> For the complete documentation index, see [llms.txt](https://blog.cweihang.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://blog.cweihang.io/android/activity/task-he-hui-tui-zhan.md).

# Task和回退栈

对Task和backStack的认识过程

1.由demo测试得到的关系图：

![task1](/files/-L_G1DZCRKKevdiwnZT9)

* 一个task中可以有多个app的Activity，
* 由于一个app可以对应一个或多个process，
* 所以一个task也可以对应多个process。

**2.由adb shell dumpsys activity测试得到的关系图：** ![task2](/files/-L_G1DZEa_CGdy5oEGfA)

* Android将所有的task维护在Main stack中，
* 每个task由TaskId唯一标识，在Hist中包含了处于同一个task内的Activity，
* 不同的process（拥有不同的PID）中，可以包含同一个app的不同Activity，
* 一个package标识一个Application。

**3.通过学习源码得到的关系图：** 

![task3](/files/-L_G1DZGzZ1-ocL47DvR)

* 在ActivityManagerService中，由一个ActivityStack对象维护回退栈，而具体到ActivityStack中，由mHistory来维护回退栈（是一个ArrayList，而非Stack）。
* 每个Task即为一个TaskRecord对象，
* TaskRecord没有为每个Task维护一个自己的stack或list，
* mHistory中，不是直接管理Activity，而是维护ActivityRecord对象，
* ActivityRecord对象通过TaskRecord与自己所属的Task关联起来，
* ActivityRecord对象通过Activity info对象与对应的Activity关联起来，
* 分析ActivityStack中的moveTaskToFrontLocked()等对Task进行操作的代码，得到，同一个task的ActivityRecord在mHistory中是放在一起的（按照先入先出的顺序），当一个task被调到前台时，这个mHistory中，属于同一个task（taskId一致）的ActivityRecord被一起移到mHistory的顶端（在这个ArrayList中，以最后一个元素所在的位置为top，top=size()-1）;


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