = mActivityMonitors.size();
1404 for (int i=0; i<N; i++) {
1405 final ActivityMonitor am = mActivityMonitors.get(i);
1406 if (am.match(who, null, intent)) {
1407 am.mHits++;
1408 if (am.isBlocking()) {
1409 return requestCode >= 0 ? am.getResult() : null;
1410 }
1411 break;
1412 }
1413 }
1414 }
1415 }
1416 try {
1417 intent.migrateExtraStreamToClipData();
1418 intent.prepareToLeaveProcess();
1419 int result = ActivityManagerNative.getDefault()//调用acitvityManger的startActivy方法，
1420 .startActivity(whoThread, who.getBasePackageName(), intent,
1421 intent.resolveTypeIfNeeded(who.getContentResolver()),
1422 token, target != null ? target.mEmbeddedID : null,
1423 requestCode, 0, null, null, options);
1424 checkStartActivityResult(result, intent);
1425 } catch (RemoteException e) {
1426 }
1427 return null;
1428 }
跟踪程序得到activty oncreate方法的调用栈。



对以上过程进行总结：

1、Launcher线程捕获onclick的点击事件，调用Launcher.startActivitySafely，进一步调用Launcher.startActivity，最后调用父类Activity的startActivity.

2、Activity和ActivityManagerService交互，引入Instrumention，将启动请求交给Instrumention,调用Instrumention.execStartActivity.

3、得到ActivityManagerService，调用其startActivity，这里做了进程切换（具体过程请查看源码）。

4、开启Activity，调用onCreate方法。