what androids are made of
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8/13/2019 What Androids Are Made Of
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Main Building Blocks
• Android building blocks make it easy to breakdown an application into conceptual units,work on them independently and easily put
them together into a complete package – Activities
– Services
– Intents
– Broadcast receivers
– Content providers
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Activities
• A single screen that a user can see
• An application typically has multiple activities
•
Concept of activities can be easily understandby comparing an Android application with a
“website”
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Activity Life Cycle
• Activity life cycle is managed by Activity
Manager
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Activity Life Cycle
• Starting State
– When an activity doesn’t exist in memory
– After starting state, an activity enters the running
state
– Transition from starting state to running state is
one of the most expensive operations in terms of
computing time and battery life
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Activity Life Cycle
• Paused State
– When an activity is not in focus
– But it is still visible
– For example, a dialog box coming in front of an
activity can cause the activity to pause
– Paused activities still have high priorities in terms
of getting memory and other resources
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Activity Life Cycle
• Stopped State
– When an activity is not visible, but still in memory
– A stopped activity can be moved back to either
the running state or it could be destroyed
– Why do you think the system keeps activities in
stopped state?
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Activity Life Cycle
• Destroyed State
– When an activity is no longer in the memory
– Before an activity gets destroyed, it can perform
certain actions, such as saving unsaved data.
– Remember, there is no guarantee that an activity
will be stopped before being destroyed
– Therefore, it is important to do important stuff,such as saving data, en route to a paused state.
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Intents
• Messages that are sent among the major
building blocks (for example, to start or stop
an activity or service)
• Intents are asynchronous
• Could be explicit (the sender clearly mentions
a specific receiver) or implicit (the sender
mentions the type of receiver)
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Intents Example
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Services
• Run in the background and don’t have any
user interface components
• Useful for actions that we want to perform for
a while, regardless of what is on the screen
• For example, listening music while flipping
between applications
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Service Life Cycle
• Services have a much simpler life cycle than
activities
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Content Providers
• Interfaces for sharing data between
applications
• Data that belongs to an application is usually
isolated
• Although we also use intents for sharing
information, content provider are suited for
large datasets
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Content Providers
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Content Provider Example
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Broadcast Receivers
• Android’s implementation of a system-wide
publish/subscribe mechanism.
• They are also called observer patterns
• The receiver is a piece of code that gets
activated once an event to which it has
subscribed occurs
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Application Context
• Android major building blocks together make up anapplication
• They live inside the same application context
• Application context refers to the application
environment• It gets created whenever first component of an
application starts (that component can be any buildingblock)
•
Reference to context can be obtained – Context.getApplicationContext()
– Activiy.getApplication()
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• Step # 1: Java
– When you write Android applications, you
typically write them in Java source code.
– That Java source code is then turned into the stuff
that Android actually runs (Dalvik bytecode in an
APK file). – Hence, the first thing you need to do is get set up
with a Java development environment and be
ready to start writing Java classes.
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• Step # 1: Java
• This is what we did in the last week
• Step # 2: Install the Android SDK
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