1. Chapter 2: Simplify! 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
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3. Objectives (continued)
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
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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
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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
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6. Using the Android User Interface (cont’d)
Figure 2-5 Linear layout with a vertical orientation (default)
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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
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8. Using the Android User Interface (cont’d)
Figure 2-6 Linear layout with a horizontal orientation
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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
Table 2-1 Measurements used for the Text size property
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10. Using the Android User Interface (cont’d)
Figure 2-10 Updated text Property
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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)
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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)
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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
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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
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15. Creating Activities (continued)
Figure 2-17 Creating the Recipe class
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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
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17. The Android Manifest File (continued)
Figure 2-22 Adding the Recipe Activity
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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
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19. Coding the Java Activity (continued)
Displaying the User Interface
Figure 2-24 Main.java code
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20. Coding the Java Activity (continued)
Figure 2-25 onCreate methods
Figure 2-26 Inserting the onCreate method
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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);
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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 }
});
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23. Coding the Java Activity (continued)
Coding a Button Event Handler
Figure 2-30 Complete Code
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24. Coding the Java Activity (continued)
Correcting Errors in Code
Figure 2-31 Syntax Error
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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
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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
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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
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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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