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.  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.  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.  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. 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