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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 861
830
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
be started and given the picture to be shared.
(Later we will see how the camera
application is able to find the email application’s
ComposeActivity
.)
Performing that share option while in the activity state seen in Fig. 10-54 will
lead to the new state in Fig. 10-55. There are a number of important things to note:
1.
The email app’s process must be started again, to run its
ComposeAc-
tivity
.
2. However, the old
MailMainActivity
is
not
started at this point, since it
is not needed. This reduces RAM use.
3.
The camera’s task now has two records: the original
CameraMainAc-
tivity
we had just been in, and the new
ComposeActivity
that is now
displayed. To the user, these are still one cohesive task: it is the cam-
era currently interacting with them to email a picture.
4. The new
ComposeActivity
is at the top, so it is resumed; the previous
CameraMainActivity
is no longer at the top, so its state has been
saved. We can at this point safely quit its process if its RAM is need-
ed elsewhere.
Activity manager in system_server process
Email app process
ActivityRecord
(ComposeActivity)
ActivityRecord
(CameraMainActivity)
ActivityRecord
(MailMainActivity)
Saved state
Saved state
STOPPED
STOPPED
RESUMED
Task: Camera
ComposeActivity
CameraMainActivity
Camera app process
Task: Email
Figure 10-55.
Sharing a camera picture through the email application.
Finally let us look at would happen if the user left the camera task while in this
last state (that is, composing an email to share a picture) and returned to the email
Page 862
SEC. 10.8
ANDROID
831
application. Figure 10-56 shows the new state the system will be in.
Note that we
have brought the email task with its main activity back to the foreground. This
makes
MailMainActivity
the foreground activity, but there is currently no instance
of it running in the application’s process.
Activity manager in system_server process
Email app process
Camera app process
MailMainActivity
ComposeActivity
CameraMainActivity
Saved state
Saved state
STOPPED
STOPPED
RESUMED
Task: Email
Task: Camera
ActivityRecord
(MailMainActivity)
ActivityRecord
(ComposeActivity)
ActivityRecord
(CameraMainActivity)
Figure 10-56.
Returning to the email application.
To return to the previous activity, the system makes a new instance, handing it
back the previously saved state the old instance had provided. This action of
restoring an activity from its saved state
must be able to bring the activity back to
the same visual state as the user last left it.
To accomplish this, the application will
look in its saved state for the message the user was in, load that message’s data
from its persistent storage, and then apply any scroll position or other user-inter-
face state that had been saved.
Services
A
service
has two distinct identities:
1.
It can be a self-contained long-running background operation.
Com-
mon examples of using services in this way are performing back-
ground music playback, maintaining an active network connection
(such as with an IRC server) while the user is in other applications,
downloading or uploading data in the background, etc.
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