Activity 啟動(dòng)流程學(xué)習(xí)總結(jié)（附源碼）

前言

關(guān)于 Activity 啟動(dòng)，Android 中場(chǎng)景大致有兩個(gè)：

從 launcher 中啟動(dòng)應(yīng)用，觸發(fā)該應(yīng)用默認(rèn) Activity 的啟動(dòng)。這種 Activity 都是在新進(jìn)程和新的任務(wù)棧中啟動(dòng)的，所以涉及到新進(jìn)程和新任務(wù)棧的初始化
應(yīng)用程序內(nèi)部啟動(dòng)非默認(rèn) Activity, 被啟動(dòng)的 Activity 一般在原來(lái)的進(jìn)程和任務(wù)棧中啟動(dòng)

本文主要介紹第一種場(chǎng)景

背景知識(shí)

進(jìn)程與線程

由于 Activity 的啟動(dòng)流程中涉及了大量的進(jìn)程間通信芯砸，例如：ActivityManagerService 和 ActivityStack 位于同一進(jìn)程，ApplicationThread 和 ActivityThread 位于同一進(jìn)程给梅。所以有必要梳理一下進(jìn)程與線程的區(qū)別

從操作系統(tǒng)的角度看，進(jìn)程和線程有什么區(qū)別双揪？

進(jìn)程有獨(dú)立的地址空間动羽，一個(gè)進(jìn)程崩潰后，在保護(hù)模式下不會(huì)對(duì)其他進(jìn)程造成影響渔期。
線程沒(méi)有獨(dú)立的地址空間运吓，線程只是進(jìn)程所屬進(jìn)程的不同執(zhí)行路徑

從 JVM 的運(yùn)行時(shí)數(shù)據(jù)分區(qū)的角度，進(jìn)程和線程有什么關(guān)系疯趟？

JVM 運(yùn)行時(shí)數(shù)據(jù)分區(qū)如圖所示：

image

由圖可以看出如下幾點(diǎn)：

一個(gè)進(jìn)程中的多個(gè)線程共享堆區(qū)和方法區(qū)
每個(gè)線程擁有自己的程序計(jì)數(shù)器拘哨，虛擬機(jī)棧，本地方法棧

Activity 啟動(dòng)模式

啟動(dòng)模式橫向?qū)Ρ?/h4>
四中啟動(dòng)模式的概念就不詳述了信峻，這里只是對(duì)關(guān)鍵點(diǎn)做出橫向?qū)Ρ染肭啵鐖D所示

Activity 啟動(dòng)模式比較

一些鮮為人知的 Intent Flag

下面是一些 Intent Flag 及介紹，如圖所示

鮮為人知但常用的啟動(dòng)模式 flag

Activity 啟動(dòng)流程源碼分析

概念

首先介紹一下 Activity 啟動(dòng)流程涉及到的核心類

ActivityThread

ActivityThread 的作用是管理應(yīng)用程序主線程的相關(guān)流程盹舞，例如管理與處理 activity manager 發(fā)送的請(qǐng)求产镐，這些請(qǐng)求可以來(lái)自 Activity 或 BroadCast

Instrumentation

它用來(lái)監(jiān)控應(yīng)用程序與系統(tǒng)之間的交互

ActivityManagerService (AMS)

AMS(ActivityManagerService)是貫穿Android系統(tǒng)組件的核心服務(wù)，負(fù)責(zé)了系統(tǒng)中四大組件的啟動(dòng)踢步、切換癣亚、調(diào)度以及應(yīng)用進(jìn)程管理和調(diào)度工作

執(zhí)行流程

Step 1. Launcher.startActivitySafely

/**
 * Launches the intent referred by the clicked shortcut.
 *
 * @param v The view representing the clicked shortcut.
 */
public void onClick(View v) {
    // Make sure that rogue clicks don't get through while allapps is launching, or after the
    // view has detached (it's possible for this to happen if the view is removed mid touch).
    if (v.getWindowToken() == null) {
        return;
    }
    if (!mWorkspace.isFinishedSwitchingState()) { // Launcher 程序在監(jiān)聽(tīng)點(diǎn)擊事件時(shí)會(huì)判斷頁(yè)面是否正在滑動(dòng)，如果在滑動(dòng)則不響應(yīng)點(diǎn)擊應(yīng)用程序 icon 的事件
        return;
    }
    Object tag = v.getTag();
    if (tag instanceof ShortcutInfo) { // 處理點(diǎn)擊應(yīng)用程序 icon 的場(chǎng)景
        // Open shortcut
        final Intent intent = ((ShortcutInfo) tag).intent;
        int[] pos = new int[2];
        v.getLocationOnScreen(pos);
        intent.setSourceBounds(new Rect(pos[0], pos[1],
                pos[0] + v.getWidth(), pos[1] + v.getHeight()));

        boolean success = startActivitySafely(v, intent, tag); // startActivitySafely 這里主要會(huì)判斷待啟動(dòng)的 Activity 是否存在获印，若不存在則會(huì)報(bào) ActivityNotFound Exception 并捕獲

        if (success && v instanceof BubbleTextView) {
            mWaitingForResume = (BubbleTextView) v;
            mWaitingForResume.setStayPressed(true);
        }
    } else if (tag instanceof FolderInfo) {  // 處理點(diǎn)擊文件夾的場(chǎng)景
        ···
    } else if (v == mAllAppsButton) {
        ···
    }
}

Launcher 程序在監(jiān)聽(tīng)點(diǎn)擊事件時(shí)會(huì)判斷頁(yè)面是否正在滑動(dòng)述雾，如果在滑動(dòng)則不響應(yīng)點(diǎn)擊應(yīng)用程序 icon 的事件

Step 2. Activity.startActivity()

由于 Launcher 繼承于 Activity, 因此代碼執(zhí)行流程到了 startActivity()。


public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks {
 
    ......
 
    @Override
    public void startActivity(Intent intent) {
        startActivityForResult(intent, -1);
    }
 
    ......

startActivity 最終是調(diào)用 startActivityForResult, 傳入的參數(shù)為 -1 代表這次啟動(dòng) Activity 不需要獲取結(jié)果

Step 3. Activity.startActivityForResult()


public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks {
 
    ......
 
    public void startActivityForResult(Intent intent, int requestCode) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                this, mMainThread.getApplicationThread(), mToken, this,
                intent, requestCode);
            ......
        } else {
            ......
        }
 
 
    ......

這里有兩個(gè)有趣的場(chǎng)景：

startActivityForResult() 不能用于啟動(dòng) singleTask 為啟動(dòng)模式的 Activity, 否則會(huì)從 onActivityForResult() 中收到 cancel 事件
假設(shè)有這樣一個(gè)場(chǎng)景，Activity A 啟動(dòng) Activity B, 如果在 A 的 onCreate() / onResume() 中調(diào)用 startActivityForResult(), 并且傳入的 requestCode >= 1, 那么 A Activity 所掛鉤的 Window 將暫時(shí)不可見(jiàn)玻孟，這種不可見(jiàn)狀態(tài)直到收到 B Activity 的回調(diào)結(jié)果唆缴。目的是防止 Activity 跳轉(zhuǎn)時(shí) UI 閃爍

Step 4. ActivityStack.startActivityUncheckedLocked

final int startActivityUncheckedLocked(ActivityRecord r,
        ActivityRecord sourceRecord, Uri[] grantedUriPermissions,
        int grantedMode, boolean onlyIfNeeded, boolean doResume) {
        final Intent intent = r.intent;
        final int callingUid = r.launchedFromUid;
 
        int launchFlags = intent.getFlags();
 
        // We'll invoke onUserLeaving before onPause only if the launching
        // activity did not explicitly state that this is an automated launch.
        mUserLeaving = (launchFlags&Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;
        
        ......
 
        ActivityRecord notTop = (launchFlags&Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP)
            != 0 ? r : null;
 
        // If the onlyIfNeeded flag is set, then we can do this if the activity
        // being launched is the same as the one making the call...  or, as
        // a special case, if we do not know the caller then we count the
        // current top activity as the caller.
        if (onlyIfNeeded) {
            ......
        }
 
        if (sourceRecord == null) {
            ......
        } else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
            ......
        } else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE
            || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {
            ......
        }
 
        if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
            ......
        }
 
        boolean addingToTask = false;
        if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
            (launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
            || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK
            || r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
                // If bring to front is requested, and no result is requested, and
                // we can find a task that was started with this same
                // component, then instead of launching bring that one to the front.
                if (r.resultTo == null) {
                    // See if there is a task to bring to the front.  If this is
                    // a SINGLE_INSTANCE activity, there can be one and only one
                    // instance of it in the history, and it is always in its own
                    // unique task, so we do a special search.
                    ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE
                        ? findTaskLocked(intent, r.info)
                        : findActivityLocked(intent, r.info);
                    if (taskTop != null) {
                        ......
                    }
                }
        }
 
        ......
 
        if (r.packageName != null) {
            // If the activity being launched is the same as the one currently
            // at the top, then we need to check if it should only be launched
            // once.
            ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);
            if (top != null && r.resultTo == null) {
                if (top.realActivity.equals(r.realActivity)) {
                    ......
                }
            }
 
        } else {
            ......
        }
 
        boolean newTask = false;
 
        // Should this be considered a new task?
        if (r.resultTo == null && !addingToTask
            && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
                // todo: should do better management of integers.
                mService.mCurTask++;
                if (mService.mCurTask <= 0) {
                    mService.mCurTask = 1;
                }
                r.task = new TaskRecord(mService.mCurTask, r.info, intent,
                    (r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);
                ......
                newTask = true;
                if (mMainStack) {
                    mService.addRecentTaskLocked(r.task);
                }
 
        } else if (sourceRecord != null) {
            ......
        } else {
            ......
        }
 
        ......
 
        startActivityLocked(r, newTask, doResume);
        return START_SUCCESS;
    }

由于我們是總 launcher 啟動(dòng) Activity, 因此當(dāng)前的前臺(tái)任務(wù)棧棧頂是 launcher Activity, 因此需要?jiǎng)?chuàng)建一個(gè)新的 ActivityStack, 和 ActivityRecord 對(duì)象，將 ActivityRecord 放入其中取募，最終這個(gè)新創(chuàng)建的 ActivityStack 被保存到 AMS 中

Step 5. Activity.resumeTopActivityLocked

   // If the top activity is the resumed one, nothing to do.
    if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
    ......
    }
 
    // If we are sleeping, and there is no resumed activity, and the top
    // activity is paused, well that is the state we want.
    if ((mService.mSleeping || mService.mShuttingDown)
    && mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
    ......
    }

現(xiàn)在我們已經(jīng)準(zhǔn)備好了新的 ActivityStack 和新的 ActivityRecord, 然后我們就需要處理 launcher Activity 的生命周期琐谤，將其置為 pause 狀態(tài)。根據(jù)源碼玩敏，這里首先會(huì)檢查被啟動(dòng)的 Activity 是否在棧頂斗忌，是否就是啟動(dòng)者，如果是的話旺聚，就什么都不做织阳，如果都不滿足的話，就需要將目前棧頂?shù)?Activity 置為 Pause 狀態(tài)砰粹，在這之前唧躲，首先要檢查是否有正在 Pausing 的 Activity, 如果有的話launcher Activity 需要被掛起，

總結(jié)

由于 Activity 啟動(dòng)流程較復(fù)雜碱璃，剩余流程將不再詳述弄痹，總結(jié)如下圖所示：