Android App Development

Display the message Now you will modify the second activity to display the message that was passed by the first activity. In  DisplayMessageActivity.java , add the following code to the  onCreate()  method: @Override protected void onCreate ( Bundle savedInstanceState ) {     super . onCreate ( savedInstanceState );     setContentView ( R . layout . activity_display_message );         // Get the Intent that started this activity and extract the string     Intent intent = getIntent ();     String message = intent . getStringExtra ( MainActivity . EXTRA_MESSAGE );     // Capture the layout's TextView and set the string as its text     TextView textView = ( TextView ) findViewById ( R . id . textView );     textView . setText ( message ); } Press Alt + Enter (or Option + Return on Mac) to import missing classes. Your imports should end up as the following: import android . content . Intent ; import android . support . v7 . app . AppCompatActivity ;

Create the second activity In the  Project  window, right-click the  app  folder and select  New > Activity > Empty Activity . In the  Configure Activity  window, enter "DisplayMessageActivity" for  Activity Name  and click  Finish  (leave all other properties set to the defaults). Android Studio automatically does three things: Creates the  DisplayMessageActivity.java  file. Creates the corresponding  activity_display_message.xml  layout file. Adds the required  <activity>  element in  AndroidManifest.xml . If you run the app and tap the button on the first activity, the second activity starts but is empty. This is because the second activity uses the empty layout provided by the template. Add a text view Figure 1.  The text view centered at the top of the layout The new activity includes a blank layout file, so now you'll add a text view where the message will appear. Open the file  app > res > layout > activity_display_m

Start Another Activity After completing the previous lesson, you have an app that shows an activity (a single screen) with a text field and a button. In this lesson, you’ll add some code to  MainActivity that starts a new activity to display the message when the user taps  Send . Note:  This lesson expects you are using Android Studio 2.3 or higher. Respond to the send button Add a method in  MainActivity.java  that's called by the button as follows: In the file  app > java > com.example.myfirstapp > MainActivity.java , add the  sendMessage()  method stub as shown below: public class MainActivity extends AppCompatActivity {     @Override     protected void onCreate ( Bundle savedInstanceState ) {         super . onCreate ( savedInstanceState );         setContentView ( R . layout . activity_main );     }     /** Called when the user taps the Send button */     public void sendMessage ( View view ) {         // Do something in response to b

Make the text box size flexible To create a layout that's responsive to different screen sizes, you'll now make the text box stretch to fill all remaining horizontal space (after accounting for the button and margins). Figure 8.  Click to change the width and remove the right margin Figure 9.  The text box stretches to fill the remaining space Create a constraint from the right side of the button to the right side of the parent layout. This now defines the total width that the two views have available (which you can now fill with the text box). Add a constraint from the right side of the text box to the left side of the button. It might look like that's already there, but you're actually adding a bidirectional constraint between the two views. So both views are constrained to each other. This is called a  chain  (as indicated by the chain betwen the views), and it enables some extra layout options. Open the  Properties  window for the text box and the

Change the UI strings Click  Show Design   in the toolbar to preview the UI. Notice that the text input is pre-filled with "Name" and the button is labeled "Button." So now you'll change these strings. Open the  Project  window and then select  res > values > strings.xml . This is a string resources file where you should specify all your UI strings. Doing so allows you to manage all UI strings in a single location, which makes it easier to find, update, and localize (compared to hard-coding strings in your layout or app code). Click  Open editor  at the top of the editor window. This opens the Translations Editor, which provides a simple interface for adding and editing your default strings, and helps keep all your translated strings organized. Figure 7.  The dialog to add a new string Click  Add Key   to create a new string as the "hint text" for the text box. Enter "edit_message" for the key name. Enter "Enter a

Add a text box Figure 5.  The text box is constrained to the top and left of the parent layout First, you need to remove what's already in the layout. So click  TextView  in the  Component Tree  window, and then press Delete. From the  Palette  window on the left, click  Text  in the left pane, and then drag  Plain Text  into the design editor and drop it near the top of the layout. This is an  EditText  widget that accepts plain text input. Click the view in the design editor. You can now see the resizing handles on each corner (squares), and the constraint anchors on each side (circles). For better control, you might want to zoom in on the editor to 75% or higher using the buttons in the toolbar. Click-and-hold the anchor on the top side, and then drag it up until it snaps to the top of the layout and release. That's a constraint—it specifies the view should be 16dp from the top of the layout (because you set the default margins to 16dp). Similarly, create a c