Lesson 3.3 Support Libraries

The Android SDK includes the Android Support Library, which is a collection of several libraries. These libraries provide features that aren’t built into the Android framework, including the following:

• Backward-compatible versions of framework components, so that apps running on older versions of the Android platform can support features made available in newer versions of the platform
• Additional layout and user interface elements
• Support for different device form factors, such as TV devices or wearables
• Components to support Material Design elements
• Other features, including palette support, annotations, percentage-based layout dimensions, and preferences

What you’ll do

• Create a new app with one TextView and one Button.
• Verify that the Android Support Repository (containing the Android Support Library) is available in your Android Studio installation.
• Explore the build.gradle files for your app project.
• Manage class or method calls that are unavailable for the version of Android your app supports.
• Use a compatibility class from the support library to provide backward-compatibility for your app.

1.1 Verify that the Android Support Repository is available

1. In Android Studio, select Tools > Android > SDK Manager, or click the SDK Manager Icon

The Android SDK Default Preferences pane appears.

Click the SDK Tools tab and expand Support Repository,

If Installed appears in the Status column, you’re all set. Click Cancel.

1.2 Set up the project and examine build.gradle

1. Create a new project called HelloCompat.

On the Target Android Devices page, API 15: Android 4.0.3 (IceCreamSandwich) is selected for the minimum SDK. As you’ve learned in previous lessons, this is the oldest version of the Android platform your app will support.

Explore build.gradle (Module:app)

1. In Android Studio, make sure the Project > Android pane is open.
2. Expand Gradle Scripts and open the build.gradle (Module: app) file.

1. Locate the compileSdkVersion line near the top of the file. For example:

compileSdkVersion 26

Locate the minSdkVersion line in the defaultConfig section a few lines down.

Locate the targetSdkVersion line in the defaultConfig section. For example:

targetSdkVersion 26

Locate the dependencies section of build.gradle, near the end of the file. For example:

1. Update the compileSdkVersion number, if necessary
2. Update the version numbers, if necessary.

Task 2: Implement the layout and MainActivity

2.1 Change the layout and colors

1. Open activity_main.xml in the Project > Android pane.
2. Click the Design tab (if it is not already selected) to show the layout editor.
3. Select the “Hello World” TextView in the layout and open the Attributes pane.

Delete the constraint that stretches from the bottom of the hello_textview TextView to the bottom of the layout, so that the TextView snaps to the top of the layout, and choose 8 (8dp) for the top margin as shown below.

Drag a Button to the bottom of the layout, and add constraints to the left and right sides and bottom of the layout, as shown in the figure below.

1. Change the layout_width attribute in the Attributes pane for the Button to match_constraint.
2. Change the other attributes in the Attributes pane for the Button as follows:

Add the following android:onClick attribute to the Button:

android:onClick="changeColor"

To add colors, expand res and values in the Project > Android pane, and open colors.xml.

<color name="red">#F44336</color>
<color name="pink">#E91E63</color>
<color name="purple">#9C27B0</color>
<color name="deep_purple">#673AB7</color>
<color name="indigo">#3F51B5</color>
<color name="blue">#2196F3</color>
<color name="light_blue">#03A9F4</color>
<color name="cyan">#00BCD4</color>
<color name="teal">#009688</color>
<color name="green">#4CAF50</color>
<color name="light_green">#8BC34A</color>
<color name="lime">#CDDC39</color>
<color name="yellow">#FFEB3B</color>
<color name="amber">#FFC107</color>
<color name="orange">#FF9800</color>
<color name="deep_orange">#FF5722</color>
<color name="brown">#795548</color>
<color name="grey">#9E9E9E</color>
<color name="blue_grey">#607D8B</color>
<color name="black">#000000</color>

2.2 Add behavior to MainActivity

Open MainActivity.

Add a private variable at the top of the class to hold the TextView object.

private TextView mHelloTextView;

Add the following color array just after the private variable:

private String[] mColorArray = {"red", "pink", "purple", "deep_purple",
          "indigo", "blue", "light_blue", "cyan", "teal", "green", 
          "light_green", "lime", "yellow", "amber", "orange", "deep_orange",
          "brown", "grey", "blue_grey", "black" };

Each color name corresponds to the name of a color resource in color.xml.In the onCreate() method, use findViewById() to get a reference to the TextView instance and assign it to that private variable:

mHelloTextView = findViewById(R.id.hello_textview);

Also in onCreate(), restore the saved instance state, if any:

// restore saved instance state (the text color)
if (savedInstanceState != null) {
    mHelloTextView.setTextColor(savedInstanceState.getInt("color"));
}

Add the onSaveInstanceState() method to MainActivity to save the text color:

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    // save the current text color
    outState.putInt("color", mHelloTextView.getCurrentTextColor());
}

he final code would be like this :

Task 3: Implement Button behavior

2.1 Add the changeButton() click handler

1. Open activity_main.xml, if it is not already open. Click the Text tab to show the XML code.
2. Click on "changeColor" in the android:onClick attribute inside the Button element.
3. Press Alt+Enter (Option+Enter on a Mac), and select Create onClick event handler.
4. Choose MainActivity and click OK.

2.2 Implement the Button action

1. Switch to MainActivity.
2. In the changeColor() method, create a random number object by using the Random class (a Java class) to generate simple random numbers.

Random random = new Random();

Use the random instance to pick a random color from the mColorArray array:

String colorName = mColorArray[random.nextInt(20)];

The nextInt() method with the argument 20 gets another random integer between 0 and 19. You use that integer as the index of the array to get a color name.

Get the resource identifier (an integer) for the color name from the resources:

int colorResourceName = getResources().getIdentifier(colorName,
          "color", getApplicationContext().getPackageName());

Get the integer ID for the actual color from the resources and assign it to a colorRes variable, and use the getTheme() method to get the theme for the current application context.

int colorRes = 
   getResources().getColor(colorResourceName, this.getTheme());

Change the colorRes assignment line to use the ContextCompat class:

int colorRes = ContextCompat.getColor(this, colorResourceName);

Set the color of the TextView to the color resource ID:

mHelloTextView.setTextColor(colorRes);

Run the app on a device or emulator, and click the Change Color Button.

The final code would be like this :

Run :

yeah :))