Transcript Designing an Android App

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
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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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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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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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Using the Android User Interface
(cont’d)
Figure 2-5 Linear layout with a vertical orientation (default)
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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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Using the Android User Interface
(cont’d)
Figure 2-6 Linear layout with a horizontal orientation
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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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Using the Android User Interface
(cont’d)
Figure 2-10 Updated text Property
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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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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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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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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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Creating Activities
(continued)
Figure 2-17 Creating the Recipe class
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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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The Android Manifest File
(continued)
Figure 2-22 Adding the Recipe Activity
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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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Coding the Java Activity
(continued)
• Displaying the User Interface
Figure 2-24 Main.java code
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Coding the Java Activity
(continued)
Figure 2-25 onCreate methods
Figure 2-26 Inserting the onCreate method
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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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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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Coding the Java Activity
(continued)
• Coding a Button Event Handler
Figure 2-30 Complete Code
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Coding the Java Activity
(continued)
• Correcting Errors in Code
Figure 2-31 Syntax Error
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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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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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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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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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