Managing Activity Backstack

Managing Android
Back stack
Rajdeep Dua
June 2012

Monday, March 18, 13

Agenda
• Activity Back Stack explained

• Tasks

• Bringing activities to the foreground

• Modifying Activity launch behavior using manifest
ﬁle attributes

• Other advanced behaviors

• No History

• Task Reset

• Task Afﬁnity

Android Back Stack

• An Android application is composed of multiple
activities
• New Activities are launched using Intents
• Older ones are either destroyed or placed on a
back stack


Android Back Stack

• New Activity can be launched in the same task or
a new task
• Back stack is a data structure which holds activities
in the background which user can navigate to using
back button.
• Back Stack works in the last in ﬁrst out mode.
• It can be cleared in case of low memory


Tasks

• Task is a collection of activities that user performs while
doing a job
• An activity is launched in a new task from the Home
screen
• By default all the activities in an application belong to the
same task
• Activities in a task can exist in the foreground,
background as well as on the back stack.


Tasks

Activity A and Activity B are launched in Task A.
By default all activities in the same application
belong to a single task.


Bringing Activities
to the Foreground
• Activities pop back to
foreground as the user
presses back button
• As an example in the
ﬁgure
• Activity B gets
destroyed as soon as
user presses back
button
• Activity A comes to
the foreground

Task in the Background

• Application 1 Launches in Task A
• Home Screen Task A Goes to the Background
• Application 2 Launches in Task B in the foreground

Modifying Activity Launch
Behavior using Manifest Flags


Activity Launch Mode

• Activities can be conﬁgured to launch with the
following modes in the Manifest ﬁle

Use Case Launch Mode

standard
Normal
singleTop

singleTask
Specialized
singleInstance


standard
<activity android: launchMode=”standard”/>

• Default Launch
Mode

• All the Activities
belong to the
same task in the
application


singleTop
<activity
android:launchMode=”singleTop”/>

• Single instance of an
Activity can exist at
the top of the back
stack

• Multiple instances can
exist if there are
activities between
them


singleTask

<activity android:launchMode=”singleTask”/>

• Activity is the root of a
task

• Other activities are part
of this task if they are
launched from this activity

• If this activity already
exists Intent is routed to
that Instance


singleTask...contd

• If this activity
already exists
Intent is routed to
that instance and
onNewIntent()
Lifecycle method
is called


singleInstance
<activity
android:launchMode=”singleInstance”/>

• Activity B is launched
with launchMode
singleInstance, it is
always the only
activity of its task


SingleTop using Intent Flags

• Single Instance of an Activity at the top of the back
stack can also be achieved using Intent Flags.
Step 1 : Create Activity A using an Intent with no flag

Intent intentA = new Intent(MainActivity.this,
ActivityA.class);
startActivity(intentA);

Step 2 : Create Activity A again with the Intent flag FLAG_ACTIVITY_SINGLE_TOP
ActivityA.class);
intentA.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP)


Other Behaviors..

• No History on the Back stack

• Task Reset and clearing the
Back Stack

• Task Afﬁnity attribute in
Manifest


No History on the Back Stack
• An Activity can be set to have no history on the back stack - it will be destroyed
as soon as user moves away from it using the the intent ﬂag
FLAG_ACTIVITY_NO_HISTORY


Clear Activity on Task Reset

• Activities can be cleared from the back stack when a
task is reset.
• Use the Intent Flags FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET
and FLAG_ACTIVITY_RESET_TASK_IF_NEEDED are used together
to clear activities on a task reset.

• Flag FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET to indicate
that the activity will be destroyed when the task is reset.

• Flag FLAG_ACTIVITY_RESET_TASK_IF_NEEDED refers to the activity in
the back stack beyond which activities are destroyed.


..contd


Sample Code

//Code below shows how the Activity A gets launched with
the appropriate flags.
ActivityA.class);
intentA.addFlags(Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)
;

//sample code below shows the flag with with Activity B and
Activity C get launched.
Intent intentB = new Intent(ActivityA.this,
ActivityB.class);
intentB.addFlags(Intent.FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET
);
startActivity(intentB);


TaskAfﬁnity

• Task Afﬁnity can be used to launch activities in a new task, e.g
having two Launcher Activities which need to have their own task

• EntryActivityA and EntryActivityB will Launch in their own tasks


TaskAfﬁnity
..contd

• EntryActivityA and EntryActivityB will Launch in
their own tasks

• Manifest ﬁle entries for the two activities
<activity android:name=".EntryActivityA"
android:label="@string/activitya"
android:taskAffinity="stacksample.activitya">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".EntryActivityB"
android:label="@string/activityb"
android:taskAffinity="stacksample.activityb">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>


Summary

• Default behavior of using the back stack serves
most of the use cases

• Default behavior can be modified using Manifest
file attributes and Intent flags to launch activities in
their own task, having a single instance or having
no history


Managing Activity Backstack

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