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Chapter 2

Chapter 2. The Android User Interface. Objectives. In this chapter, you learn to: Develop a user interface using the TextView , ImageView , and Button controls Create an Android project that includes a Button event Select a Linear of Relative layout for the user interface

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Chapter 2

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  1. Chapter 2 The Android User Interface

  2. Objectives In this chapter, you learn to: Develop a user interface using the TextView, ImageView, and Button controls Create an Android project that includes a Button event Select a Linear of Relative layout for the user interface Create multiple Android Activities Add activities to the Android Manifest file Add a Java class file

  3. Objectives Write code using the onCreate method Display content using the setContentView command Open a second screen using a Button event handler Use an OnClickListener to detect user interaction Launch a second screen using a startActivity method Correct errors in Java code Run the completed app in the emulator

  4. Designing an Android App Designing apps is like constructing a building The Big Picture Follow these steps: Create the user interface for every screen Create an Activity for every screen Update the Android Manifest file Code each Java class with objects and actions

  5. Using the Android User Interface The interface is a window on the screen of any mobile device The layout is designed with XML code Special Android-formatted XML code is extremely compact Linear Layouts and Relative Layouts A Linear Layout organizes layout components in a vertical column or horizontal row Objects are placed directly below each other Can be switched from vertical to horizontal orientation Linear layout is the default layout

  6. Using the Android User Interface (cont’d)

  7. Using the Android User Interface (cont’d) A Relative Layout organizes layout components in relation to each other Provides more flexibility in positioning than Linear layouts Must be changed from the linear default

  8. Using the Android User Interface (cont’d)

  9. Using the Android User Interface (cont’d) Android Text Properties Text Property – changes the text written in the control Text Size Property- can be adjusted in inches, millimeters, pixels, density-independent pixels, and scaled-independent pixels

  10. Using the Android User Interface (cont’d)

  11. Using the Android User Interface (cont’d) Adding a File to the Resources Folder Before you can use images, they must be placed in the resources folder Res folder contains three subfolders All folder names begin with drawable hdpi (resources for high-density screens) mdpi (resources for medium-density screens) ldpi (resources for low-density screens) Android supports three graphic formats .png (preferred), .jpg (acceptable), .gif(discouraged)

  12. Using the Android User Interface (cont’d) Adding an ImageView Control An ImageView control displays icons or graphics on the Android screen Adding a Button Control There are three types of Buttons Button (buttons that perform a function) ToggleButton (buttons that can be on or off) ImageButton (buttons that have a picture on them)

  13. Using the Android User Interface (cont’d) Planning a Program Gather and analyze program requirements Design the user interface Design the program processing objects Code the program Test the program

  14. Creating Activities Each screen is considered an activity Constructed using XML layout files and a Java class Creating an XML Layout file All layout files are placed in the res/layout directory Adding a Class File A class describes a group of objects and serves as a blueprint, or template, for creating those objects An object is a specific, concrete instance of a class When you create an object, you instantiate it; meaning you define one particular variation of the object

  15. Creating Activities (continued)

  16. The Android Manifest File The Android Manifest file contains: the name of the Java application a listing of each activity permissions needed to access other Android functions (like accessing the Internet) the minimum level of the Android APL Adding an Activity to the Android Manifest When applications have more than one activity, the Manifest must have an intent to navigate among multiple activities

  17. The Android Manifest File (continued)

  18. Coding the Java Activity A method is a set of Java statements that can be included inside a Java class Methods perform specific tasks Coding an onCreate Method public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); } Note the use of Curly braces { that begin and end the method code and the use of the semi-colon ; SetContentView(R.layout.main); is added to display the screen

  19. Coding the Java Activity (continued) Displaying the User Interface

  20. Coding the Java Activity (continued)

  21. Coding the Java Activity (continued) Creating a Button Event Handler An event handler is part of a program coded to respond to a specific event Tapping the button is called a click event Java code must contain the following sections Class property to hold a reference to the Button object onClickListener() method to await the button click action onClick() method to respond to the click event Code Syntax: Button b=(Button)findViewById(R.id.btnRecipe);

  22. Coding the Java Activity (continued) When you import the Button type as an Android widget, the button package becomes available throughout the app An import statement makes more Java functions available to your app A stub is a code placeholder module b.setOnClickListener(new OnClickListener() { public void onClick(View v) { // TODO Auto-generated method stub } });

  23. Coding the Java Activity (continued) Coding a Button Event Handler

  24. Coding the Java Activity (continued) Correcting Errors in Code

  25. Coding the Java Activity (continued) Saving and Running the Application Testing the App automatically saves it The Save All button will save the project Select Android Application from the dialog window the first time an app runs

  26. Summary Linear layouts arrange screen components in a vertical column or horizontal row Relative layouts arrange components freely on the screen Text Property and TextSize property are used when creating text To display graphics (pictures and icons), use the ImageView control An Activity is when the app makes contact with the user and is a core component of the app

  27. Summary (continued) Each screen in your app is an Activity Every app has an Android Manifest file containing the name if the app and a list of each activity When an app has more than one activity, it must have an intent so the app can navigate among multiple activities A method is a set of Java statements included in a Java class The onCreate method initializes an Activity

  28. Summary (continued) The setContentView command displays the content on the screen Tapping or clicking a button invokes the event listener and runs the code in that button Use the startActivity() method to start an Activity in an app that has multiple Activities The intent contains a context - initiating Activity class is making the request - and the name of the Activity Red error icons and red curly lines indicate errors

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