Android干货四大组件

Activity的启动流程这一篇够了

2021-01-16  本文已影响0人  酱爆大头菜
Activity启动.png

来了小伙子，先自我介绍一下吧

什么？你精通Android系统？来，你给我说下Activity的启动流程。

Activity的启动过程是系统中比较有代表意义的过程，涉及到了各个进程之间的相互交互，以及生命周期的回调控制，这也是为什么在面试过程出现频率这么高的原因之一。

Activity的启动流程在API28以后变成了事务启动的方式，相比之前版本的逻辑有了一些变化，但是万变不离其宗，大致流程还是类似的，只是增加了一些类，用来更好的划分职责，更优的处理逻辑。

在开始之前，我们先简单普及下基础知识。

Activity 启动主要涉及到3个进程。

进程之间靠什么通信？

我们都知道进程与进程之间是数据隔离的，无法相互访问数据，所以进程之间通信是靠Binder来完成的。

面试官可能会问你 为什么会用Binder通信，Binder相比Socket有什么优势呢？

然而果然被追问了 为啥Binder能做到1次拷贝，而其他的技术是2次拷贝

好，可以描述下具体的映射怎么做的么？....

这里我推荐一篇Binder的文章，我认为是大量介绍Binder的博客中描述的很透彻的，没有废话，适合急性子。有兴趣的同学可以学习以下，防止面试的时候跟我一样叫大哥。
Android Binder通信一次拷贝你真的理解了吗？

Activity启动流程主要包含几步？

我们以点击Launcher的一个icon为开始，整体扯一下Activity的启动过程，桌面其实就是LauncherApp的一个Activity

Activity启动过程主要涉及哪些类

启动一个Activity一般通过startActivity()

startActivity(new Intent(OneActivity.this,TwoActivity.class));

源码层分析整个链路

我们先看下正常启动Activity的方式，一般我们都会通过以下的方式启动一个新的Activity。

startActivity(new Intent(OneActivity.this,TwoActivity.class));

其实这是在Activity中的调用方式，调用的即是父类Activity的startActivity()方法,因参数不同分为两个方法，具体如下

  @Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

  @Override
  public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

最终调用的是startActivityForResult()

  public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                //主要看这里mInstrumentation为Instrumentation对象
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

核心逻辑是调用了Instrumentation.execStartActivity()

  public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
       ...
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            //核心在这一句
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

ActivityManager.getService()这个获取的是谁？
逻辑是从IActivityManagerSingleton.get()获取，那IActivityManagerSingleton又是谁？
IActivityManagerSingleton是这么定义的Singleton<IActivityManager> IActivityManagerSingleton
get取出来的是IActivityManager,看这个大写I开头就知道是一个接口，实际调用过的是它的实现ActivityManagerService。
ActivityManagerService. startActivity()

   @Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
    }

    @Override
    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
                true /*validateIncomingUser*/);
    }

    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
            boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivity");

        userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,
                Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

        // TODO: Switch to user app stacks here.
        return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();

    }

mActivityStartController.obtainStarter实际调用的是ActivityStarter.execute(),连带调用到ActivityStarter.startActivity()

 private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                ActivityRecord[] outActivity) {
        int result = START_CANCELED;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
        } finally {
            ...     
        }
        postStartActivityProcessing(r, result, mTargetStack);
        return result;
    }

ActivityStarter.startActivityUnchecked()连带调用ActivityStackSupervisor.resumeFocusedStackTopActivityLocked();

 boolean resumeFocusedStackTopActivityLocked(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

        if (!readyToResume()) {
            return false;
        }

        if (targetStack != null && isFocusedStack(targetStack)) {
            //主要看这里
            return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }

        final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
        if (r == null || !r.isState(RESUMED)) {
            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        } else if (r.isState(RESUMED)) {
            // Kick off any lingering app transitions form the MoveTaskToFront operation.
            mFocusedStack.executeAppTransition(targetOptions);
        }
        return false;
    }

targetStack为ActivityStack对象，ActivityStack.resumeTopActivityUncheckedLocked()

 boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mStackSupervisor.inResumeTopActivity) {
            // 防止递归的
            return false;
        }

        boolean result = false;
        try {
            // Protect against recursion.
            mStackSupervisor.inResumeTopActivity = true;
            //主要看这
            result = resumeTopActivityInnerLocked(prev, options);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }

        return result;
    }

ActivityStack.resumeTopActivityInnerLocked()调用了mStackSupervisor.startSpecificActivityLocked(next, true, true);其中mStackSupervisor为ActivityStackSupervisor。
ActivityStackSupervisor.startSpecificActivityLocked()中调用
ActivityStackSupervisor.realStartActivityLocked()

  final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
...
  // Schedule transaction.
   mService.getLifecycleManager().scheduleTransaction(clientTransaction);
}

注意这个clientTransaction对象，通过这种方式初始化

    //app.thread为IApplicationThread  
    final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,  r.appToken);

     // 注意下这个LaunchActivityItem.obtain
    clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                        System.identityHashCode(r), r.info,
                        // TODO: Have this take the merged configuration instead of separate global
                        // and override configs.
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo));

ClientLifecycleManager.scheduleTransaction(clientTransaction);

    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) { 
            transaction.recycle();
        }
    }

transaction.schedule();就要找到

public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }

mClient即是以上描述的ApplicationThread,因此我们跟进ApplicationThread.scheduleTransaction（）

 @Override
        public void scheduleTransaction
                (ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);
        }

我靠，调用了ActivityThread.scheduleTransaction(transaction),但是ActivityThread并没有scheduleTransaction(),所以我们找他继承的类ClientTransactionHandler，发现ClientTransactionHandler果然有scheduleTransaction()

   /** Prepare and schedule transaction for execution. */
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }

以上消息通过ActivityThread H对象进行发送具体解析也在ActivityThread中

  case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) { 
                        transaction.recycle();
                    }
                    // TODO(lifecycler): Recycle locally scheduled transactions.
                    break;

我们可以看下TransactionExecutor.execute(transaction);

    //首先，所有回调将按照它们在列表中出现的顺序执行。如果回调需要特定的执行前或执行后状态，
   //则客户端将相应地进行转换。然后客户端将循环到最终的生命周期状态（如果提供）。
   //否则，它将保持在回调所需的初始状态或最后状态。
    public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);

        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

这里我们主要看下executeCallbacks()方法

  /** Transition to the final state if requested by the transaction. */
  public void executeCallbacks(ClientTransaction transaction) {
//transaction.getCallbacks()会获取clientTransaction.addCallbacks()的数据。
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null) {
            // No callbacks to execute, return early.
            return;
        }
        log("Resolving callbacks");

        final IBinder token = transaction.getActivityToken();
        ActivityClientRecord r = mTransactionHandler.getActivityClient(token);

        // In case when post-execution state of the last callback matches the final state requested
        // for the activity in this transaction, we won't do the last transition here and do it when
        // moving to final state instead (because it may contain additional parameters from server).
        final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
        final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
                : UNDEFINED;
        // Index of the last callback that requests some post-execution state.
        final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);

        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            log("Resolving callback: " + item);
            final int postExecutionState = item.getPostExecutionState();
            final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,
                    item.getPostExecutionState());
            if (closestPreExecutionState != UNDEFINED) {
                cycleToPath(r, closestPreExecutionState);
            }

            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            if (r == null) {
                // Launch activity request will create an activity record.
                r = mTransactionHandler.getActivityClient(token);
            }

            if (postExecutionState != UNDEFINED && r != null) {
                // Skip the very last transition and perform it by explicit state request instead.
                final boolean shouldExcludeLastTransition =
                        i == lastCallbackRequestingState && finalState == postExecutionState;
                cycleToPath(r, postExecutionState, shouldExcludeLastTransition);
            }
        }
    }

transaction.getCallbacks()就是上述过程中ClientTransaction创建过程赋值的LaunchActivityItem对象，因此主要看下LaunchActivityItem.execute()

    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
        ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
                mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
                mPendingResults, mPendingNewIntents, mIsForward,
                mProfilerInfo, client);
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }

此时的client为ActivityThread，因此调用ActivityThread.handleLaunchActivity()

   @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        if (!ThreadedRenderer.sRendererDisabled) {
            GraphicsEnvironment.earlyInitEGL();
        }
        WindowManagerGlobal.initialize();

        final Activity a = performLaunchActivity(r, customIntent);
...
}

 /**  Core implementation of activity launch. */
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
        ...
            }
        }

        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
          ...
                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {     
                }
                r.activity = activity;
            }
            r.setState(ON_CREATE);
            mActivities.put(r.token, r);
        } catch (SuperNotCalledException e) {
            throw e;
        } catch (Exception e) {
            }
        }

        return activity;
    }

主要跟进mInstrumentation.callActivityOnCreate()

    public void callActivityOnCreate(Activity activity, Bundle icicle,
            PersistableBundle persistentState) {
        prePerformCreate(activity);
        activity.performCreate(icicle, persistentState);
        postPerformCreate(activity);
    }

Activity.performCreate()

 final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        mCanEnterPictureInPicture = true;
        restoreHasCurrentPermissionRequest(icicle);
        if (persistentState != null) {
            onCreate(icicle, persistentState);
        } else {
            onCreate(icicle);
        }
        writeEventLog(LOG_AM_ON_CREATE_CALLED, "performCreate");
        mActivityTransitionState.readState(icicle);

        mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
                com.android.internal.R.styleable.Window_windowNoDisplay, false);
        mFragments.dispatchActivityCreated();
        mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
    }

至此已经回调到了Activity的onCreate()方法，Activity也就正式启动了，后续就是对应的声明周期回调。

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