1.前言

• 最近一直在看 《Android進階解密》 的一本書，這本書編寫邏輯、流程都非常好，而且很容易看懂，非常推薦大家去看看（沒有收廣告費，單純覺得作者寫的很好）。
• 上一篇簡單的介紹了Android進階（三）：Application啟動過程(最詳細&最簡單)。
• 今天就介紹Application啟動之後，是如何啟動應用程式中的 第一個Activity （基於Android 8.0 系統）。
• 文章中例項 linhaojian的Github

2.Activity啟動過程的時序圖

3.原始碼分析

3.1 ActivityManagerService

：

private final boolean attachApplicationLocked(IApplicationThread thread,
      int pid, int callingUid, long startSeq) {
    //...
    //通知ActivityThread啟動application
    thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
                    null, null, null, testMode,
                    mBinderTransactionTrackingEnabled,
 
 enableTrackAllocation,
                    isRestrictedBackupMode || !normalMode, app.persistent,
                    new Configuration(getGlobalConfiguration()), app.compat,
                    getCommonServicesLocked(app.isolated),
                    mCoreSettingsObserver.getCoreSettingsLocked()
 
,
                    buildSerial, isAutofillCompatEnabled);// 1
     // 啟動第一個Activity
    if (normalMode) {
        try {
            //Application初始化之後，初始化&啟動第一個Activity
            if (mStackSupervisor.attachApplicationLocked(app)) {// 2
                didSomething = true;
            }
        } catch (Exception e) {
            Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
            badApp = true;
        }
    }     
}

• 註釋1：通知ActivityThread啟動application；
• 註釋2：呼叫ActivityStackSupervisor，啟動第一個Activity；

3.2 ActivityStackSupervisor 啟動Activity：

    boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
        //獲取應用的進行名稱
        final String processName = app.processName;
        boolean didSomething = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            final ActivityDisplay display = mActivityDisplays.valueAt(displayNdx);
            for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
                final ActivityStack stack = display.getChildAt(stackNdx);
                if (!isFocusedStack(stack)) {
                    continue;
                }
                stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
                final ActivityRecord top = stack.topRunningActivityLocked();
                final int size = mTmpActivityList.size();
                for (int i = 0; i < size; i++) {
                    final ActivityRecord activity = mTmpActivityList.get(i);
                    if (activity.app == null && app.uid == activity.info.applicationInfo.uid
                            && processName.equals(activity.processName)) {
                        try {
                            // Application初始化之後，啟動第一個Activity
                            if (realStartActivityLocked(activity, app,
                                    top == activity /* andResume */, true /* checkConfig */)) { // 1
                                didSomething = true;
                            }
                        } catch (RemoteException e) {
                            Slog.w(TAG, "Exception in new application when starting activity "
                                    + top.intent.getComponent().flattenToShortString(), e);
                            throw e;
                        }
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
        }
        return didSomething;
    }

• 註釋1：呼叫自身的realStartActivityLocked()，真正的啟動Activity；

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
                //...
                // Create activity launch transaction.
                //通知ActivityThread建立activity的例項 ；呼叫Activity的OnCreate； 建立對應PhoneWindow例項
                final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
                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)); // 2
                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward()); // 3
                } else {
                    lifecycleItem = PauseActivityItem.obtain(); // 4
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);
                // 編制啟動Activity例項的事務，並按順序呼叫onCreate();onStart();onResume();
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);// 5
    }

• 註釋2：封裝啟動Activity的相關資料與Activity的啟動流程（LaunchActivityItem 下面會介紹該類）；
• 註釋3：封裝Activity啟動之後生命週期變化為Resume的流程（ResumeActivityItem下面會介紹該類）；
• 註釋4：封裝Activity啟動之後生命週期變化為Pause的流程；
• 註釋5：真正開始啟動Activity與呼叫相關的生命週期方法；

3.3 ClientLifecycleManager管理ClientTransaction：

    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();// 1
        transaction.schedule();// 2
        if (!(client instanceof Binder)) {
            // If client is not an instance of Binder - it's a remote call and at this point it is
            // safe to recycle the object. All objects used for local calls will be recycled after
            // the transaction is executed on client in ActivityThread.
            transaction.recycle();
        }
    }

• 註釋1：獲取需要啟動Activity程序的代理物件IApplicationThread；
• 註釋2：呼叫ClientTransaction中的schedule()；

3.4 ClientTransaction中schedule函式：

    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);// 1
    }

• 註釋1：mClient就是與ActivityThread通訊的代理物件(IApplicationThread)，所以這裡其實是呼叫ActivityThread類中ApplicationThread內部類的scheduleTransaction()；

3.5 ApplicationThread的scheduleTransaction函式：

    private class ApplicationThread extends IApplicationThread.Stub {
        @Override
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);// 1
        }    
    }

• 註釋1：這裡其實最後還是呼叫了ActivityThread的scheduleTransaction()，而這個scheduleTransaction()其實真正的實現是ClientTransactionHandler，因為ActivityThread是繼承ClientTransactionHandler類的；

public abstract class ClientTransactionHandler {
    /** Prepare and schedule transaction for execution. */
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);// 2
    }
}

• 註釋2：呼叫ActivityThread的sendMessage()，將ClientTransaction引數通過Handler機制切換至主執行緒進行處理；

3.6 ActivityThread的H類：

    class H extends Handler {
        //...
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case EXECUTE_TRANSACTION: // 1
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;// 2
                    mTransactionExecutor.execute(transaction);// 3
                    if (isSystem()) {
                        // Client transactions inside system process are recycled on the client side
                        // instead of ClientLifecycleManager to avoid being cleared before this
                        // message is handled.
                        transaction.recycle();
                    }
                    // TODO(lifecycler): Recycle locally scheduled transactions.
                    break;              
            }        
        }
    }

• 註釋1：接收ClientTransactionHandler傳送的EXECUTE_TRANSACTION事件；
• 註釋2：接收ClientTransactionHandler傳送的ClientTransaction引數；
• 註釋3：通過TransactionExecutor真正處理ClientTransaction中封裝的Activity相關資訊；

3.7 TransactionExecutor處理ClientTransaction：

    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");
    }

3.8 executeCallbacks()啟動Activity：

    @VisibleForTesting
    public void executeCallbacks(ClientTransaction transaction) {
        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);// 1
            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);// 2
            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);
            }
        }
    }

• 註釋1：獲取ClientTransaction中的ClientTransactionItem物件（其實這裡的物件就上面3.2介紹中的LaunchActivityItem）；
• 註釋2：呼叫LaunchActivityItem類的execute()，下面我們看看LaunchActivityItem；

public class LaunchActivityItem extends ClientTransactionItem {
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,// 1
            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 */);// 2
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
}

• 註釋1：剛剛說了ClientTransactionHandler是被ActivityThread繼承，所以這裡傳入進來就是ActivityThread;
• 註釋2：看到了吧，經過多次不同層次相互呼叫，最終真正呼叫ActivityThread的handleLaunchActivity()，啟動應用的第一個Activity;

3.8 executeLifecycleState()改變Activity的生命週期：

    private void executeLifecycleState(ClientTransaction transaction) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();// 1
        if (lifecycleItem == null) {
            // No lifecycle request, return early.
            return;
        }
        log("Resolving lifecycle state: " + lifecycleItem);
        final IBinder token = transaction.getActivityToken();
        final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        if (r == null) {
            // Ignore requests for non-existent client records for now.
            return;
        }
        // Cycle to the state right before the final requested state.
        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
        // Execute the final transition with proper parameters.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions); // 2
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }

• 註釋1：獲取ClientTransaction中的ActivityLifecycleItem 物件（其實這裡的物件就上面3.2介紹中的ResumeActivityItem）；
• 註釋2：呼叫ResumeActivityItem的execute()，下面我們看看ResumeActivityItem；

public class ResumeActivityItem extends ActivityLifecycleItem {
    //...
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
        client.handleResumeActivity(token, true /* finalStateRequest */, mIsForward, // 1
                "RESUME_ACTIVITY");
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
}

• 註釋1：其實就是呼叫Activity的handleResumeActivity()，最終就會呼叫Activity的onStart()與onResume()；

4.類關係

• 通過上圖發現，Activity啟動過程就是AMS與ActivityThread不同的程序互動實現的。

5.總結

• 到此，**Activity啟動過程**介紹完畢。
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  -  linhaojian的Github

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