Android Development Course (Part 2) Running Your App/Building a Simple User Interface

Android Development Course (Part 2) Running Your App/Building a Simple User Interface

Author:
Price:
Add to CartProduct Added View Cart Checkout
Product Tags:

Continue Reading>>>>

 Image result for making android app

 

1.  Running Your App

If you followed the previous lesson to create an Android project, it includes a default set of "Hello World" source files that allow you to immediately run the app.
How you run your app depends on two things: whether you have a real device running Android and whether you're using Android Studio. This lesson shows you how to install and run your app on a real device and on the Android emulator, and in both cases with either Android Studio or the command line tools.

Run on a Real Device

If you have a device running Android, here's how to install and run your app.

Set up your device

  1. Plug in your device to your development machine with a USB cable. If you're developing on Windows, you might need to install the appropriate USB driver for your device. For help installing drivers, see the OEM USB Drivers document.
  2. Enable USB debugging on your device.
    • On most devices running Android 3.2 or older, you can find the option under Settings > Applications > Development.
    • On Android 4.0 and newer, it's in Settings > Developer options.
      Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options.

Run the app from Android Studio

  1. Select one of your project's files and click Run from the toolbar.
  2. In the Choose Device window that appears, select the Choose a running device radio button, select your device, and click OK .
Android Studio installs the app on your connected device and starts it.

Run the app from a command line

Open a command-line and navigate to the root of your project directory. Use Gradle to build your project in debug mode, invoke the assembleDebug build task using the Gradle wrapper script (gradlew assembleRelease).
This creates your debug .apk file inside the module build/ directory, named MyFirstApp-debug.apk.
On Windows platforms, type this command:
> gradlew.bat assembleDebug
On Mac OS and Linux platforms, type these commands:
$ chmod +x gradlew
$ ./gradlew assembleDebug
After you build the project, the output APK for the app module is located in app/build/outputs/apk/
Note: The first command (chmod) adds the execution permission to the Gradle wrapper script and is only necessary the first time you build this project from the command line.
Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable, then execute: 
adb install app/build/outputs/MyFirstApp-debug.apk

On your device, locate MyFirstApp and open it.
That's how you build and run your Android app on a device! To start developing, continue to the next lesson.

Run on the Emulator

Whether you're using Android Studio or the command line, to run your app on the emulator you need to first create an Android Virtual Device (AVD). An AVD is a device configuration for the Android emulator that allows you to model a specific device.

Create an AVD

  1. Launch the Android Virtual Device Manager:
    • In Android Studio, select Tools > Android > AVD Manager, or click the AVD Manager icon in the toolbar.
    • Or, from the command line, change directories to sdk/ and execute: 
      tools/android avd
      Note: The AVD Manager that appears when launched from the command line is different from the version in Android Studio, so the following instructions may not all apply.
    Figure 1. The AVD Manager main screen shows your current virtual devices.
  2. On the AVD Manager main screen (figure 1), click Create Virtual Device.
  3. In the Select Hardware window, select a device configuration, such as Nexus 6, then click Next.
  4. Select the desired system version for the AVD and click Next.
  5. Verify the configuration settings, then click Finish.
For more information about using AVDs, see Managing AVDs with AVD Manager.

Run the app from Android Studio

  1. In Android Studio, select your project and click Run from the toolbar.
  2. In the Choose Device window, click the Launch emulator radio button.
  3. From the Android virtual device pull-down menu, select the emulator you created, and click OK.
It can take a few minutes for the emulator to load itself. You may have to unlock the screen. When you do, My First App appears on the emulator screen.

Run your app from the command line

  1. Build the project from the command line. The output APK for the app module is located in app/build/outputs/apk/.
  2. Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable.
  3. Execute this command: 
    adb install app/build/outputs/MyFirstApp-debug.apk
  4. On the emulator, locate MyFirstApp and open it.
That's how you build and run your Android app on the emulator!

2.   Building a Simple User Interface

In this lesson, you create a layout in XML that includes a text field and a button. In the next lesson, your app responds when the button is pressed by sending the content of the text field to another activity.
The graphical user interface for an Android app is built using a hierarchy of View and ViewGroup objects. View objects are usually UI widgets such as buttons or text fields. ViewGroup objects are invisible view containers that define how the child views are laid out, such as in a grid or a vertical list.
Android provides an XML vocabulary that corresponds to the subclasses of View and ViewGroup so you can define your UI in XML using a hierarchy of UI elements.
Layouts are subclasses of the ViewGroup. In this exercise, you'll work with a LinearLayout.

Alternative Layouts

Declaring your UI layout in XML rather than runtime code is useful for several reasons, but it's especially important so you can create different layouts for different screen sizes. For example, you can create two versions of a layout and tell the system to use one on "small" screens and the other on "large" screens. For more information, see the class about Supporting Different Devices.

Figure 1. Illustration of how ViewGroup objects form branches in the layout and contain other View objects.

Create a Linear Layout

  1. In Android Studio, from the res/layout directory, open the activity_my.xml file. The BlankActivity template you chose when you created this project includes the activity_my.xml file with a RelativeLayout root view and a TextView child view.
  2. In the Preview pane, click the Hide icon to close the Preview pane. In Android Studio, when you open a layout file, you’re first shown the Preview pane. Clicking elements in this pane opens the WYSIWYG tools in the Design pane. For this lesson, you’re going to work directly with the XML.
  3. Delete the <TextView> element.
  4. Change the <RelativeLayout> element to <LinearLayout>.
  5. Add the android:orientation attribute and set it to "horizontal".
  6. Remove the android:padding attributes and the tools:context attribute.
The result looks like this:
res/layout/activity_my.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >
</LinearLayout>
LinearLayout is a view group (a subclass of ViewGroup) that lays out child views in either a vertical or horizontal orientation, as specified by the android:orientation attribute. Each child of a LinearLayout appears on the screen in the order in which it appears in the XML.
Two other attributes, android:layout_width and android:layout_height, are required for all views in order to specify their size.
Because the LinearLayout is the root view in the layout, it should fill the entire screen area that's available to the app by setting the width and height to "match_parent". This value declares that the view should expand its width or height to match the width or height of the parent view.
For more information about layout properties, see the Layout guide.

Add a Text Field

As with every View object, you must define certain XML attributes to specify the EditText object's properties.
  1. In the activity_my.xml file, within the <LinearLayout> element, define an <EditText> element with the id attribute set to @+id/edit_message.
  2. Define the layout_width and layout_height attributes as wrap_content.
  3. Define a hint attribute as a string object named edit_message.
The <EditText> element should read as follows:
res/layout/activity_my.xml
<EditText android:id="@+id/edit_message"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" />
Here are the <EditText> attributes you added:
android:id
This provides a unique identifier for the view, which you can use to reference the object from your app code, such as to read and manipulate the object (you'll see this in the next lesson). The at sign (@) is required when you're referring to any resource object from XML. It is followed by the resource type (id in this case), a slash, then the resource name (edit_message).

Resource Objects

A resource object is a unique integer name that's associated with an app resource, such as a bitmap, layout file, or string.
Every resource has a corresponding resource object defined in your project's gen/R.java file. You can use the object names in the R class to refer to your resources, such as when you need to specify a string value for the android:hint attribute. You can also create arbitrary resource IDs that you associate with a view using the android:id attribute, which allows you to reference that view from other code.
The SDK tools generate the R.java file each time you compile your app. You should never modify this file by hand.
For more information, read the guide to Providing Resources.
The plus sign (+) before the resource type is needed only when you're defining a resource ID for the first time. When you compile the app, the SDK tools use the ID name to create a new resource ID in your project's gen/R.java file that refers to the EditText element. With the resource ID declared once this way, other references to the ID do not need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not needed for concrete resources such as strings or layouts. See the sidebox for more information about resource objects.
android:layout_width and android:layout_height
Instead of using specific sizes for the width and height, the "wrap_content" value specifies that the view should be only as big as needed to fit the contents of the view. If you were to instead use "match_parent", then the EditText element would fill the screen, because it would match the size of the parent LinearLayout. For more information, see the Layouts guide.
android:hint
This is a default string to display when the text field is empty. Instead of using a hard-coded string as the value, the "@string/edit_message" value refers to a string resource defined in a separate file. Because this refers to a concrete resource (not just an identifier), it does not need the plus sign. However, because you haven't defined the string resource yet, you’ll see a compiler error at first. You'll fix this in the next section by defining the string.
Note: This string resource has the same name as the element ID: edit_message. However, references to resources are always scoped by the resource type (such as id or string), so using the same name does not cause collisions.

Add String Resources

By default, your Android project includes a string resource file at res/values/strings.xml. Here, you'll add a new string named "edit_message" and set the value to "Enter a message."
  1. In Android Studio, from the res/values directory, open strings.xml.
  2. Add a line for a string named "edit_message" with the value, "Enter a message".
  3. Add a line for a string named "button_send" with the value, "Send". You'll create the button that uses this string in the next section.
  4. Remove the line for the "hello world" string.
The result for strings.xml looks like this:
res/values/strings.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="app_name">My First App</string>
    <string name="edit_message">Enter a message</string>
    <string name="button_send">Send</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>
</resources>
For text in the user interface, always specify each string as a resource. String resources allow you to manage all UI text in a single location, which makes the text easier to find and update. Externalizing the strings also allows you to localize your app to different languages by providing alternative definitions for each string resource.
For more information about using string resources to localize your app for other languages, see the Supporting Different Devices class.

Add a Button

  1. In Android Studio, from the res/layout directory, edit the activity_my.xml file.
  2. Within the <LinearLayout> element, define a <Button> element immediately following the <EditText> element.
  3. Set the button's width and height attributes to "wrap_content" so the button is only as big as necessary to fit the button's text label.
  4. Define the button's text label with the android:text attribute; set its value to the button_send string resource you defined in the previous section.
Your <LinearLayout> should look like this:
res/layout/activity_my.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >
      <EditText android:id="@+id/edit_message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
      <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
</LinearLayout>
Note: This button doesn't need the android:id attribute, because it won't be referenced from the activity code.
The layout is currently designed so that both the EditText and Button widgets are only as big as necessary to fit their content, as shown in figure 2.

Figure 2. The EditText and Button widgets have their widths set to "wrap_content".
This works fine for the button, but not as well for the text field, because the user might type something longer. It would be nice to fill the unused screen width with the text field. You can do this inside a LinearLayout with the weight property, which you can specify using the android:layout_weight attribute.
The weight value is a number that specifies the amount of remaining space each view should consume, relative to the amount consumed by sibling views. This works kind of like the amount of ingredients in a drink recipe: "2 parts soda, 1 part syrup" means two-thirds of the drink is soda. For example, if you give one view a weight of 2 and another one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining space and the second view fills the rest. If you add a third view and give it a weight of 1, then the first view (with weight of 2) now gets 1/2 the remaining space, while the remaining two each get 1/4.
The default weight for all views is 0, so if you specify any weight value greater than 0 to only one view, then that view fills whatever space remains after all views are given the space they require.

Make the Input Box Fill in the Screen Width

To fill the remaining space in your layout with the EditText element, do the following:
  1. In the activity_my.xml file, assign the <EditText> element's layout_weight attribute a value of 1.
  2. Also, assign <EditText> element's layout_width attribute a value of 0dp.
    res/layout/activity_my.xml
    <EditText
    android:layout_weight="1"
    android:layout_width="0dp"
    ... />
    To improve the layout efficiency when you specify the weight, you should change the width of the EditText to be zero (0dp). Setting the width to zero improves layout performance because using "wrap_content" as the width requires the system to calculate a width that is ultimately irrelevant because the weight value requires another width calculation to fill the remaining space.
    Figure 3 shows the result when you assign all weight to the EditText element.
    Figure 3. The EditText widget is given all the layout weight, so it fills the remaining space in the LinearLayout.
Here’s how your complete activity_my.xmllayout file should now look:
res/layout/activity_my.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">
    <EditText android:id="@+id/edit_message"
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
</LinearLayout>

Run Your App

This layout is applied by the default Activity class that the SDK tools generated when you created the project. Run the app to see the results:
  • In Android Studio, from the toolbar, click Run .
  • Or from a command line, change directories to the root of your Android project and execute: 
    ant debug
adb install bin/MyFirstApp-debug.apk
     
    next tutorial will be here
    stay updated for the next tutorial! 
     

0 Reviews