Android线程间通信(补充):UI线程
2015-11-19 22:57
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前文提到,Android只允许UI线程更新应用界面。当我们在其他线程中想要更改应用界面时,就需要用到线程间通信,这是最常见的线程通信场景。在UI线程或者使用主Looper创建Handler,其他线程通过这个Handler往UI线程发送操作消息,UI线程根据消息做出相应的反应。
我们前面说接收线程是需要申请和绑定Looper对象才能接收和处理消息的。但是在我们用Handler+Message更新UI线程时,却没有申请和绑定过任何的Looper对象。这是为什么呢?
不知道诸君眼里的UI线程是怎么样的?我们在Looper类提到过,Looper类中prepareMainLooper()的注释强调了只能UI线程调用。我尝试查看这个方法的调用列表,然后发现在一个叫做ActivityThread类的方法中调用了它。这个方法的代码如下:
没错,这居然是一个Java的主方法!在这里我们不展开分析ActivityThread类,来看一下前辈们对这个类的定义:
它管理应用进程的主线程的执行(相当于普通Java程序的main入口函数),并根据AMS的要求(通过IApplicationThread接口,AMS为Client、ActivityThread.ApplicationThread为Server)负责调度和执行activities、broadcasts和其它操作。
—— Android ActivityThread(主线程或UI线程)简介
不严格地说,可以理解成Android是一个Java应用,ActivityThread是这个Java应用的主类,它对应的线程可以叫做主/UI线程。再进一步说,Android的UI线程其实和我们之前提到的receiver线程一样,都需要申请和绑定Looper,都会无限循环地从消息队列中取消息。只不过这些操作藏在了Android源码中。这意味着UI线程涉及的所有包括创建Activity、销毁Activity、开始应用和退出应用等基础操作都是通过消息类传达的!
在ActivityThread类中,还定义了一个Handler的子类H,H重写了Handler的handleMessage()。很显然这个H适用于其他线程与UI线程通信的。我们来看一下它的代码:
虽然大部分调用的方法我们不知道其作用,但是可以根据消息表示可以看出大部分方法都涉及到Android系统的管理。看到这些我们可以断定,Android的UI线程其实就是一个Message机制的接收线程,其他线程遇到需要更新界面的情况,都只需通过H对象往UI线程发送一个更新界面的消息。当然,因为Looper.getMainLooper()方法的存在,我们也可以自己定义绑定主Looper的Handler类并实现更新界面消息的发送。
我们前面说接收线程是需要申请和绑定Looper对象才能接收和处理消息的。但是在我们用Handler+Message更新UI线程时,却没有申请和绑定过任何的Looper对象。这是为什么呢?
不知道诸君眼里的UI线程是怎么样的?我们在Looper类提到过,Looper类中prepareMainLooper()的注释强调了只能UI线程调用。我尝试查看这个方法的调用列表,然后发现在一个叫做ActivityThread类的方法中调用了它。这个方法的代码如下:
public static void main(String[] args) { ……(省略) Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
没错,这居然是一个Java的主方法!在这里我们不展开分析ActivityThread类,来看一下前辈们对这个类的定义:
它管理应用进程的主线程的执行(相当于普通Java程序的main入口函数),并根据AMS的要求(通过IApplicationThread接口,AMS为Client、ActivityThread.ApplicationThread为Server)负责调度和执行activities、broadcasts和其它操作。
—— Android ActivityThread(主线程或UI线程)简介
不严格地说,可以理解成Android是一个Java应用,ActivityThread是这个Java应用的主类,它对应的线程可以叫做主/UI线程。再进一步说,Android的UI线程其实和我们之前提到的receiver线程一样,都需要申请和绑定Looper,都会无限循环地从消息队列中取消息。只不过这些操作藏在了Android源码中。这意味着UI线程涉及的所有包括创建Activity、销毁Activity、开始应用和退出应用等基础操作都是通过消息类传达的!
在ActivityThread类中,还定义了一个Handler的子类H,H重写了Handler的handleMessage()。很显然这个H适用于其他线程与UI线程通信的。我们来看一下它的代码:
private class H extends Handler { public static final int LAUNCH_ACTIVITY = 100; public static final int PAUSE_ACTIVITY = 101; public static final int PAUSE_ACTIVITY_FINISHING= 102; public static final int STOP_ACTIVITY_SHOW = 103; public static final int STOP_ACTIVITY_HIDE = 104; public static final int SHOW_WINDOW = 105; public static final int HIDE_WINDOW = 106; public static final int RESUME_ACTIVITY = 107; public static final int SEND_RESULT = 108; public static final int DESTROY_ACTIVITY = 109; public static final int BIND_APPLICATION = 110; public static final int EXIT_APPLICATION = 111; public static final int NEW_INTENT = 112; public static final int RECEIVER = 113; public static final int CREATE_SERVICE = 114; public static final int SERVICE_ARGS = 115; public static final int STOP_SERVICE = 116; public static final int CONFIGURATION_CHANGED = 118; public static final int CLEAN_UP_CONTEXT = 119; public static final int GC_WHEN_IDLE = 120; public static final int BIND_SERVICE = 121; public static final int UNBIND_SERVICE = 122; public static final int DUMP_SERVICE = 123; public static final int LOW_MEMORY = 124; public static final int ACTIVITY_CONFIGURATION_CHANGED = 125; public static final int RELAUNCH_ACTIVITY = 126; public static final int PROFILER_CONTROL = 127; public static final int CREATE_BACKUP_AGENT = 128; public static final int DESTROY_BACKUP_AGENT = 129; public static final int SUICIDE = 130; public static final int REMOVE_PROVIDER = 131; public static final int ENABLE_JIT = 132; public static final int DISPATCH_PACKAGE_BROADCAST = 133; public static final int SCHEDULE_CRASH = 134; public static final int DUMP_HEAP = 135; public static final int DUMP_ACTIVITY = 136; public static final int SLEEPING = 137; public static final int SET_CORE_SETTINGS = 138; public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139; public static final int TRIM_MEMORY = 140; public static final int DUMP_PROVIDER = 141; public static final int UNSTABLE_PROVIDER_DIED = 142; public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143; public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144; public static final int INSTALL_PROVIDER = 145; public static final int ON_NEW_ACTIVITY_OPTIONS = 146; public static final int CANCEL_VISIBLE_BEHIND = 147; public static final int BACKGROUND_VISIBLE_BEHIND_CHANGED = 148; public static final int ENTER_ANIMATION_COMPLETE = 149; String codeToString(int code) { if (DEBUG_MESSAGES) { switch (code) { case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY"; case PAUSE_ACTIVITY: return "PAUSE_ACTIVITY"; case PAUSE_ACTIVITY_FINISHING: return "PAUSE_ACTIVITY_FINISHING"; case STOP_ACTIVITY_SHOW: return "STOP_ACTIVITY_SHOW"; case STOP_ACTIVITY_HIDE: return "STOP_ACTIVITY_HIDE"; case SHOW_WINDOW: return "SHOW_WINDOW"; case HIDE_WINDOW: return "HIDE_WINDOW"; case RESUME_ACTIVITY: return "RESUME_ACTIVITY"; case SEND_RESULT: return "SEND_RESULT"; case DESTROY_ACTIVITY: return "DESTROY_ACTIVITY"; case BIND_APPLICATION: return "BIND_APPLICATION"; case EXIT_APPLICATION: return "EXIT_APPLICATION"; case NEW_INTENT: return "NEW_INTENT"; case RECEIVER: return "RECEIVER"; case CREATE_SERVICE: return "CREATE_SERVICE"; case SERVICE_ARGS: return "SERVICE_ARGS"; case STOP_SERVICE: return "STOP_SERVICE"; case CONFIGURATION_CHANGED: return "CONFIGURATION_CHANGED"; case CLEAN_UP_CONTEXT: return "CLEAN_UP_CONTEXT"; case GC_WHEN_IDLE: return "GC_WHEN_IDLE"; case BIND_SERVICE: return "BIND_SERVICE"; case UNBIND_SERVICE: return "UNBIND_SERVICE"; case DUMP_SERVICE: return "DUMP_SERVICE"; case LOW_MEMORY: return "LOW_MEMORY"; case ACTIVITY_CONFIGURATION_CHANGED: return "ACTIVITY_CONFIGURATION_CHANGED"; case RELAUNCH_ACTIVITY: return "RELAUNCH_ACTIVITY"; case PROFILER_CONTROL: return "PROFILER_CONTROL"; case CREATE_BACKUP_AGENT: return "CREATE_BACKUP_AGENT"; case DESTROY_BACKUP_AGENT: return "DESTROY_BACKUP_AGENT"; case SUICIDE: return "SUICIDE"; case REMOVE_PROVIDER: return "REMOVE_PROVIDER"; case ENABLE_JIT: return "ENABLE_JIT"; case DISPATCH_PACKAGE_BROADCAST: return "DISPATCH_PACKAGE_BROADCAST"; case SCHEDULE_CRASH: return "SCHEDULE_CRASH"; case DUMP_HEAP: return "DUMP_HEAP"; case DUMP_ACTIVITY: return "DUMP_ACTIVITY"; case SLEEPING: return "SLEEPING"; case SET_CORE_SETTINGS: return "SET_CORE_SETTINGS"; case UPDATE_PACKAGE_COMPATIBILITY_INFO: return "UPDATE_PACKAGE_COMPATIBILITY_INFO"; case TRIM_MEMORY: return "TRIM_MEMORY"; case DUMP_PROVIDER: return "DUMP_PROVIDER"; case UNSTABLE_PROVIDER_DIED: return "UNSTABLE_PROVIDER_DIED"; case REQUEST_ASSIST_CONTEXT_EXTRAS: return "REQUEST_ASSIST_CONTEXT_EXTRAS"; case TRANSLUCENT_CONVERSION_COMPLETE: return "TRANSLUCENT_CONVERSION_COMPLETE"; case INSTALL_PROVIDER: return "INSTALL_PROVIDER"; case ON_NEW_ACTIVITY_OPTIONS: return "ON_NEW_ACTIVITY_OPTIONS"; case CANCEL_VISIBLE_BEHIND: return "CANCEL_VISIBLE_BEHIND"; case BACKGROUND_VISIBLE_BEHIND_CHANGED: return "BACKGROUND_VISIBLE_BEHIND_CHANGED"; case ENTER_ANIMATION_COMPLETE: return "ENTER_ANIMATION_COMPLETE"; } } return Integer.toString(code); } public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { case LAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); final ActivityClientRecord r = (ActivityClientRecord) msg.obj; r.packageInfo = getPackageInfoNoCheck( r.activityInfo.applicationInfo, r.compatInfo); handleLaunchActivity(r, null); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case RELAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart"); ActivityClientRecord r = (ActivityClientRecord)msg.obj; handleRelaunchActivity(r); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case PAUSE_ACTIVITY: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2, (msg.arg1&2) != 0); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case PAUSE_ACTIVITY_FINISHING: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); handlePauseActivity((IBinder)msg.obj, true, (msg.arg1&1) != 0, msg.arg2, (msg.arg1&1) != 0); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case STOP_ACTIVITY_SHOW: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop"); handleStopActivity((IBinder)msg.obj, true, msg.arg2); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case STOP_ACTIVITY_HIDE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop"); handleStopActivity((IBinder)msg.obj, false, msg.arg2); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case SHOW_WINDOW: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityShowWindow"); handleWindowVisibility((IBinder)msg.obj, true); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case HIDE_WINDOW: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityHideWindow"); handleWindowVisibility((IBinder)msg.obj, false); 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; case SEND_RESULT: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDeliverResult"); handleSendResult((ResultData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case DESTROY_ACTIVITY: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDestroy"); handleDestroyActivity((IBinder)msg.obj, msg.arg1 != 0, msg.arg2, false); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case BIND_APPLICATION: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication"); AppBindData data = (AppBindData)msg.obj; handleBindApplication(data); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case EXIT_APPLICATION: if (mInitialApplication != null) { mInitialApplication.onTerminate(); } Looper.myLooper().quit(); break; case NEW_INTENT: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityNewIntent"); handleNewIntent((NewIntentData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case RECEIVER: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp"); handleReceiver((ReceiverData)msg.obj); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case CREATE_SERVICE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate"); handleCreateService((CreateServiceData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case BIND_SERVICE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind"); handleBindService((BindServiceData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case UNBIND_SERVICE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind"); handleUnbindService((BindServiceData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case SERVICE_ARGS: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStart"); handleServiceArgs((ServiceArgsData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case STOP_SERVICE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop"); handleStopService((IBinder)msg.obj); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case CONFIGURATION_CHANGED: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "configChanged"); mCurDefaultDisplayDpi = ((Configuration)msg.obj).densityDpi; handleConfigurationChanged((Configuration)msg.obj, null); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case CLEAN_UP_CONTEXT: ContextCleanupInfo cci = (ContextCleanupInfo)msg.obj; cci.context.performFinalCleanup(cci.who, cci.what); break; case GC_WHEN_IDLE: scheduleGcIdler(); break; case DUMP_SERVICE: handleDumpService((DumpComponentInfo)msg.obj); break; case LOW_MEMORY: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "lowMemory"); handleLowMemory(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case ACTIVITY_CONFIGURATION_CHANGED: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityConfigChanged"); handleActivityConfigurationChanged((IBinder)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case PROFILER_CONTROL: handleProfilerControl(msg.arg1 != 0, (ProfilerInfo)msg.obj, msg.arg2); break; case CREATE_BACKUP_AGENT: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupCreateAgent"); handleCreateBackupAgent((CreateBackupAgentData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case DESTROY_BACKUP_AGENT: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupDestroyAgent"); handleDestroyBackupAgent((CreateBackupAgentData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case SUICIDE: Process.killProcess(Process.myPid()); break; case REMOVE_PROVIDER: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "providerRemove"); completeRemoveProvider((ProviderRefCount)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case ENABLE_JIT: ensureJitEnabled(); break; case DISPATCH_PACKAGE_BROADCAST: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastPackage"); handleDispatchPackageBroadcast(msg.arg1, (String[])msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case SCHEDULE_CRASH: throw new RemoteServiceException((String)msg.obj); case DUMP_HEAP: handleDumpHeap(msg.arg1 != 0, (DumpHeapData)msg.obj); break; case DUMP_ACTIVITY: handleDumpActivity((DumpComponentInfo)msg.obj); break; case DUMP_PROVIDER: handleDumpProvider((DumpComponentInfo)msg.obj); break; case SLEEPING: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "sleeping"); handleSleeping((IBinder)msg.obj, msg.arg1 != 0); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case SET_CORE_SETTINGS: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "setCoreSettings"); handleSetCoreSettings((Bundle) msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case UPDATE_PACKAGE_COMPATIBILITY_INFO: handleUpdatePackageCompatibilityInfo((UpdateCompatibilityData)msg.obj); break; case TRIM_MEMORY: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "trimMemory"); handleTrimMemory(msg.arg1); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; case UNSTABLE_PROVIDER_DIED: handleUnstableProviderDied((IBinder)msg.obj, false); break; case REQUEST_ASSIST_CONTEXT_EXTRAS: handleRequestAssistContextExtras((RequestAssistContextExtras)msg.obj); break; case TRANSLUCENT_CONVERSION_COMPLETE: handleTranslucentConversionComplete((IBinder)msg.obj, msg.arg1 == 1); break; case INSTALL_PROVIDER: handleInstallProvider((ProviderInfo) msg.obj); break; case ON_NEW_ACTIVITY_OPTIONS: Pair<IBinder, ActivityOptions> pair = (Pair<IBinder, ActivityOptions>) msg.obj; onNewActivityOptions(pair.first, pair.second); break; case CANCEL_VISIBLE_BEHIND: handleCancelVisibleBehind((IBinder) msg.obj); break; case BACKGROUND_VISIBLE_BEHIND_CHANGED: handleOnBackgroundVisibleBehindChanged((IBinder) msg.obj, msg.arg1 > 0); break; case ENTER_ANIMATION_COMPLETE: handleEnterAnimationComplete((IBinder) msg.obj); break; } if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what)); } private void maybeSnapshot() { if (mBoundApplication != null && SamplingProfilerIntegration.isEnabled()) { // convert the *private* ActivityThread.PackageInfo to *public* known // android.content.pm.PackageInfo String packageName = mBoundApplication.info.mPackageName; android.content.pm.PackageInfo packageInfo = null; try { Context context = getSystemContext(); if(context == null) { Log.e(TAG, "cannot get a valid context"); return; } PackageManager pm = context.getPackageManager(); if(pm == null) { Log.e(TAG, "cannot get a valid PackageManager"); return; } packageInfo = pm.getPackageInfo( packageName, PackageManager.GET_ACTIVITIES); } catch (NameNotFoundException e) { Log.e(TAG, "cannot get package info for " + packageName, e); } SamplingProfilerIntegration.writeSnapshot(mBoundApplication.processName, packageInfo); } } }
虽然大部分调用的方法我们不知道其作用,但是可以根据消息表示可以看出大部分方法都涉及到Android系统的管理。看到这些我们可以断定,Android的UI线程其实就是一个Message机制的接收线程,其他线程遇到需要更新界面的情况,都只需通过H对象往UI线程发送一个更新界面的消息。当然,因为Looper.getMainLooper()方法的存在,我们也可以自己定义绑定主Looper的Handler类并实现更新界面消息的发送。
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