What Androids Are Made Of

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



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”



Activity Life Cycle

• Activity life cycle is managed by Activity

Manager



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





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



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?



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.



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)



Intents Example



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



Service Life Cycle

• Services have a much simpler life cycle than

activities



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



Content Providers



Content Provider Example



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



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()





• 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.



• Step # 1: Java

• This is what we did in the last week

• Step # 2: Install the Android SDK

Documents

What Androids Are Made Of