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5 tricky activity life cycle interview questions. After learning, go and hang the interviewer!

2022-04-23 13:10:00 【InfoQ】

A.onPause -> B.onCreate -> B.onStart -> B.onResume -> A.onStop

Naive ! The answer is just a pass , Because only in B Activity Of launchMode by standard perhaps B Activity This is what happens when there are no reusable instances .

When B Activity Of launchMode by singleTop And B Activity Already at the top of the stack （ Some special cases, such as clicking on the notification bar 、 Connecting point ）, At this time only B The page itself has a lifecycle change :

B.onPause -> B.onNewIntent -> B.onResume

When B Activity Of launchMode by singleInstance ,singleTask And the corresponding B Activity When there are reusable instances , The lifecycle callback looks like this :

A.onPause -> B.onNewIntent -> B.onRestart -> B.onStart -> B.onResume -> A.onStop -> ( If A If it is removed from the stack, there is another one A.onDestory)

You can add points by answering all the situations , At the same time, we should also have a good chat launchMode To prepare .

[](() eject Dialog What's the impact on the life cycle

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We know , Lifecycle callbacks are all AMS adopt Binder Notify the application process to call ; And pop up Dialog、Toast、PopupWindow It's all directly through WindowManager.addView() According to the （ Not through AMS）, So it doesn't have any impact on the life cycle .

If it's starting a Theme by Dialog Of Activity , Then the life cycle is ：

A.onPause -> B.onCreate -> B

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.onStart -> B.onResume

Notice that there is no previous one here Activity No callback onStop, Because only in Activity When it is invisible in the background, it will call back onStop; And pop up Dialog The theme of Activity When the previous page is still visible , It's just a loss of focus, so it's just onPause Callback .

[](()Activity stay onResume What's the reason for this later

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Although we set Activity The layout is usually in onCreate Method call setContentView . It's called directly window Of setContentView, Create a DecorView To wrap the layout we created . Details are as follows ：

PhoneWindow.java

public?void?setContentView(int?layoutResID)?{

if?(mContentParent?==?null)?{

installDecor();

...

//? Load layout , Add to ?mContentParent

//?mContentParent? again ?DecorView? A sub layout of ??

mLayoutInflater.inflate(layoutResID,?mContentParent);

}

However, this step just loads the layout , Generate a ViewTree , How to deal with ViewTree Show it , Here's the answer ：

ActivityThread.java

public?void?handleResumeActivity(...){

//?onResume? Callback

ActivityClientRecord?r?=?performResumeActivity(...)

final?Activity?a?=?r.activity;

if?(r.window?==?null?&&?!a.mFinished?&&?willBeVisible)?{

r.window?=?r.activity.getWindow();

View?decor?=?r.window.getDecorView();

ViewManager?wm?=?a.getWindowManager();

wm.addView(decor,?l);//? a key

}

WindowManager Of addView The method will eventually DecorView Add to WMS , Drawing to the screen 、 Receive touch screen events . The specific call chain is as follows ：

WindowManagerImpl.addView

->?WindowManagerGlobal.addView

->?ViewRootImpl.setView?????

->?ViewRootImpl.requestLayout()?//? perform ?View? Drawing process of

//? adopt ?Binder? call ?WMS?,WMS? It will add a ?Window? Related objects

//? The application end passes through ?mWindowSession? call ?WMS

//?WMS? adopt ?mWindow?( One ?Binder? object )? Call the application side ??

mWindowSession.addToDisplay(mWindow)?

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