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Regardless of your prior programming experiences, be it Windows, Mac OS X, Linux or even iOS based, the chances are good that Android development is quite unlike anything you have encountered before.

The objective of this chapter, therefore, is to provide an understanding of the high level concepts behind the architecture of Android applications. In so doing, we will explore in detail both the various components that can be used to construct an application and the mechanisms that allow these to work together to create a cohesive application.

== Android Activities ==

Those familiar with object-oriented programming languages such as Java, C++ or C# will be familiar with the concept of encapsulating elements of application functionality into classes that are then instantiated as objects and manipulated to create an application. Since Android applications are written in Java, this is still very much the case. Android, however, also takes the concept of re-usable components to a higher level.

Android applications are created by bringing together one or more components known as Activities. An activity is a single, standalone module of application functionality which usually correlates directly to a single user interface screen and its corresponding functionality. An appointments application might, for example, have an activity screen that displays appointments set up for the current day. The application might also utilize a second activity consisting of a screen where new appointments may be entered by the user.

Activities are intended as fully reusable and interchangeable building blocks which can be shared amongst different applications. An existing email application, for example, might contain an activity specifically for composing and sending an email message. A developer might be writing an application that also has a requirement to send an email message. Rather than develop an email composition activity specifically for the new application, the developer can simply use the activity from the existing email application.

Activities are created as subclasses of the Android Activity class and must be implemented so as to be entirely independent of other activities in the application. In other words, a shared activity cannot rely on being called at a known point in a program flow (since other applications may make use of the activity in unanticipated ways) and one activity cannot directly call methods or access instance data of another activity. This, instead, is achieved through the use of Intents and Content Providers.

By default, an activity cannot return results to the activity from which it was invoked. If this functionality is required, the activity must be specifically started as a sub-activity of the originating activity.

== Android Intents ==

Intents are the mechanism by which one activity is able to launch another and implement the flow through the activities that make up an application. Intents consist of a description of the operation to be performed and, optionally, the data on which it is to be performed. 

Intents can be explicit, in that they request the launch of a specific activity by referencing the activity by class name, or implicit by stating either the type of action to be performed or providing data of a specific type on which the action is to be performed. In the case of implicit intents, the Android runtime will select the activity to launch that most closely matches the criteria specified by the Intent using a process referred to as ''Intent Resolution''.  

== Broadcast Intents ==

Another type of Intent, the Broadcast Intent, is a system wide intent that is sent out to all applications that have registered an “interested” Broadcast Receiver. The Android system, for example, will typically send out Broadcast Intents to indicate changes in device status such as the completion of system start up, connection of an external power source to the device or the screen being turned on or off.

A Broadcast Intent can be normal (asynchronous) in that it is sent to all interested Broadcast Receivers at more or less the same time, or ordered in that it is sent to one receiver at a time where it can be processed and then either aborted or allowed to be passed to the next Broadcast Receiver.

== Broadcast Receivers ==

Broadcast Receivers are the mechanism by which applications are able to respond to Broadcast Intents. A Broadcast Receiver must be registered by an application and configured with an Intent Filter to indicate the types of broadcast in which it is interested. When a matching intent is broadcast, the receiver will be invoked by the Android runtime regardless of whether the application that registered the receiver is currently running. The receiver then has 5 seconds in which to complete any tasks required of it (such as launching a Service, making data updates or issuing a notification to the user) before returning. Broadcast Receivers operate in the background and do not have a user interface.

== Android Services ==

Android Services are processes that run in the background and do not have a user interface. They can be started and subsequently managed from Activities, Broadcast Receivers or other Services. Android Services are ideal for situations where an application needs to continue performing tasks but does not necessarily need a user interface to be visible to the user. Although Services lack a user interface, they can still notify the user of events through the use of notifications and toasts (small notification messages that appear on the screen without interrupting the currently visible Activity) and are also able to issue Intents.

Services are given a higher priority by the Android runtime than many other processes and will only be terminated as a last resort by the system in order to free up resources. In the event that the runtime does need to kill a Service, however, it will be automatically restarted as soon as adequate resources once again become available.

Example situations where a Service might be a practical solution include the streaming of audio that should continue when the application is no longer active, or a stock market tracking application that needs to notify the user when a share hits a specified price.

== Content Providers ==

Content Providers implement a mechanism for the sharing of data between applications. Any application can provide other applications with access to its underlying data through the implementation of a Content Provider including the ability to add, remove and query the data (subject to permissions). Access to the data is provided via a Universal Resource Identifier (URI) defined by the Content Provider. Data can be shared in the form a file or an entire SQLite database.

The native Android applications include a number of standard Content Providers allowing applications to access data such as contacts and media files.

The Content Providers currently available on an Android system may be located using a ''Content Resolver''.

== The Application Manifest ==

The glue that pulls together the various elements that comprise an application is the Application Manifest file. It is within this XML based file that the application outlines the activities, services, broadcast receivers, data providers and permission that make up the complete application. 

== Application Resources ==

In addition to the manifest file and Dex files that contains the executable byte code for the Dalvik VM, an Android application will also typically contain a collection of resource files. These files contain resources such as the strings, images, fonts and colors that appear in the user interface together with the XML representation of the user interface layouts. By default, these files are stored in the /res sub-directory of the application project’s hierarchy. 

==Application Context ==

When an application is compiled, a class named R is created that contains references to the application resources. The application manifest file and these resources combine to create what is known as the Application Context. This context, represented by the Android Context class, may be used in the application code to gain access to the application resources at runtime. In addition, a wide range of methods may be called on an application’s context to gather information and make changes to the application’s environment at runtime.

== Summary ==

A number of different elements can be brought together in order to create an Android application. In this chapter we have provided an high level overview of Activities, Services, Intents and Broadcast Receivers together with an overview of the manifest file and application resources.

Maximum reuse and interoperability are promoted through the creation of individual, standalone modules of functionality in the form of activities and intents whilst data sharing between applications is achieved by the implementation of content providers. 
Whilst activities are focused on areas where the user interacts with application (an activity essentially equating to a single user interface screen), background processing is typically handled by Services and Broadcast Receivers.

The components that make up the application are outlined for the Android runtime system in a manifest file which, combined the application’s resources, represents the application’s context.

Much has been covered in this chapter that is most likely new to the average developer. Rest assured, however, that extensive exploration and practical use of these concepts will be made in  subsequent chapters to ensure a solid knowledge foundation on which to build your own applications.


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