Android is a software stack for mobile devices that includes an operating system, middleware and key applications. The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language.

Features

• Application framework enabling reuse and replacement of components
• Dalvik virtual machine optimized for mobile devices
• Integrated browser based on the open source WebKit engine
• Optimized graphics powered by a custom 2D graphics library; 3D graphics based on the OpenGL ES 1.0 specification (hardware acceleration optional)
• SQLite for structured data storage
• Media support for common audio, video, and still image formats (MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)
• GSM Telephony (hardware dependent)
• Bluetooth, EDGE, 3G, and WiFi (hardware dependent)
• Camera, GPS, compass, and accelerometer (hardware dependent)
• Rich development environment including a device emulator, tools for debugging, memory and performance profiling, and a plugin for the Eclipse IDE

Libraries

Android includes a set of C/C++ libraries used by various components of the Android system. These capabilities are exposed to developers through the Android application framework. Some of the core libraries are listed below:

• System C library – a BSD-derived implementation of the standard C system library (libc), tuned for embedded Linux-based devices
• Media Libraries – based on PacketVideo’s OpenCORE; the libraries support playback and recording of many popular audio and video formats, as well as static image files, including MPEG4, H.264, MP3, AAC, AMR, JPG, and PNG
• Surface Manager – manages access to the display subsystem and seamlessly composites 2D and 3D graphic layers from multiple applications
• LibWebCore – a modern web browser engine which powers both the Android browser and an embeddable web view
• SGL – the underlying 2D graphics engine
• 3D libraries – an implementation based on OpenGL ES 1.0 APIs; the libraries use either hardware 3D acceleration (where available) or the included, highly optimized 3D software rasterizer
• FreeType – bitmap and vector font rendering
• SQLite – a powerful and lightweight relational database engine available to all applications

Application Fundamentals

Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.

Once installed on a device, each Android application lives in its own security sandbox:

• The Android operating system is a multi-user Linux system in which each application is a different user.
• By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.
• Each process has its own virtual machine (VM), so an application’s code runs in isolation from other applications.
• By default, every application runs in its own Linux process. Android starts the process when any of the application’s components need to be executed, then shuts down the process when it’s no longer needed or when the system must recover memory for other applications.

In this way, the Android system implements the principle of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.

However, there are ways for an application to share data with other applications and for an application to access system services:

• It’s possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other’s files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).
• An application can request permission to access device data such as the user’s contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.

That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you to:

• The core framework components that define your application.
• The manifest file in which you declare components and required device features for your application.
• Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations.

Application Components

Application components are the essential building blocks of an Android application. Each component is a different point through which the system can enter your application. Not all components are actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your application’s overall behavior.

There are four different types of application components. Each type serves a distinct purpose and has a distinct lifecycle that defines how the component is created and destroyed.

Here are the four types of application components:

Activities

An activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. Although the activities work together to form a cohesive user experience in the email application, each one is independent of the others. As such, a different application can start any one of these activities (if the email application allows it). For example, a camera application can start the activity in the email application that composes new mail, in order for the user to share a picture.An activity is implemented as a subclass of Activity and you can learn more about it in the Activities developer guide.

Services

A service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity. Another component, such as an activity, can start the service and let it run or bind to it in order to interact with it.A service is implemented as a subclass of Service and you can learn more about it in the Services developer guide.

Content providers

A content provider manages a shared set of application data. You can store the data in the file system, an SQLite database, on the web, or any other persistent storage location your application can access. Through the content provider, other applications can query or even modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the user’s contact information. As such, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.Content providers are also useful for reading and writing data that is private to your application and not shared. For example, the Note Pad sample application uses a content provider to save notes.

A content provider is implemented as a subclass of ContentProvider and must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer guide.

Broadcast receivers

A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the system—for example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applications can also initiate broadcasts—for example, to let other applications know that some data has been downloaded to the device and is available for them to use. Although broadcast receivers don’t display a user interface, they may create a status bar notification to alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is just a “gateway” to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.

There are separate methods for activiting each type of component:

• You can start an activity (or give it something new to do) by passing an Intent to startActivity() or startActivityForResult() (when you want the activity to return a result).
• You can start a service (or give new instructions to an ongoing service) by passing an Intent to startService(). Or you can bind to the service by passing an Intent to bindService().
• You can initiate a broadcast by passing an Intent to methods like sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().
• You can perform a query to a content provider by calling query() on a ContentResolver.

The Manifest File

Before the Android system can start an application component, the system must know that the component exists by reading the application’s AndroidManifest.xml file (the “manifest” file). Your application must declare all its components in this file, which must be at the root of the application project directory.

The manifest does a number of things in addition to declaring the application’s components, such as:

• Identify any user permissions the application requires, such as Internet access or read-access to the user’s contacts.
• Declare the minimum API Level required by the application, based on which APIs the application uses.
• Declare hardware and software features used or required by the application, such as a camera, bluetooth services, or a multitouch screen.
• API libraries the application needs to be linked against (other than the Android framework APIs), such as the Google Maps library.
• And more

 

 

iPhone is a smartphone made by Apple that combines an iPod, a tablet PC, a digital camera and a cellular phone. The device includes Internet browsing and networking capabilities.

iPhone is extremely thin (only 11.6 millimeters thick) but wider and longer than many comparable devices. The display area is a 3.5-inch wide screen multi-touch interface with unusually high resolution (160 pixels per inch). Unlike most other smartphones, iPhone does not use a hardware keyboard or a stylus. To navigate, a user uses multiple taps and drags to navigate through a mobile version of Apple’s OS X operating system. Like iPod, iPhone synchronizes data with a user’s personal computer, using iTunes as a client software and Apple’s proprietary USB port. iPhone