An instruction on how the activity should be launched. There are four modes
that work in conjunction with activity flags (FLAG_ACTIVITY_* constants) in Intent objects to
determine what should happen when the activity is called upon to handle an
intent. They are:
"standard"
"singleTop"
"singleTask"
"singleInstance"
The default mode is "standard".
As shown in the table below, the modes fall into two main groups, with
"standard" and "singleTop" activities on one side, and
"singleTask" and "singleInstance" activities on the
other. An activity with the "standard" or "singleTop"
launch mode can be instantiated multiple times. The instances can belong to any
task and can be located anywhere in the activity stack. Typically, they're
launched into the task that called startActivity()FLAG_ACTIVITY_NEW_TASK
In contrast, "singleTask" and "singleInstance"
activities can only begin a task. They are always at the root of the activity
stack. Moreover, the device can hold only one instance of the activity at a time
— only one such task.
The "standard" and "singleTop" modes differ from
each other in just one respect: Every time there's a new intent for a
"standard" activity, a new instance of the class is created to
respond to that intent. Each instance handles a single intent. Similarly, a new
instance of a "singleTop" activity may also be created to handle a
new intent. However, if the target task already has an existing instance of the
activity at the top of its stack, that instance will receive the new intent (in
an onNewIntent() call); a new instance is not created. In other circumstances — for example, if
an existing instance of the "singleTop" activity is in the target
task, but not at the top of the stack, or if it's at the top of a stack, but not
in the target task — a new instance would be created and pushed on the stack.
Similarly, if you navigate up to an
activity on the current stack, the behavior is determined by the parent
activity's launch mode. If the parent activity has launch mode
singleTop (or the up intent contains FLAG_ACTIVITY_CLEAR_TOP),
the parent is brought to the top of the stack, and its state is preserved. The
navigation intent is received by the parent activity's onNewIntent() method. If the parent activity has launch mode standard (and the
up intent does not contain FLAG_ACTIVITY_CLEAR_TOP),
the current activity and its parent are both popped off the stack, and a new
instance of the parent activity is created to receive the navigation intent.
The "singleTask" and "singleInstance" modes also
differ from each other in only one respect: A "singleTask" activity
allows other activities to be part of its task. It's always at the root of its
task, but other activities (necessarily "standard" and
"singleTop" activities) can be launched into that task. A
"singleInstance" activity, on the other hand, permits no other
activities to be part of its task. It's the only activity in the task. If it
starts another activity, that activity is assigned to a different task — as if
FLAG_ACTIVITY_NEW_TASK was in the intent.
| Use Cases | Launch Mode | Multiple Instances? | Comments |
|---|
| Normal launches for most activities | "standard" | Yes | Default. The system always creates a new instance of the activity in the
target task and routes the intent to it. |
"singleTop" | Conditionally | If an instance of the activity already exists at the top of the target task,
the system routes the intent to that instance through a call to its onNewIntent() method, rather than creating a new instance of the activity. |
Specialized launches
(not recommended for general
use) | "singleTask" | No | The system creates the activity at the root of a new task and routes the
intent to it. However, if an instance of the activity already exists, the system
routes the intent to existing instance through a call to its onNewIntent() method, rather than creating a new one. |
"singleInstance" | No | Same as "singleTask", except that the system doesn't launch any
other activities into the task holding the instance. The activity is always the
single and only member of its task. |
As shown in the table above, standard is the default mode and is
appropriate for most types of activities. SingleTop is also a
common and useful launch mode for many types of activities. The other modes —
singleTask and singleInstance — are not appropriate for most applications, since they
result in an interaction model that is likely to be unfamiliar to users and is
very different from most other applications.
Regardless of the launch mode that you choose, make sure to test the
usability of the activity during launch and when navigating back to it from
other activities and tasks using the Back button.
For more information on launch modes and their interaction with Intent flags,
see the Tasks and Back
Stack document