how to develop an android apps

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How to Develop Android Apps

Deepak Halder 09:04

Building a Simple User Interface Previous Next This lesson teaches you to Create a Linear Layout Add a Text Field Add String...

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Learn How to Develop Your Own Android Apps

Deepak Halder 08:26

Creating an Android Project Previous Next This lesson teaches you to Create a Project with Android Studio Create a Project wi...

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Showing posts with label how to develop an android apps. Show all posts

08 October 2015

Building a Simple User Interface

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Layouts

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. ViewGroupobjects 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

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.
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.
Delete the <TextView> element.
Change the <RelativeLayout> element to <LinearLayout>.
Add the android:orientation attribute and set it to "horizontal".
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 LinearLayoutappears 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.

In the activity_my.xml file, within the <LinearLayout> element, define an <EditText> element with theid attribute set to @+id/edit_message.
Define the layout_width and layout_height attributes as wrap_content.
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.javafile each time you compile your app. You should never modify this file by hand.

For more information, read the guide toProviding 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 EditTextelement 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."

In Android Studio, from the res/values directory, open strings.xml.
Add a line for a string named "edit_message" with the value, "Enter a message".
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.
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

In Android Studio, from the res/layout directory, edit the activity_my.xml file.
Within the <LinearLayout> element, define a <Button> element immediately following the <EditText>element.
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.
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 theweight 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:

In the activity_my.xml file, assign the <EditText> element's layout_weight attribute a value of 1.
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 EditTextto 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
```

Continue to the next lesson to learn how to respond to button presses, read content from the text field, start another activity, and more.

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Learn How to Develop Your Own Android Apps

Creating an Android Project

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Managing Projects

An Android project contains all the files that comprise the source code for your Android app.

This lesson shows how to create a new project either using Android Studio or using the SDK tools from a command line.

Note: You should already have the Android SDK installed, and if you're using Android Studio, you should also haveAndroid Studio installed. If you don't have these, follow the guide to Installing the Android SDK before you start this lesson.

Create a Project with Android Studio

In Android Studio, create a new project:
- If you don't have a project opened, in the Welcome screen, click New Project.
- If you have a project opened, from the File menu, select New Project.

Figure 1. Configuring a new project in Android Studio.

Under Configure your new project, fill in the fields as shown in figure 1 and click Next.
It will probably be easier to follow these lessons if you use the same values as shown.
- Application Name is the app name that appears to users. For this project, use "My First App."
- Company domain provides a qualifier that will be appended to the package name; Android Studio will remember this qualifier for each new project you create.
- Package name is the fully qualified name for the project (following the same rules as those for naming packages in the Java programming language). Your package name must be unique across all packages installed on the Android system. You can Edit this value independently from the application name or the company domain.
- Project location is the directory on your system that holds the project files.
Under Select the form factors your app will run on, check the box for Phone and Tablet.
For Minimum SDK, select API 8: Android 2.2 (Froyo).
The Minimum Required SDK is the earliest version of Android that your app supports, indicated using the API level. To support as many devices as possible, you should set this to the lowest version available that allows your app to provide its core feature set. If any feature of your app is possible only on newer versions of Android and it's not critical to the app's core feature set, you can enable the feature only when running on the versions that support it (as discussed in Supporting Different Platform Versions).
Leave all of the other options (TV, Wear, and Glass) unchecked and click Next.

Activities

An activity is one of the distinguishing features of the Android framework. Activities provide the user with access to your app, and there may be many activities. An application will usually have a main activity for when the user launches the application, another activity for when she selects some content to view, for example, and other activities for when she performs other tasks within the app. See Activities for more information.

Under Add an activity to <template>, select Blank Activity and click Next.
Under Choose options for your new file, change the Activity Name to MyActivity. The Layout Name changes to activity_my, and the Title to MyActivity. The Menu Resource Name ismenu_my.
Click the Finish button to create the project.

Your Android project is now a basic "Hello World" app that contains some default files. Take a moment to review the most important of these:

app/src/main/res/layout/activity_my.xml

This is the XML layout file for the activity you added when you created the project with Android Studio. Following the New Project workflow, Android Studio presents this file with both a text view and a preview of the screen UI. The file includes some default settings and a TextView element that displays the message, "Hello world!"

app/src/main/java/com.mycompany.myfirstapp/MyActivity.java

A tab for this file appears in Android Studio when the New Project workflow finishes. When you select the file you see the class definition for the activity you created. When you build and run the app, the Activityclass starts the activity and loads the layout file that says "Hello World!"

app/src/main/AndroidManifest.xml

The manifest file describes the fundamental characteristics of the app and defines each of its components. You'll revisit this file as you follow these lessons and add more components to your app.

app/build.gradle

Android Studio uses Gradle to compile and build your app. There is a build.gradle file for each module of your project, as well as a build.gradle file for the entire project. Usually, you're only interested in thebuild.gradle file for the module, in this case the app or application module. This is where your app's build dependencies are set, including the defaultConfig settings:

compiledSdkVersion is the platform version against which you will compile your app. By default, this is set to the latest version of Android available in your SDK. (It should be Android 4.1 or greater; if you don't have such a version available, you must install one using the SDK Manager.) You can still build your app to support older versions, but setting this to the latest version allows you to enable new features and optimize your app for a great user experience on the latest devices.
applicationId is the fully qualified package name for your application that you specified during the New Project workflow.
minSdkVersion is the Minimum SDK version you specified during the New Project workflow. This is the earliest version of the Android SDK that your app supports.
targetSdkVersion indicates the highest version of Android with which you have tested your application. As new versions of Android become available, you should test your app on the new version and update this value to match the latest API level and thereby take advantage of new platform features. For more information, read Supporting Different Platform Versions.

See Building Your Project with Gradle for more information about Gradle.

Note also the /res subdirectories that contain the resources for your application:

drawable<density>/: Directories for drawable objects (such as bitmaps) that are designed for various densities, such as medium-density (mdpi) and high-density (hdpi) screens. Other drawable directories contain assets designed for other screen densities. Here you'll find the ic_launcher.png that appears when you run the default app.
layout/: Directory for files that define your app's user interface like activity_my.xml, discussed above, which describes a basic layout for the MyActivity class.
menu/: Directory for files that define your app's menu items.
values/: Directory for other XML files that contain a collection of resources, such as string and color definitions. The strings.xml file defines the "Hello world!" string that displays when you run the default app.

To run the app, continue to the next lesson.

Create a Project with Command Line Tools

If you're not using the Android Studio IDE, you can instead create your project using the SDK tools from a command line:

Change directories into the Android SDK’s sdk/ path.
Execute:
```
tools/android list targets
```
This prints a list of the available Android platforms that you’ve downloaded for your SDK. Find the platform against which you want to compile your app. Make a note of the target ID. We recommend that you select the highest version possible. You can still build your app to support older versions, but setting the build target to the latest version allows you to optimize your app for the latest devices.

If you don't see any targets listed, you need to install some using the Android SDK Manager tool. See Adding SDK Packages.
Execute:
```
android create project --target <target-id> --name MyFirstApp \
--path <path-to-workspace>/MyFirstApp --activity MyActivity \
--package com.example.myfirstapp
```
Replace <target-id> with an ID from the list of targets (from the previous step) and replace <path-to-workspace> with the location in which you want to save your Android projects.

Tip: Add the platform-tools/ as well as the tools/ directory to your PATH environment variable.

Your Android project is now a basic "Hello World" app that contains some default files. To run the app, continue to the next lesson.

Next: Running Your Application

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How to Develop Android Apps

Learn How to Develop Your Own Android Apps

How to Develop Android Apps

Building a Simple User Interface

This lesson teaches you to

You should also read

Alternative Layouts

Create a Linear Layout

Add a Text Field

Resource Objects

Add String Resources

Add a Button

Make the Input Box Fill in the Screen Width

Run Your App

Learn How to Develop Your Own Android Apps

Creating an Android Project

This lesson teaches you to

You should also read

Create a Project with Android Studio

Activities

Create a Project with Command Line Tools