chapter 2: simplify! the android user interface. objectives in this chapter, you learn to: develop a...

Chapter 2: Simplify! 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• Create multiple Android Activities• Add activities to the Android Manifest file• Add a Java class file

2Android Boot Camp for Developers using Java

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


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


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


Using the Android User Interface (cont’d)


Figure 2-5 Linear layout with a vertical orientation (default)


– 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




Figure 2-6 Linear layout with a horizontal orientation


• 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



Figure 2-10 Updated text Property


• 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)



• 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)



• Planning a Program– Gather and analyze program requirements– Design the user interface– Design the program processing objects– Code the program– Test the program


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


Creating Activities (continued)


Figure 2-17 Creating the Recipe class

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


The Android Manifest File (continued)


Figure 2-22 Adding the Recipe Activity

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

Coding the Java Activity (continued)

• Displaying the User Interface


Figure 2-24 Main.java code



Figure 2-25 onCreate methods

Figure 2-26 Inserting the onCreate method


• 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);



– 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}});



• Coding a Button Event Handler


Figure 2-30 Complete Code


• Correcting Errors in Code


Figure 2-31 Syntax Error


• 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


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


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


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


chapter 2: simplify! the android user interface. objectives in this chapter, you learn to: develop a...

Documents

android screenadding

android project

android manifest fileadd

android manifest filecode

android appdesigning

emulator3android boot

user interactionlaunch

linear of relative layout