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It is also possible to go to Idle state of the phone and leave an application running.
This can be done by pressing the End key, or by selecting the Phone application from
the Application shell. (The Phone application and Idle state are mutually exclusive
states in the phone: when there is a voice call going on, there is no Idle state.) During
a phone calls, however, the End key is used for terminating the call and cannot be
used as a shortcut to Idle.
The number of simultaneously running (different) applications is limited only by the
available memory in the device. When a new application process can't be created
because of limited memory, the system can automatically shut down applications to
gain more memory space.
Application interactions
There are two different models of interaction between applications:
1)
Use of modules (services) that can be called and run within several
different applications. In this model, a service or library function is running
within the application the user was originally working with. The
applications do not conflict with each other when this model is used.
It should be noted that from the user's point of view, each item in the
Application shell is seen as an application. A specific service can be run in
any number of these applications at the same time, so the user may see a
similar screen in many applications running simultaneously. But the user
cannot launch a new instance of any of the applications in the Application
shell before terminating the existing one first: selecting a running
application will simply revert to the existing one.
The Back function works normally in this model: the user can step back
from an embedded module to the calling application; the modules may
even be nested.
Application
process
return
Figure 6-4:
Nested services running within an application process. The Back function
leads to the parent process.
2)
Actually switching from one application to another to accomplish a task.
In this model, the other application may need to be interrupted if it is
already running. This model is needed when links from one application to
another are used.
Whenever an application needs to be interrupted, the system takes care of
all pending data. If there's information that needs to be saved, the system
call
Service A
sub-process
call
Service B
sub-process
return
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