Android Activity啟動模式全面解析

在Android應用中, Activity是最核心的組件, 如何生成一個Activity實例, 可以選擇不同的啟動模式, 即LaunchMode. 啟動模式主要包括: standard, singleTop, singleTask, singleInstance.

標准模式在每次啟動時, 都會創建實例; 三種單例模式, 會根據情況選擇創建還是復用實例. 在Activity啟動中, 創建實例的生命周期: onCreate -> onStart -> onResume; 重用實例的生命周期: onNewIntent -> onResume.

在AndroidManifest的Activity中, 使用launchMode屬性, 可以設置啟動模式, 默認是standard模式; 使用taskAffinity屬性, 並添加包名, 可以設置Activity棧, 默認是當前包名, 只能應用於single模式.

希望通過本文, 可以更好的理解Activity的啟動模式(LaunchMode).

觀察Activity棧的腳本, 參考第5點 .

adb shell dumpsys activity | sed -n -e '/Stack #/p' -e '/Running activities/,/Run #0/p'

本文示例的GitHub 下載地址

1. Standard

標准模式, 啟動Activity的默認模式, 被啟動的Activity 會運行於 啟動的Activity 棧, 因此必須使用Activity的Context啟動, 不能使用Application, 否則會報錯.

如MainActivity啟動TestAActivity.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{3caa65e3 #2711 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}
        Run #1: ActivityRecord{36b06e99 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2711}
        Run #0: ActivityRecord{27396226 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2711}
  Stack #0:
    Running activities (most recent first):
      TaskRecord{27d796c9 #2695 A=com.miui.home U=0 sz=1}
        Run #0: ActivityRecord{2e5712cb u0 com.miui.home/.launcher.Launcher t2695}

棧內由下到上: MainActivity -> TestAActivity.

2. SingleTop

棧頂復用模式. 只有Activity位於棧頂, 重復啟動時, 會使用默認實例, 即單例模式; 如果位於棧內, 則仍然會創建實例.

MainActivity啟動TestA, TestA啟動TestB, TestB啟動自身, TestB是單例. 觀察棧內情況, TestB只有一份實例, 第二次創建復用.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{12abf566 #2712 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=3}
        Run #2: ActivityRecord{187d7ff7 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2712}
        Run #1: ActivityRecord{a551034 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2712}
        Run #0: ActivityRecord{22f9cce4 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2712}

棧內: MainActivity -> TestAActivity -> TestBActivity

MainActivity啟動TestA, TestA啟動TestB, TestB啟動TestC, TestC啟動TestB, TestB是單例. 觀察棧內情況, 由於TestC是棧頂, TestC啟動TestB, TestB不是棧頂, 重新創建TestB實例, 則保留兩份TestB.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{1792f5f0 #2715 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=5}
        Run #4: ActivityRecord{1e70110b u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2715}
        Run #3: ActivityRecord{c7f4dce u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestCActivity t2715}
        Run #2: ActivityRecord{254536cd u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2715}
        Run #1: ActivityRecord{36b2da15 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2715}
        Run #0: ActivityRecord{3a1c4a6a u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2715}

棧內: MainActivity -> TestAActivity -> TestBActivity ->TestCActivity -> TestBActivity

3. SingleTask

棧內復用模式, 只要Activity在一個棧中存在, 多次調用時, 都不會創建實例, 即單例模式.

情況包含以下幾種:

(1) 任務棧不存在, 初次啟動SingleTask實例, 會創建任務棧和實例.

MainActivity啟動TestA, TestA啟動TestB, TestB是SingleTask, 並且任務棧不同. 觀察可知, 系統包含兩個任務棧, TestB位於其他任務棧中.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{d5d53d4 #2727 A=me.chunyu.spike.stack U=0 sz=1}
        Run #2: ActivityRecord{1d720e55 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2727}
      TaskRecord{a3f797d #2726 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}
        Run #1: ActivityRecord{ffd689d u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2726}
        Run #0: ActivityRecord{192310ac u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2726}

使用taskAffinity屬性, 添加新的Activity棧, 與SingleTask配合使用, Standard模式無效.

新任務棧是 me.chunyu.spike.stack .

(2) 任務棧存在, 初次啟動SingleTask實例, 會直接入棧, 與Standard模式相同.

(3) 任務棧相同, 再次啟動SingleTask實例, 實例會置於棧頂, 並清除其上面實例, 具有clearTop的效果.

MainActivity啟動TestA, TestA啟動TestB, TestB是SingleTask, TestB啟動TestC, TestC重新啟動TestB, 則TestC會出棧. 觀察可知, TestC出棧, TestB位於棧頂.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{18230815 #2737 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=3}
        Run #4: ActivityRecord{1126c300 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2737}
        Run #3: ActivityRecord{3114fee8 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2737}
        Run #2: ActivityRecord{f8e235d u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2737}

TestC啟動TestB, SingleTask模式, 導致clearTop, TestC出棧.

(4) 任務棧不同, 再次啟動SingleTask實例, 會導致任務棧切換, 後台置於前台.

這比較難理解.MainActivity啟動TestA, TestA啟動TestB(SingleTask實例, 不同任務棧), TestB啟動TestC(與B類似), 則MainActivity和TestA相同棧, TestB和TestC相同棧, 此時棧頂是TestC. 按Home鍵, 再次啟動應用, 則默認任務棧會啟動, TestA啟動, TestA啟動TestC. 應用當前狀態如下.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{1d05e6c9 #2754 A=me.chunyu.spike.stack U=0 sz=2}
        Run #4: ActivityRecord{3f77e822 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestCActivity t2754}
      TaskRecord{3fe736d0 #2753 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}
        Run #3: ActivityRecord{15f0470e u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2753}
      TaskRecord{1d05e6c9 #2754 A=me.chunyu.spike.stack U=0 sz=2}
        Run #2: ActivityRecord{181229e6 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2754}
      TaskRecord{3fe736d0 #2753 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}
        Run #1: ActivityRecord{28628d61 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2753}
      TaskRecord{2d646058 #2719 A=com.android.incallui U=0 sz=1}

TestC至於棧頂, 點擊Back鍵, 不是返回TestA(啟動TestC的實例), 而是TestB, 即優先返回相同棧的實例. 再次是TestA, 然後是MainActivity, 依次出棧.

4. SingleInstance

單實例模式, 啟動時, 系統會為其創造一個單獨的任務棧, 以後每次使用, 都會使用這個單例, 直到其被銷毀, 屬於真正的單例模式.

示例: MainActivity啟動TestA, TestA啟動TestB(SingleInstance模式),

TestB啟動TestC, TestC再啟動TestB, 則仍啟動上一次的TestB,

TestC合並入默認棧(MainActivity+TestA).

  Stack #1:
    Running activities (most recent first):
      TaskRecord{384e3928 #2765 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=1}
        Run #3: ActivityRecord{1ffc5b6b u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2765}
      TaskRecord{2ad03544 #2764 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=3}
        Run #2: ActivityRecord{293d8c37 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestCActivity t2764}
        Run #1: ActivityRecord{158bc0f3 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2764}
        Run #0: ActivityRecord{77691cf u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2764}

5. startActivityForResult

Thx@回調的幸福時光

startActivityForResult不同於startActivity, 使用LaunchMode模式啟動Activity時, 也會有一些不同, 可以正常傳遞數據, 但是無法連續創建自己時, 會生成多份實例.

TestB(singleTask模式)使用startActivity創建自己時, 會使用默認實例, 即單例; 而使用startActivityForResult創建自己時, 會生成一份新的示例.

  Stack #1:
    Running activities (most recent first):
      TaskRecord{323200ac #2786 A=me.chunyu.clwang.stack U=0 sz=3}
        Run #4: ActivityRecord{3f9e14f3 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2786}
        Run #3: ActivityRecord{30d8f17b u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2786}
        Run #2: ActivityRecord{11b95b5c u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestBActivity t2786}
      TaskRecord{c86e175 #2785 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}
        Run #1: ActivityRecord{3558d7c4 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2785}
        Run #0: ActivityRecord{1b8620c u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2785}

由此可知, 因為startActivityForResult需要返回值, 會保留實例, 覆蓋單例效果.

注意: 4.x版本通過startActivityForResult啟動singleTask, 無法正常獲取返回值, 參考 .

5.x以上版本修復此問題, 考慮兼容性, 不推薦使用startActivityForResult和singleTask.

啟動模式做為Activity的重要屬性, 還是需要比較透徹的掌握.