Detailed explanation of activity life cycle and startup mode

One 、Activity Life cycle

1. Normal condition ：

(1) onCreate:

Express Activty Being created , This is a Activity The first method of life cycle , Can do some initialization work , such as ： Load layout , Binding controls , Initialization data, etc .

(2) onRestart:

Express Activity is again start-up , Usually by Activty Called when it changes from invisible to visible .

Triggering scenarios ： Press Home Key to enter the desktop , Or start a new Activity, Now Activity Be suspended , And then back to the Activity , It's going to trigger onRestart.

(3) onStart:

At present Activity Is being activated , here Activity It's already visible , But it has not yet appeared in The front desk , Can't interact with users , Users can't see .

(4) onResume:

At present Activity Already visible , Appear at the front desk And start activities ,onStart and onResume All means Activity so ,onStart When Activity Still backstage ,onResume When Activity To the front desk .

(5) onPause:

At present Activity Stopping , It is still visible at this time , Under normal circumstances , Then it will call onStop Method .

Can do some data storage , Animation stop and other lightweight recycling work , Not too time consuming . Because it will affect the new Activity Display of , Only onPause After execution , new Activity Of onResume Method will be executed .

Be careful For example, pop up a non full screen Dialog Or start a transparent Activty, Then the current onStop Method does not call .

(6) onStop:

Express Activity About to stop , invisible , You can do some slightly heavyweight post meeting work , It can't be too time consuming .

(7) onDestroy:

Express Activity About to be destroyed , This is a Activity The last callback in the life cycle , You can do some recycling and finally release resources .

Life cycle flowchart

Expand the problem ：

1.onStart and onResume,onPause and onStop What is the substantive difference ？

onStart and onStop It's from Activity Whether to callback from the visible angle , and onResume and onPause It's from Activity Whether the callback is in the foreground .

2.A start-up B, that B Of onResume and A Of onPause Which comes first ？

Conclusion ：A Execute first onPause() Method , Then B perform onCreate() -> onSatar() -> onResume() Method , Then perform A Of onStop()

reason ：

start-up Activity The request for Instrumentation adopt Binder Interface to AMS Send a request ,AMS One is maintained internally ActivityStack And be responsible for the stack Activity State synchronization , next AMS adopt IApplicationThread( In the current application process ) Of Binder Interface desynchronization Activity To complete the call to the lifecycle method .

call ApplicationThread.scheduleLaunchActivity adopt handle.sendMessage notice ActivityThread Complete the new Activity Of onCreate,onStart,onResume Call procedure for .

ActivtyStack.resumeTopActivityInnerLocked();//
---TaskDisplayArea.pauseBackStacks();// Pause all stacks or only all activities in the post stack 
----ActivtyStack.startPausingLocked();// Start pause is currently in  resumed  The activities of 
------ActivityStackSupervisor.realStartActivityLocked();

.... app  process 
ActivtyThread.handleLaunchActivity(){
----ActivtyThread.performLaunchActivity();
----1.ContextImpl appContext = createBaseContextForActivity(r);// Create the  context Implementation class 
----2.mInstrumentation.newActivity();// adopt appContext Class loader reflection creation Activity
----3.LoadedApk.makeApplication();// establish application
----4.appContext And activity Related to 
----5. perform activity.attach() Method .
----6.mInstrumentation.callActivityOnCreate();// perform oncreate Method  

2. Abnormal life cycle ：

1. Changes in system configuration result in Activity Killed and recreated

Abnormal termination ： I hit the back button , Lock screen , Click on Home key , There are others APP Enter the front desk （ Like answering the phone ） , We started a new Activity , The direction of the screen rotates ,APP Be killed .

All in all ： System is only in Acitivity It will be triggered only when it is abnormally terminated onSaveInstanceState and onRestoreInstanceState

When the system configuration changes ,Activity The life cycle process after being destroyed and rebuilt :

Can be in onSaveInstanceState Of Bundle Save data in , stay onRestoreInstanceState Recovering data in . The system automatically does some recovery work for us . When Activity When rebuilding , The system will save the current... For us Activity The view structure of , And in Activity Recover data after restart .

View View data recovery ：

Every View There are onSaveInstanceState and onRestoreInstanceState Method .

Activity When terminated maliciously ,Activity Would call onSaveInstanceState To save data , then Activity Will commission Window To save data , next Window Then delegate the top-level container above it to save data , Usually the top-level container is a ViewGroup, Generally speaking, it is likely to be DecorVew. Finally, the bottom container notifies its children one by one View To save the data , That is to call ViewGroup Of dispatchSaveInstanceState() Finally, call the specific View Of onSaveInstanceState To save data .

Be careful ：View Data recovery must be done at the current time xml Set... For it in ID, Otherwise, it cannot be recovered .

The specific saved data can be seen in View The concrete implementation of subclasses .

Activity State saving and recovery in

2. Insufficient resource memory resulted in Activty Be killed

Acitivty The priority of the ：

1. The front desk Activity, Interacting with users , The highest priority .

2. Visible but not foreground , Like pop ups , transparent Activity start-up .

3. backstage Activity, Suspended Activity, The lowest priority .

Background work is not suitable for running independently of the four components , Such a process can easily be killed , Process priorities can be provided to reduce the risk of being killed .

3. How to avoid Activity The reconstruction

stay AndroidManifest In Chinese, it means Activity To configure configChanges Configuration to avoid... Due to configuration changes Acitivty Destroyed and rebuilt .

This will only go Activity Of onConfigurationChanged Method .

android:configChanges="screenSize|orientation|keyboardHidden|screenLayout" 

2.Activity Start mode of

1.LaunchMode

standard： The standard model

The default mode of the system , One at a time Activity Metropolis Recreate a new instance , Whether this instance exists or not . A task stack can have multiple tasks Activity Example , Each instance can also be on a different task stack .

In this mode , Who started the Acitivty, At present Activity It's running on the one that started it Activity In the task stack .

Be careful ： stay standard In mode , If you pass ApplicationContext Start up Activity, Abnormal transactions , because ApplicationContext Starting up Activity There is no task stack . The solution is for the to be started Activity Appoint FLAG_ACTIVITY_NEW_TASK Marker bit , At startup, a new task stack will be created for it , It's time to start Activity In fact, it is based on singleTask Mode enabled .

Application scenarios ： majority Activity Applicable scenarios .

singleTop： Stack top reuse mode

If you want to create a new Activity Already at the top of the task stack , So many times Activity Will not be recreated , At the same time, its onNewIntent Method will be called , Corresponding onCreate and onStart Method will not be called .

If the newly created Activity Instance already exists , But not at the top of the stack , So new Activity Will still be recreated .

Application scenarios ： Click on the received notification in the notification bar , Need to start a Activity, You can use it singleTop, Otherwise, each click will create a new Activity

singleTask： In stack reuse mode

A singleton mode , as long as Activity Exists in a stack , that Start this more than once Activity Will not recreate the instance , The system calls it onNewIntent Method . When starting a mode is singleTask Of Acitivity A When , The system first looks for A Desired task stack , If it doesn't exist , Then restart to create a task stack , take A Put it in . If the task stack exists , Check whether there is... In the task stack A Example , without , Create put . If there is A Example , Then the system will A Other above Activity remove , Give Way A Stack top , And call onNewIntent Method .

Application scenarios ： majority App The home page of . For most applications , When we click the back button on the main interface, we exit the application. Then when we enter the main interface for the first time , The main interface is at the bottom of the stack , No matter how many we open later Activity, As long as we go back to the main interface again , You should use the main interface Activity On all the Activity Remove the way to make the main interface Activity At the top of the stack , Instead of adding a main interface to the top of the stack Activity Example , In this way, you can guarantee all of the Activity All can be reported for destruction .

Check... By command Activity Task stack ：adb shell dumpsys activity

Scene one ：A->B->C,B by singleTask Pattern , The current task stack is as follows ：

stay 【A,B,C】 Start in B, The task stack is as follows ：

Scene two ： The current task stack is ：A->B->C, Then with singTask + taskAffinity = “com.ubtech.taskS2” start-up ActivityD

Directly by Back Key fallback ：

D -> C -> B -> A -> desktop

Check the background task window of the mobile phone as follows ：

1. At this point, if you open the task stack S2, The fallback stack becomes : D -> desktop

2. At this point, if you open the task stack S1, The fallback stack becomes ：C->B->A-> desktop

Uniapp There's a problem with development ：

uniapp The Lord of Activity The task stack name of is actually the unique identification code of the packaged application

singleInstance： Single instance mode

intensive singleTask, With this pattern Activity Can only be in a single task stack , There is only one instance of the whole system .

** Application scenarios ：** System interface , Such as incoming interface, etc .

TaskAffinity

• This parameter identifies a Activity The name of the required task stack , By default , be-all Activity The name of the task stack is the package name of the application .

• The task stack is divided into foreground task stack , Background task stack , You can switch the background task stack to the foreground .

2.Activity in Flags meaning

FLAG_ACTIVITY_NEW_TASK

• The purpose of this flag bit is to Activity Appoint “singleTask” Pattern , Effect and in xml This mode is the same as that specified in

FLAT_ACTIVITY_SINGLE_TOP

• The purpose of this flag bit is to Activity Appoint “singleTop” Pattern , Effect and in xml This mode is the same as that specified in

FLAG_ACTIVITY_CLEAR_TOP

• With this flag bit Activity, When it starts , In the same task stack, all the Activity They all have to go out , Need and FLAG_ACTIVITY_NEW_TASK In combination with , If the instance exists , Would call onNewIntent Method .

FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS

• With this flag bit Activity Not in history Acitivity List of , If you don't want the user to go back to this... Through the history list Activity Use this marker position when , It's equivalent to being in xml It is specified in android:excludeFromRecents=“true”. Not the same as not having it in the fallback stack .

3. IntentFilter Matching rules

1. Show / Implicit call

1. Display call

//1. Display the calling method 1 
Intent intent1 = new Intent(this,ActivityB.class);
startActivity(intent1);

//2. Display call mode 2 
ComponentName component = new ComponentName(this,ActivityB.class);
Intent intent2 = new Intent();
intent2.setComponent(component);
startActivity(intent2);

//3. Display the calling method 3 
Intent intent3 = new Intent();
intent3.setClass(this,ActivityB.class);
startActivity(intent3); 

2. Implicit call

Be careful ： You must add the default

2. Matching rules

• IntentFilter The filtering information in is action,category,data.

• One IntentFilter There can be multiple action,category,data. The information of the same category restricts the matching process of the current category . only one Intent Simultaneous matching action Category ,category Category ,data Category is a perfect match .

• One Activity There can be multiple IntentFilter, One Intent As long as you can match any group intent-filter that will do .

1. action Matching rules

Intent Medium action Must be able to post and intent-filter Medium action matching , One intent-filter There are multiple action,Intent Medium action Just match one of them . If Intent Not specified in action, The match fails .action Case sensitive .

2.category Matching rules

• Intent If any category, be-all category And intent-filter One of the matches , If Intent in The emergence of category Must be intent-filter As defined in .
• Intent in There can be no category, Can still match successfully . The system is calling startActivity The default will be Intent add to "android.intent.category.DEFAULT" This category.
• in order to activity Can accept implicit calls , Must be in intent-filter It is specified in "android.intent.category.DEFAULT" This category.

3. data Matching rules

• If intent-filter It defines data, be Intent You must also define a matching data.
• data It consists of two parts ： mimeType and URI
• mimeType Media type , such as imgage/jpeg,audo/* and video/* etc. , Can represent a picture , Text , Different media formats such as video .
• URI Each part of the is a separate attribute ：scheme、host、port and path
• :: ,eg:content://com.example.project:200/folder/subfolder/etc
• Scheme: URI Pattern , such as http,file,content etc. , If URI Not specified , It doesn't work
• Host: URI The host name , If not specified, the whole URI It doesn't work .
• Port：URI Port number in , Only when the URI It is specified in scheme and host Parameters should be port It works .
• path： Path information

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3. Implicit Activity Whether there is judgment

• adopt PackageManager.resolveActivity Judge
• adopt PackageManager.queryIntentActivities() Judge , This method returns a list of collections .

4. Expand the problem

1.Activity and Fragment The difference between

• Activity Is one of the four components of the system , from ActivityManager management , The life cycle is controlled by the system .

• Fragment Is in 3.0 Components introduced after , from FragmentManager management , Can be Activity Free control , Introduce or delete , More convenient .

• because Fragment yes Activity management , So when using , Pay special attention to , Before creating, you need to getFragmentByTag perhaps ById Check to see if there are already ,FragmentManager Also with ActivityManager There is also a caching mechanism . The same TAG Of fragment If added to more than once activity in , Then through the getFragmentBytag obtain Fragment What is returned is the last added Fragment

• fragment In the process of display to destruction, it will execute its own life cycle .onAttach -> onCreate ->onCreateView -> onActivityCreate -> onStart -> onResume -> onPause -> onStop -> onDestroyView -> onDestroy -> onDetach

• At the same time activity The impact of the life cycle ,activity onPause Fragment The corresponding... Will also be executed onPause

2. Why setArgument The ginseng , Instead of using constructors with parameters ？

• You can use constructors and setArgument Way to pass parameters , Once you write a parameter constructor , It is necessary to build a parameterless constructor
• Pass arguments through the constructor ： When Fragment Be recreated , The system will call Fragment The default constructor in , As a result, the previously passed parameters are missing , The official recommendation setArgument,fragment After being destroyed and rebuilt , Finally, a new one will be instantiated through the reflection parameterless construction Fragment, And give mArgments Initialize to the original value , And the original Fragment The data of the instance is lost , And re initialize
• Activity On re creation , Will rebuild what it manages Fragment, The original Fragment All field values will be lost , But by Fragment.setArguments(Bundle bundle) Method set bundle Will keep it , And restore it during reconstruction .

3.window、view and Activity The relationship between ?

Window yes Android Macro definition of window in , Mainly management View The creation of , As well as ViewRootImpl Interaction , take Activity And View decoupling .
Activity And PhoneWindow And DecorView What's the relationship between ？
One Activity Corresponding to one Window That is to say PhoneWindow, One PhoneWindow Hold one DecorView Example ,DecorView Itself is a FrameLayout.

4.finish How to get to onDestroy Of ?

stay Activity Call in finish() Method , What life cycle approaches will be taken ?

If FirstActivt Start in SecondActivity, Will go first FisrtActivity Of onPause Method , Then go SecondActivity Of onCreate->onStart->onResume-> Until then FirstActivity Of onStop->onDestory

interviewer ： Why? Activity.finish() after 10s only onDestroy ？

Activity Of onStop/onDestroy It's dependence IdleHandler Come back . Normally, when the main thread is idle, it will call . However, due to the problems in some special scenes , This causes the main thread to be idle for a long time ,onStop/onDestroy It will also be delayed in getting the call . But that doesn't mean Activity Never get recycled , The system provides a thorough mechanism , When onResume Callback 10s after , If still not called , It will trigger .

5.dialog Will it affect Activity Life cycle , Why? ?

Does not affect the ,Activity 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 .

Unless it starts Theme by Dialog Of Activity

Destroy It will also be delayed in getting the call . But that doesn't mean Activity Never get recycled , The system provides a thorough mechanism , When onResume Callback 10s after , If still not called , It will trigger .

5.dialog Will it affect Activity Life cycle , Why? ?

Does not affect the ,Activity 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 .

Unless it starts Theme by Dialog Of Activity

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