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This article introduces you how to interpret the basic concepts of Android system application, the content is very detailed, interested friends can refer to, hope to be helpful to you.

Friends who often follow our 51CTO should know that in the previous article we have a detailed tracking introduction to the mobile phone system Android, which is convenient for you to learn and apply this new powerful open source mobile phone system, such as the parsing of Android source code, and so on. So today we will have a more detailed understanding of the basic architecture of Android system applications.

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Understanding these concepts is more effective for developers to develop Android system applications.

For an Android application, it is organized by four building blocks as follows:

Activity

Intent Receiver

Service

Content Provider

However, not every Android system application needs these four building blocks, which is not necessary, and sometimes we only need a few of these four to combine into our application.

Once we have identified which building blocks our application needs, we need to register a list of these building blocks in AndroidManifest.xml. This is a XML configuration file that defines the components of our application, the functions and requirements of the components, and so on. This configuration file is required for every Android application. For AndroidMainfest.xml 's Schema, refer to the documentation that came with the SDK package. Here are some descriptions of the four building blocks:

1 、 Activity

Activity is one of the most basic building blocks in Android system applications. In applications, an activity is usually a separate screen. Each activity is implemented as a separate class and inherits from the base class Activity. This activity class will display the user interface made up of several Views controls and respond to events. Most applications include multiple screens. For example, a short message application will have a screen for displaying a contact list, a second screen for writing short messages, and a screen for browsing old short messages and making system settings. Every such screen is an activity. It is easy to navigate from one screen to another. In some applications, a screen may even return a value to the previous screen.

When a new screen opens, the previous screen will be paused and saved in the history stack. The user can return to the previous screen in the history stack. When the screen is no longer in use, it can also be removed from the history stack. By default, Android will keep the running screen from the home screen to each application.

Android system applications use the special class Intent to move from screen to screen. The Intent class is used to describe what an application will do. In the description structure of Intent, there are two most important parts: actions and data corresponding to actions. Typical action types are: MAIN (activity's portal), VIEW, PICK, EDIT, and so on. The data corresponding to the action is expressed in the form of URI. For example, to view a person's contact information, you need to create an intent with an action type of VIEW and a URI that represents that person.

A class related to it is called IntentFilter. As opposed to intent is a valid request to do something, an intent filter is used to describe which intent an activity (or Intent Receiver) can operate on. An activity needs to declare an IntentFilter if it wants to display a person's contact information, and the IntentFilter needs to know how to handle the VIEW action and represent a person's URI. IntentFilter needs to be defined in AndroidManifest.xml.

Navigating from one screen to another is simple by parsing various intent. When navigating forward, activity will call the startActivity (Intent myIntent) method. The system then looks in the IntentFilter defined in all installed applications to find the activity corresponding to the Intent that best matches the myIntent. When the new activity receives the notification from myIntent, it starts running. When the startActivity method is called, the action to parse the myIntent is triggered, and this mechanism provides two key benefits:

A, Activities can reuse a request in the form of Intent from other components

B, Activities can be replaced at any time by a new Activity with the same IntentFilter.

2 、 Intent Receiver

You can use an Intent Receiver when you want your application to respond to an external event, such as when the phone calls, or when the data network is available, or at night. Although Intent Receiver uses NotificationManager to notify users when events of interest occur, it does not generate a UI. Intent Receiver is registered in AndroidManifest.xml, but you can also register using Context.registerReceiver () in your code. When an intent receiver is triggered, your application does not have to call intent receiver on the request, the system will launch your application when needed. Various applications can also broadcast their own intent receiver to other applications by using Context.broadcastIntent ().

3 、 Service

A Service is a long-life program without a user interface. A good example is a media player that is playing songs from a playlist. In a media player application, there should be multiple activity that allow users to select and play songs. However, the music playback feature does not have a corresponding activity, because users will of course think that the music should still be playing when navigating to other screens. In this example, the media player the activity uses Context.startService () to launch a service so that the music can be kept playing in the background. At the same time, the system will keep the service running until the service is finished. In addition, we can connect to a service by using the Context.bindService () method (which will start if the service is not already running). After connecting to a service, we can also communicate with it through the interface provided by service. Take the media player as an example, we can also pause, replay and other operations.

4 、 Content Provider

Android system applications can save their data to files, SQL databases, or even any valid device. Content Provider is useful when you want to share your application data with other applications. A Content Provider class implements a set of standard methods that allow other applications to save or read the various data types processed by this Content Provider.

On how to interpret the basic concepts of Android system applications to share here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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