Android：启动模式总结

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android启动模式的实现一般分两种，一种在代码里面startActivity的时候添加flag实现，一种是在AndroidManifest中添加launchMode实现。 如果两个地方都加了，就会是叠加的效果。

一.flag例子：桌面点击图标是什么参数？

默认情况点击图标时：

ActivityManager: START u0 {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.example.wenfengtou.firstmacapp/.MainActivity bnds=[539,717][790,1001]} from uid 10028 and from pid 2458

可以看到flag时0x10200000，对应的flag就是
FLAG_ACTIVITY_NEW_TASK
FLAG_ACTIVITY_RESET_TASK_IF_NEEDED

FLAG_ACTIVITY_NEW_TASK标志，如果要启动的Activity在后台，就会把Activity所在的Task推到前台显示，要注意不一定显示要启动的Activity，而是显示Task上处于TOP位置的Activity。例如点击桌面图标，启动A，在A中启动B，点击home键返回桌面，再次点击桌面图标，会显示B，因为B处于TOP的位置。
使用adb模拟桌面点击：

am start -n com.example.wenfengtou.firstmacapp/.MainActivity -f  0x10200000 -c android.intent.category.LAUNCHER -a android.intent.action.MAIN                                   

注意一定注意保证action和catroy一样，否则会启动一下新的A，栈的情况就会是A－B－A

二.Launchmode例子：如何实现点击桌面图标会回到首页？

Launchmode有四种

    public static final int LAUNCH_MULTIPLE = 0;
    /**
     * Constant corresponding to <code>singleTop</code> in
     * the {@link android.R.attr#launchMode} attribute.
     */
    public static final int LAUNCH_SINGLE_TOP = 1;
    /**
     * Constant corresponding to <code>singleTask</code> in
     * the {@link android.R.attr#launchMode} attribute.
     */
    public static final int LAUNCH_SINGLE_TASK = 2;
    /**
     * Constant corresponding to <code>singleInstance</code> in
     * the {@link android.R.attr#launchMode} attribute.
     */
    public static final int LAUNCH_SINGLE_INSTANCE = 3;

可以通过dumpsys activity a 来查看启动的Activity的launchmode。

    Task id #28869
    mFullscreen=true
    mBounds=null
    mMinWidth=-1
    mMinHeight=-1
    mLastNonFullscreenBounds=null
    * TaskRecord{51e69b4 #28869 A=com.example.wenfengtou.firstmacapp U=0 StackId=1 sz=1}
      userId=0 effectiveUid=u0a218 mCallingUid=u0a218 mUserSetupComplete=true mCallingPackage=com.example.wenfengtou.firstmacapp
      affinity=com.example.wenfengtou.firstmacapp
      intent={flg=0x10104000 cmp=com.example.wenfengtou.firstmacapp/.Sencond}
      realActivity=com.example.wenfengtou.firstmacapp/.Sencond
      autoRemoveRecents=false isPersistable=true numFullscreen=1 taskType=0 mTaskToReturnTo=1
      rootWasReset=false mNeverRelinquishIdentity=true mReuseTask=false mLockTaskAuth=LOCK_TASK_AUTH_PINNABLE
      Activities=[ActivityRecord{8fcae2a u0 com.example.wenfengtou.firstmacapp/.Sencond t28869}]
      askedCompatMode=false inRecents=false isAvailable=true
      lastThumbnail=null lastThumbnailFile=/data/system_ce/0/recent_images/28869_task_thumbnail.png
      stackId=1
      hasBeenVisible=true mResizeMode=RESIZE_MODE_RESIZEABLE_VIA_SDK_VERSION mSupportsPictureInPicture=false isResizeable=true firstActiveTime=1532758042288 lastActiveTime=1532758200753 (inactive for 1310s)
      * Hist #0: ActivityRecord{8fcae2a u0 com.example.wenfengtou.firstmacapp/.Sencond t28869}
          packageName=com.example.wenfengtou.firstmacapp processName=com.example.wenfengtou.firstmacapp
          launchedFromUid=10218 launchedFromPackage=com.example.wenfengtou.firstmacapp userId=0
          app=ProcessRecord{cfd9c93 27860:com.example.wenfengtou.firstmacapp/u0a218}
          Intent { flg=0x10104000 cmp=com.example.wenfengtou.firstmacapp/.Sencond }
          frontOfTask=true task=TaskRecord{51e69b4 #28869 A=com.example.wenfengtou.firstmacapp U=0 StackId=1 sz=1}
          taskAffinity=com.example.wenfengtou.firstmacapp
          realActivity=com.example.wenfengtou.firstmacapp/.Sencond
          baseDir=/data/app/com.example.wenfengtou.firstmacapp-eLkTGBVSXt9xqMyC0HtQKQ==/base.apk
          dataDir=/data/user/0/com.example.wenfengtou.firstmacapp
          stateNotNeeded=false componentSpecified=true mActivityType=0
          compat={480dpi} labelRes=0x7f060021 icon=0x7f030000 theme=0x0
          mLastReportedConfigurations:
           mGlobalConfig={1.0 ?mcc?mnc [zh_CN] ldltr sw360dp w360dp h685dp 480dpi nrml long port finger -keyb/v/h -nav/h appBounds=Rect(0, 0 - 1080, 2136) s.28mThemeChanged = 0mThemeChangedFlags = 0mFlipFont = 0}
           mOverrideConfig={1.0 ?mcc?mnc [zh_CN] ldltr sw360dp w360dp h685dp 480dpi nrml long port finger -keyb/v/h -nav/h appBounds=Rect(0, 0 - 1080, 2136) s.28mThemeChanged = 0mThemeChangedFlags = 0mFlipFont = 0}
          CurrentConfiguration={1.0 ?mcc?mnc [zh_CN] ldltr sw360dp w360dp h685dp 480dpi nrml long port finger -keyb/v/h -nav/h appBounds=Rect(0, 0 - 1080, 2136) s.28mThemeChanged = 0mThemeChangedFlags = 0mFlipFont = 0}
          taskDescription: iconFilename=null label="null" primaryColor=ff212121
            backgroundColor=fffafafa
            statusBarColor=ff000000
            navigationBarColor=0
          launchFailed=false launchCount=0 lastLaunchTime=-22m24s866ms
          haveState=true icicle=Bundle[mParcelledData.dataSize=860]
          state=STOPPED stopped=true delayedResume=false finishing=false
          keysPaused=false inHistory=true visible=false sleeping=false idle=true mStartingWindowState=STARTING_WINDOW_REMOVED
          fullscreen=true noDisplay=false immersive=false launchMode=0

四种launchmode简介：
standard：普通模式，启动activity都会新建。
singletop：如果要启动的Activity在top了，就不会新建Activity。如果不在，就新建一个。
singletask：如果要启动的activity在top了，就不会新建Activity（这种情况跟singletop一样）。如果不在top，而是在中间，就会清空他头上的所有activity。如果没创建过该activity，就会新建一个在top。
singleinstance：会在一个单独的task里面。

所以，如何如何实现点击桌面图标会回到首页？
例如首页时A，然后A启动了B，B启动了C。然后点击home键，如何实现点击桌面图片回到A呢？

答案就是设置A的Launchmode为singletask就可以了。因为他会清空他头上的B和C。