一、前言
SystemUI 主要负责显示系统、应用状态，主要有以下几部分组成：导航栏(NavigationBar)，快捷开关面板(QSPanel)和最近开启的应用面板(RecentPanel)状、态栏(StatusBar)及锁屏音量调节对话框、RingTonePlayer播发器界面、通知栏（Notification Panel）、PowerUI界面等等。
图片
StatusBar：状态显示（时间、电量、信号、蓝牙、GPS，Wifi、应用Icon）及通知消息提示。
NavigationBar：返回，HOME键，Recent（最近任务栏）
Recents：近期应用管理，以堆叠栈的形式展现。
KeyGuard：锁屏模块，保护个人信息。
Notification Panel：展示系统或应用通知信息，提供便捷的系统设置开关（亮度、wifi、蓝牙、数据、飞行模式、手电筒、位置信息、设置等内容）。
screenshot：长按电源键+音量下键后截屏，用以展示截取的屏幕照片/内容
PowerUI：主要处理和Power相关的事件，比如省电模式切换、电池电量变化和开关屏事件等。
RingtonePlayer：铃声播放
VolumeUI：来用展示或控制音量的变化：媒体音量、铃声音量与闹钟音量
二、启动分析
Android系统启动流程:
按下电源，系统上电
从固定地址启动固化在ROM的BootLoader程序
启动Linux内核，加载init进程
启动init进程，fork出zygote进程
启动SystemServer，启动Binder线程池以及各种服务
AMS启动Launcher， 然后你就看到该看到的了
SystemUI作为系统主要进程，就是在SystemServer启动的服务其中之一。
话不多说，read the fucking source code:
SystemServer.java
public static void main(String[] args) { new SystemServer().run(); }
private void run() { ...... // Start services. try { traceBeginAndSlog("StartServices"); startBootstrapServices(); startCoreServices(); startOtherServices(); SystemServerInitThreadPool.shutdown(); } catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex; } finally { traceEnd(); } ......
}
private void startOtherServices() { ...... mActivityManagerService.systemReady(() -> { Slog.i(TAG, "Making services ready"); traceBeginAndSlog("StartActivityManagerReadyPhase"); ...... try { startSystemUi(context, windowManagerF); } catch (Throwable e) { reportWtf("starting System UI", e); } ...... } } static final void startSystemUi(Context context, WindowManagerService windowManager) { Intent intent = new Intent(); intent.setComponent(new ComponentName("com.android.systemui", "com.android.systemui.SystemUIService")); intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING); //Slog.d(TAG, "Starting service: " + intent); context.startServiceAsUser(intent, UserHandle.SYSTEM); windowManager.onSystemUiStarted(); }
总结下代码流程：SystemServermain()方法会调用SystemServer.run()方法，在run方法中，会启动bootStrapService，coreService，以及其他服务otherService，在startOtherServices()中，会注册AMS的systemReady回调，在回调中启动SystemUI，其实就是启动了SystemUIService。
SystemUIService.java
@Override public void onCreate() { super.onCreate(); ((SystemUIApplication) getApplication()).startServicesIfNeeded(); ..... } SystemUIApplication.java @Override public void startServicesIfNeeded() { String[] names = getResources().getStringArray(R.array.config_systemUIServiceComponents); startServicesIfNeeded(names); } private void startServicesIfNeeded(String[] services) { if (mServicesStarted) { return; } mServices = new SystemUI[services.length];
if (!mBootCompleted) { // check to see if maybe it was already completed long before we began // see ActivityManagerService.finishBooting() if ("1".equals(SystemProperties.get("sys.boot_completed"))) { mBootCompleted = true; if (DEBUG) Log.v(TAG, "BOOT_COMPLETED was already sent"); } } Log.v(TAG, "Starting SystemUI services for user " + Process.myUserHandle().getIdentifier() + "."); TimingsTraceLog log = new TimingsTraceLog("SystemUIBootTiming", Trace.TRACE_TAG_APP); log.traceBegin("StartServices"); final int N = services.length; for (int i = 0; i < N; i++) { String clsName = services[i]; if (DEBUG) Log.d(TAG, "loading: " + clsName); log.traceBegin("StartServices" + clsName); long ti = System.currentTimeMillis(); Class cls; try { cls = Class.forName(clsName); mServices[i] = (SystemUI) cls.newInstance(); } catch(ClassNotFoundException ex){ throw new RuntimeException(ex); } catch (IllegalAccessException ex) { throw new RuntimeException(ex); } catch (InstantiationException ex) { throw new RuntimeException(ex); } mServices[i].mContext = this; mServices[i].mComponents = mComponents; if (DEBUG) Log.d(TAG, "running: " + mServices[i]); mServices[i].start(); ...... } }
<string-array name="config_systemUIServiceComponents" translatable="false"> <item>com.android.systemui.Dependency</item> <item>com.android.systemui.util.NotificationChannels</item> <item>com.android.systemui.statusbar.CommandQueue$CommandQueueStart</item> <item>com.android.systemui.keyguard.KeyguardViewMediator</item> <item>com.android.systemui.recents.Recents</item> <item>com.android.systemui.volume.VolumeUI</item> <item>com.android.systemui.stackdivider.Divider</item> <item>com.android.systemui.SystemBars</item> <item>com.android.systemui.usb.StorageNotification</item> <item>com.android.systemui.power.PowerUI</item> <item>com.android.systemui.media.RingtonePlayer</item> <item>com.android.systemui.keyboard.KeyboardUI</item> <item>com.android.systemui.pip.PipUI</item> <item>com.android.systemui.shortcut.ShortcutKeyDispatcher</item> <item>@string/config_systemUIVendorServiceComponent</item> <item>com.android.systemui.util.leak.GarbageMonitor$Service</item> <item>com.android.systemui.LatencyTester</item> <item>com.android.systemui.globalactions.GlobalActionsComponent</item> <item>com.android.systemui.ScreenDecorations</item> <item>com.android.systemui.fingerprint.FingerprintDialogImpl</item> <item>com.android.systemui.SliceBroadcastRelayHandler</item> </string-array> <!-- SystemUI vender service, used in config_systemUIServiceComponents. --> <string name="config_systemUIVendorServiceComponent" translatable="false">com.android.systemui.VendorServices</string>
SystemUIService的onCreate()方法中，直接调用SystemUIApplication.startServicesIfNeeded()方法，最终会将所有服务一一加载并通过start()方法启动起来。
至此，SystemUI基本启动流程就结束了。
