Android study notes 05: Activity overview

Zero, learning goals

1. Can tell the role of Activity
2. Will create Activity based on template
3. Will register Activity in the project manifest file
4. Will be set to start Activity in the project manifest file

One, the role of Activity

• Activity is one of the four core components of Android. Its main function is to provide an interface and interact with users.
• A program is generally composed of multiple activities, the relationship between each activity is very loose, there is no direct relationship between them. There must be an activity designated as the main activity, which is the first interface displayed when the program starts. Each activity can start other activities at will. Whenever an activity is started, the previous activity is stopped. All started activities in a program are placed in a stack, so the stopped activity is not destroyed, but stored in the stack. The newly started activity is first stored in the stack and then gets the input focus. Click the return button on the currently active activity, it is removed from the stack, and then destroyed, and then the previous activity is restored.
• When an activity is stopped because a new activity is started, it will be notified of a state change. There are multiple such changes, each of which will cause the system to call a corresponding callback method to notify the activity. These callback methods are collectively referred to as "Life cycle callback method". These callback methods are called when the Activity is created, stopped, resumed, and destroyed.

Two, create a custom Activity

• Create Android application [ActivityOverview]
  

1. Create based on common class inheriting Activity

• New Java class
  
• Enter the class name
  
• Add documentation comments to the class
  
• Create a layout file activity_login for LoginActivity as the user interface
  
• Set the layout file name-activity_login
  
  
• Switch to Code view
  
• Set linear layout attributes, add a label, set attributes
  
• Modify LoginActivity, inherit AppCompatActivity
  
• Create a callback method onCreate(), use the layout resource file to set the user interface
  

2. Created based on the Activity template of Android Studio

• Create a custom activity based on the Empty Activity template
  
• Configure Activity information
  
  
• Add a label control to the layout file
  

Three, register a custom Activity

1. View the project list file

• Only the custom Activity created by inheriting the Activity itself needs to be registered in the project manifest file. The LoginActivity we created just now needs to be registered in the project manifest file by ourselves, and the RegisterActivity created based on the template has been registered for us by the system.

2. Register LoginActivity

• You can set the properties of LoginActivity
  
• Define the login variable in the string resource file strings.xml
  
• Check the project list file again
  
• Start the application and check the effect
  
• What you see is the MainActivity window. If you want to see the LoginActivity window first when you want to start the application, you must set LoginActivity to start the Activity (Launcher Activity) in the project manifest file.

Fourth, set to start Activity

• Set to start Activity by adding intent filter
  

1. Set LoginActivity to start Activity

• Set up intent filter
  
• Start the application and check the effect
  

2. Set RegisterActivity to start Activity

• Set up intent filter
  
• Start the application and check the effect
  
• Questions: If the MainActivity is started, how can I jump to LoginActivity or RegisterActivity?