Shulou(Shulou.com)06/02 Report--

In this issue, the editor will bring you about how to understand the basic concepts of Android application. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

A prominent feature of the Android platform is "low coupling". Activity is one of the most basic user UI modules of Android applications. If you use the Windows Form application as a reference, Activity is equivalent to WinForm in Windows. Unlike Windows applications, the interaction between running an Activity or Activity is achieved through messages. In other words, if you want to start an Activity or start another Activity in an Activity, it is triggered by sending an Intent message, unlike the Windows WinForm application, which needs to call the Show or Load method of the Form example. If the interaction between Activity is realized through Intent message, the coupling degree between modules is reduced to a certain extent. This mechanism class is the same as the Subscriber/Publisher mechanism.

Another important feature of the Android platform is "reuse". An Android application can be made up of multiple Activity. Take playing cards as an example, the Android application is equivalent to a box of playing cards, and each card in the box is a relatively independent Activity. When the Android application runs, it wants to draw cards from the playing cards and put them together. The card drawn is the main Activity of the Android application. The main Activity can call other Activity (by sending Intent messages), and the triggered Activity is issued on the main Activity just like playing cards. This forms a "Activity" stack. Press "Back" on the device to go back to the previous Activity like a browser. Every application on the Android phone has the same structure. "reuse" means that when the Android application is running, it can trigger the Activity defined in other applications. For example, in GTalk, you want to show the location of a friend on the map. The GoogleMap app can display maps. GTalk does not need to repeat the same code or for similar Activity. You can start MapViewActivity in GoogleMap directly through Intent messages.

The following figure shows the basic components of the Android application.

In addition to Activity, Android can also implement Service, which is the same as Windows Service and generally runs in the background without a user interface. Brodcast Receiver can be used to respond to some system messages. The basic functionality is somewhat similar to PushRegistry in Java ME. For example, if you want to trigger your application when you receive a text message, you can define a Broadcast Receiver in the Manifest file of the Android application to trigger an Activity.

As shown in the figure above, in Android applications, the Application object is like a container that can contain multiple Activity, multiple Service, or multiple Broadcast Receiver. These Activity,Service,Broadcast Receiver are relatively independent and interact with each other only through Intent messages. Like Java ME's MIDlet JAD file, each Android application has a Manifest file with a fixed file name of AndroidManifest.xml. The Activity,Service,Broadcast Receiver defined in the Android application needs to be defined in this Mainifest file before it can be called by this application or other applications. Here we still borrow the concept of Publisher/Subscriber to illustrate. An Activity,Service, etc., needs to Subscriber some kind of message in Mainifest if it can be called.

Above is the definition of the main Activity in HelloWorld in AndroidManifest.xml, which defines the corresponding class of the Activity and the intent-filter that can trigger the Activity (equivalent to some kind of Subscriber message), but when the user clicks on the application icon, the Android operating system will send an Intent message, and the Android system checks the Activity,Service or Broadcast Receiver of the subscribe Intent message, and if found, it moves the Activity,Service or Broadcast Receiver. For HelloWorld, "HelloWorld" is displayed on the screen. In addition to the fact that the system can send Intent, Android references can also launch other Activity or Service through startActivity (Intent), StartService (Intent). Intent can take incoming data (parameters). Even in the same application, information needs to be transmitted through Intent, which greatly reduces the coupling between modules in the application, so that one module in the application can be seamlessly replaced without affecting other parts. When you first come into contact with the mechanism of Android, you may feel that it is not as convenient as WinForm's (new Form1 ()). Show (). However, from the point of view of the maintainability and scalability of the application, the low-coupling design of Android is very favorable. In addition, if you need to share some data among multiple Activity, you can do this by extending the Application class, where the variables defined in the Application class can be accessed by all Activity in the application.

The above is the editor for you to share how to understand the basic concepts of Android applications, if you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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