Android Activity Launch Mode
Launch
mode is an instruction for Android OS which specifies how the activity should
be launched. It instructs how any new activity should be associated with the
current task. Before moving further you first need to understand about very
important topics-
- Tasks
- Back Stack
Tasks
A
task is a collection of activities that users interact with when performing a
certain job. In general an application contains number of activities. Normally when
user launch an application a new task will be created and the first activity
instance is called root of the task.
When
user launches an app from home icon, it navigates through different screens so
different activities placed on the top of one another. This collection of
activities is known as tasks.
Back Stack
Activities
are arranged with the order in which each activity is opened. This maintained stack
called Back Stack. When you start a new activity using startActivity(), it “pushes”
a new activity onto your task, and put the previous Activity in the back stack.
Once
you press back button then “pops” the top most activity and remove it from the
back stack and taking you back to the previous activity.
Above
figure explains representation of how each new activity in a task adds an item
to the back stack. When the user presses the Back button, the current activity
is destroyed and the previous activity resumes. For more details you can also
refer.
There
are four launch modes for activity. They are:
- standard
- singleTop
- singleTask
- singleInstance
In
the AndroidManifest you can use “launchMode” attribute inside the
<activity> element to declare the activity’s launch mode like-
Now
let’s look at the differences between launch modes.
1. standard
This
is the default launch mode of an activity (If not specified). It creates a new
instance of an activity in the task from which it was started. Multiple
instances of the activity can be created and multiple instances can be added to
the same or different tasks. In other words you can create the same activity
multiple times in the same task as well as in different tasks.
<activity
android:launchMode="standard" />
Example:
Suppose
you have A, B, C and D activities and your activity B has “launch mode = standard”.
Now you again launching activity B –
State
of Activity Stack before launch B
A -> B -> C -> D
State
of Activity Stack after launch B
A -> B -> C -> D -> B
2. singleTop
In
this launch mode if an instance of activity already exists at the top of the
current task, a new instance will not be created and Android system will route the
intent information through onNewIntent(). If an instance is not present on top
of task then new instance will be created.
Using
this launch mode you can create multiple instance of the same activity in the
same task or in different tasks only if the same instance does not already
exist at the top of stack.
<activity
android:launchMode="singleTop" />
Example:
Case 1:
Suppose
you have A, B and C activities and your activity D has “launch mode = singleTop”.
Now you launching activity D -
State
of Activity Stack before launch D
A -> B -> C
State
of Activity Stack after launch D activity
A -> B -> C -> D (Here D launch as
usual)
Case 2:
Suppose
you have A, B, C and D activities and your activity D has “launch mode = singleTop”.
Now you again launching activity D -
State
of Activity Stack before launch D
A -> B -> C -> D
State
of Activity Stack after launch D activity
A -> B -> C -> D (Here old instance gets
called and intent data route through onNewIntent() callback)
3. singleTask
In
this launch mode a new task will always be created and a new instance will be
pushed to the task as the root one. If an instance of activity exists on the
separate task, a new instance will not be created and Android system routes the
intent information through onNewIntent() method. At a time only one instance of
activity will exist.
<activity
android:launchMode="singleTask" />
Example:
Case 1:
Suppose
you have A, B and C activities and your activity D has “launch mode = singleTask”.
Now you launching activity D -
State
of Activity Stack before launch D
A -> B -> C
State
of Activity Stack after launch D activity
A -> B -> C -> D (Here D launch as
usual)
Case 2:
Suppose
you have A, B, C and D activities and your activity B has “launch mode = singleTask”.
Now you again launching activity B-
State
of Activity Stack before launch D
A -> B -> C -> D
State
of Activity Stack after launch B activity
A -> B (Here old instance gets called and
intent data route through onNewIntent() callback)
Also
notice that C and D activities get destroyed here.
4. singleInstance
This
is very special launch mode and only used in the applications that has only one
activity. It is similar to singleTask except that no other activities will be
created in the same task. Any other activity started from here will create in a
new task.
<activity
android:launchMode="singleInstance" />
Example:
Case 1:
Suppose
you have A, B and C activities and your activity D has “launch mode = singleInstance”.
Now you launching activity D -
State
of Activity Stack before launch D
A -> B -> C
State
of Activity Stack after launch D activity
Task1
- A -> B -> C
Task2
- D (here D will be in different task)
Now
if you continue this and start E and D then Stack will look like-
Task1
- A -> B -> C -> E
Task2
- D
Case 2:
Suppose
you have A, B, C activities in one task and activity D is in another task with “launch
mode = singleInstance”. Now you again launching activity D-
State
of Activity Stack before launch D
Task1
- A -> B -> C
Task2
- D
State
of Activity Stack after launch B activity
Task1
- A -> B -> C
Task2
- D (Here old instance gets called and intent data route through onNewIntent()
callback)
Intent Flags
Android
also provides Activity flags by which you can change the default behavior of
Activity association with Tasks while starting it via startActivity() method.
Thease flags values can be pass through Intent extra data.
FLAG_ACTIVITY_NEW_TASK
This
flag works similar to “launchMode = singleTask”.
FLAG_ACTIVITY_CLEAR_TASK
This
flag will cause any existing task that would be associated with the activity to
be cleared before the activity is started. The activity becomes the new root of
an otherwise empty task, and any old activities are finished.
FLAG_ACTIVITY_SINGLE_TOP
This
flag works similar to “launchMode = singleTop”.
FLAG_ACTIVITY_CLEAR_TOP
If
set, and the activity being launched is already running in the current task,
then instead of launching a new instance of that activity, all of the other
activities on top of it will be closed and this Intent will be delivered to the
(now on top) old activity as a new Intent.
There
are quite a lot on flags. You could find more about it at Intent.
Hope
you find this article useful.
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