The launcher process starts and the user interface is presented, revealing the key steps of each stage

Launcher

Launcher (launcher, desktop) is the user interface on the Android operating system used to display application icons, search for applications, manage desktop shortcuts, and perform other tasks related to the device's home screen. The layout and appearance of the device's home screen is the primary way users interact with the device.

Launcher features:

1. 「Customizability」: Most launchers allow users to customize icons, wallpapers, desktop layouts, etc.
2. 「Performance」: An efficient Launcher can improve the overall performance of the device because it needs to respond to touch input and load icons and layouts quickly.
3. 「Compatibility」: As the Android version is updated, Launcher needs to ensure compatibility with the latest version of Android.
4. 「Security」: Launcher must ensure the security of user data and prevent malware attacks.
5. 「Diversity」: There are many different Launcher apps on the market, each with its own unique functions and interface design.

Launcher process startup process

1. 「SystemServer process start」:

SystemServer is a core process in the Android system, responsible for starting and initializing various system services.

During the startup of SystemServer, other services such as PackageManagerService (PMS) and ActivityManagerService (AMS) initialization methods are called.

 public final class SystemServer { private void run() { ... startBootstrapServices(); startOtherServices(); ... } private void startBootstrapServices() { ... mActivityManagerService = mSystemServiceManager.startService(ActivityManagerService.Lifecycle.class).getService(); mActivityManagerService.setSystemServiceManager(mSystemServiceManager); mActivityManagerService.setInstaller(installer); ... } private void startOtherServices() { ... mActivityManagerService.systemReady(() -> { }, BOOT_TIMINGS_TRACE_LOG); } }

When SystemServer is started, the startOtherServices() method is executed to call the systemReady() method of AMS, and the Launcher is started through this method.

 // Tag for timing measurement of main thread. private static final String SYSTEM_SERVER_TIMING_TAG = "SystemServerTiming"; private static final TimingsTraceLog BOOT_TIMINGS_TRACE_LOG = new TimingsTraceLog(SYSTEM_SERVER_TIMING_TAG, Trace.TRACE_TAG_SYSTEM_SERVER); private void startOtherServices() { ... mActivityManagerService.systemReady(() -> { Slog.i(TAG, "Making services ready"); traceBeginAndSlog("StartActivityManagerReadyPhase"); mSystemServiceManager.startBootPhase(SystemService.PHASE_ACTIVITY_MANAGER_READY); ... }, BOOT_TIMINGS_TRACE_LOG); }

2. 「PMS service initialization」:

The PMS service will complete the installation and management of applications in the system.

PMS will scan the /data/app directory and load the installed application information.

3. 「AMS service initialization」:

AMS is a service in the Android system that is responsible for managing the application life cycle and activity status.

During the initialization of AMS, various system broadcast receivers are registered, including broadcasts related to Launcher startup.

4.「Launcher application registration」:

The Launcher application is a special system application that configures a specific Intent Filter in the AndroidManifest.xml file so that the system can recognize and start it.

Typically, the Action of a Launcher application is set to Intent.ACTION_MAIN and the Category is set to Intent.CATEGORY_HOME.

5. "SystemReady Phase":

When the system has completed initialization and is ready to start the desktop, AMS calls its systemReady() method.

In the systemReady() method, AMS checks whether the system is ready to start the Launcher and calls the relevant method to start it.

 public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) { ... synchronized (this) { ... startHomeActivityLocked(currentUserId, "systemReady"); ... } ... }

In the startHomeActivityLocked() method, getHomeIntent() method is used to get the intent object of HomeActivity to be started. mTopAction defaults to INTENT.ACTION_MAIN, and adds the category flag of CATEGORY_HOME. Use PackageManager to get the corresponding Activity, the corresponding ActivityInfo, and the corresponding process record. At this time, the corresponding process has not been started. Add FLAG_ACTIVITY_NEW_TASK startup parameter to intent to start a new stack, and then call the startHomeActivity() method of ActivityStartController class to execute the startup.

 boolean startHomeActivityLocked(int userId, String reason) { ... Intent intent = getHomeIntent(); ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId); if (aInfo != null) { intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name)); // Don't do this if the home app is currently being instrumented. aInfo = new ActivityInfo(aInfo); aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId); ProcessRecord app = getProcessRecordLocked(aInfo.processName, aInfo.applicationInfo.uid, true); if (app == null || app.instr == null) { intent.setFlags(intent.getFlags() | FLAG_ACTIVITY_NEW_TASK); final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid); // For ANR debugging to verify if the user activity is the one that actually launched. final String myReason = reason + ":" + userId + ":" + resolvedUserId; mActivityStartController.startHomeActivity(intent, aInfo, myReason); } } ... return true; } Intent getHomeIntent() { Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null); intent.setComponent(mTopComponent); intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING); if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) { intent.addCategory(Intent.CATEGORY_HOME); } return intent; }

6. 「Start Launcher process」:

AMS starts the Launcher process.

This method creates a new process (if the Launcher is not already running) to start the Launcher application.

 void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason) { mSupervisor.moveHomeStackTaskToTop(reason); mLastHomeActivityStartResult = obtainStarter(intent, "startHomeActivity: " + reason) .setOutActivity(tmpOutRecord) .setCallingUid(0) .setActivityInfo(aInfo) .execute(); mLastHomeActivityStartRecord = tmpOutRecord[0]; if (mSupervisor.inResumeTopActivity) { // If we are in resume section already, home activity will be initialized, but not // resumed (to avoid recursive resume) and will stay that way until something pokes it // again. We need to schedule another resume. mSupervisor.scheduleResumeTopActivities(); } } int execute() { try { // TODO(b/64750076): Look into passing request directly to these methods to allow // for transactional diffs and preprocessing. if (mRequest.mayWait) { return startActivityMayWait(mRequest.caller, mRequest.callingUid, ...); } else { return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent, ...); } } finally { onExecutionComplete(); } }

7. 「Operations after Launcher process is started」:

After the Launcher process is started, it will request information about installed applications from the PMS and display the information on the desktop.

Users can launch corresponding applications by clicking the application icon on the desktop.

 @TargetApi(23) public InvariantDeviceProfile(Context context) { WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE); Display display = wm.getDefaultDisplay(); DisplayMetrics dm = new DisplayMetrics(); display.getMetrics(dm); ... ArrayList<InvariantDeviceProfile> closestProfiles = findClosestDeviceProfiles(minWidthDps, minHeightDps, getPredefinedDeviceProfiles(context)); ... } ArrayList<InvariantDeviceProfile> getPredefinedDeviceProfiles(Context context) { ArrayList<InvariantDeviceProfile> profiles = new ArrayList<>(); try (XmlResourceParser parser = context.getResources().getXml(R.xml.device_profiles)) { final int depth = parser.getDepth(); int type; while (((type = parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { if ((type == XmlPullParser.START_TAG) && "profile".equals(parser.getName())) { TypedArray a = context.obtainStyledAttributes(Xml.asAttributeSet(parser), R.styleable.InvariantDeviceProfile); int numRows = a.getInt(R.styleable.InvariantDeviceProfile_numRows, 0); int numColumns = a.getInt(R.styleable.InvariantDeviceProfile_numColumns, 0); float iconSize = a.getFloat(R.styleable.InvariantDeviceProfile_iconSize, 0); profiles.add(new InvariantDeviceProfile( a.getString(R.styleable.InvariantDeviceProfile_name), a.getFloat(R.styleable.InvariantDeviceProfile_minWidthDps, 0), a.getFloat(R.styleable.InvariantDeviceProfile_minHeightDps, 0), numRows, numColumns, a.getInt(R.styleable.InvariantDeviceProfile_numFolderRows, numRows), a.getInt(R.styleable.InvariantDeviceProfile_numFolderColumns, numColumns), iconSize, a.getFloat(R.styleable.InvariantDeviceProfile_landscapeIconSize, iconSize), a.getFloat(R.styleable.InvariantDeviceProfile_iconTextSize, 0), a.getInt(R.styleable.InvariantDeviceProfile_numHotseatIcons, numColumns), a.getResourceId(R.styleable.InvariantDeviceProfile_defaultLayoutId, 0), a.getResourceId(R.styleable.InvariantDeviceProfile_demoModeLayoutId, 0))); a.recycle(); } } } catch (IOException|XmlPullParserException e) { throw new RuntimeException(e); } return profiles; }

The InvariantDeviceProfile object mainly stores basic configuration information of the App, such as the size of the App icon, the text size, and how many Apps can be displayed in each workspace or folder.

In the startLoader() method of LauncherModel, a new LoaderResults object is created, and a Runnable task of LoaderTask is created through the startLoaderForResults() method.

 public boolean startLoader(int synchronousBindPage) { ... synchronized (mLock) { // Don't bother to start the thread if we know it's not going to do anything if (mCallbacks != null && mCallbacks.get() != null) { ... LoaderResults loaderResults = new LoaderResults(mApp, sBgDataModel, mBgAllAppsList, synchronousBindPage, mCallbacks); if (mModelLoaded && !mIsLoaderTaskRunning) { ... return true; } else { startLoaderForResults(loaderResults); } } } return false; } public void startLoaderForResults(LoaderResults results) { synchronized (mLock) { stopLoader(); mLoaderTask = new LoaderTask(mApp, mBgAllAppsList, sBgDataModel, results); runOnWorkerThread(mLoaderTask); } } private static void runOnWorkerThread(Runnable r) { if (sWorkerThread.getThreadId() == Process.myTid()) { r.run(); } else { // If we are not on the worker thread, then post to the worker handler sWorker.post(r); } }

In the run() method of LoaderTask, load the information of the apps installed on the phone, query the database to obtain the relevant information of the installed apps, load the Launcher layout, convert the data into View, bind it to the interface, and finally you can see the desktop icons in the grid list displayed on the desktop.

 public void run() { ... try (LauncherModel.LoaderTransaction transaction = mApp.getModel().beginLoader(this)) { // 查询数据库整理App信息，转化为View绑定到界面loadWorkspace(); mResults.bindWorkspace(); loadAllApps(); mResults.bindAllApps(); loadDeepShortcuts(); mResults.bindDeepShortcuts(); mBgDataModel.widgetsModel.update(mApp, null); mResults.bindWidgets(); transaction.commit(); } catch (CancellationException e) { // Loader stopped, ignore TraceHelper.partitionSection(TAG, "Cancelled"); } TraceHelper.endSection(TAG); }

As the Android system continues to develop and update, the launcher process may also change and optimize accordingly. The Android system also supports multiple ways to launch the Launcher, such as automatic launch after booting, short press the Home button to launch, and automatic restart after abnormal crash. The implementation process of these launch methods is also different, but the basic process is similar to the above steps.