Android Starting New Activity Without Registering Receiver Again

When you complete the previous lesson, y'all take an app that shows an activeness that consists of a single screen with a text field and a Send push button. In this lesson, you add some code to the MainActivity that starts a new activity to display a message when the user taps the Transport button.

Reply to the Ship push

Follow these steps to add together a method to the MainActivity class that's called when the Send button is tapped:

1. In the file app > java > com.case.myfirstapp > MainActivity, add together the following sendMessage() method stub:

   Kotlin

   class MainActivity : AppCompatActivity() {     override fun onCreate(savedInstanceState: Bundle?) {         super.onCreate(savedInstanceState)         setContentView(R.layout.activity_main)     }                  /** Chosen when the user taps the Ship push */     fun sendMessage(view: View) {         // Practice something in response to push button     }                  }                

   Java

   public course MainActivity extends AppCompatActivity {     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }                  /** Called when the user taps the Send push button */     public void sendMessage(View view) {         // Do something in response to button     }                  }                

   You might see an error because Android Studio cannot resolve the View class used as the method argument. To clear the error, click the View proclamation, identify your cursor on information technology, and then press Alt+Enter, or Option+Enter on a Mac, to perform a Quick Set up. If a menu appears, select Import class.

2. Render to the activity_main.xml file to call the method from the button:
   1. Select the button in the Layout Editor.
   2. In the Attributes window, locate the onClick belongings and select sendMessage [MainActivity] from its drop-down list.

   Now when the button is tapped, the organisation calls the sendMessage() method.

   Accept note of the details in this method. They're required for the organisation to recognize the method as compatible with the android:onClick aspect. Specifically, the method has the post-obit characteristics:

   • Public admission.
   • A void or, in Kotlin, an implicit unit of measurement return value.
   • A View every bit the only parameter. This is the View object yous clicked at the cease of Step 1.
3. Adjacent, fill in this method to read the contents of the text field and deliver that text to some other action.

Build an intent

An Intent is an object that provides runtime binding betwixt split up components, such equally two activities. The Intent represents an app'south intent to exercise something. Y'all can employ intents for a wide variety of tasks, but in this lesson, your intent starts another activity.

In MainActivity, add the EXTRA_MESSAGE constant and the sendMessage() code, every bit shown:

Kotlin

              const val EXTRA_MESSAGE = "com.instance.myfirstapp.Message"              class MainActivity : AppCompatActivity() {     override fun onCreate(savedInstanceState: Bundle?) {         super.onCreate(savedInstanceState)         setContentView(R.layout.activity_main)     }      /** Called when the user taps the Ship button */     fun sendMessage(view: View) {              val editText = findViewById<EditText>(R.id.editTextTextPersonName)         val bulletin = editText.text.toString()         val intent = Intent(this, DisplayMessageActivity::grade.java).apply {             putExtra(EXTRA_MESSAGE, message)         }         startActivity(intent)              } }            

Java

public class MainActivity extends AppCompatActivity {              public static final String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";              @Override     protected void onCreate(Package savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }      /** Called when the user taps the Send push */     public void sendMessage(View view) {              Intent intent = new Intent(this, DisplayMessageActivity.class);         EditText editText = (EditText) findViewById(R.id.editTextTextPersonName);         String message = editText.getText().toString();         intent.putExtra(EXTRA_MESSAGE, message);         startActivity(intent);              } }            

Look Android Studio to run into Cannot resolve symbol errors once more. To clear the errors, press Alt+Enter, or Option+Return on a Mac. You should end upwards with the following imports:

Kotlin

import androidx.appcompat.app.AppCompatActivity import android.content.Intent import android.os.Bundle import android.view.View import android.widget.EditText            

Java

import androidx.appcompat.app.AppCompatActivity; import android.content.Intent; import android.bone.Packet; import android.view.View; import android.widget.EditText;            

An fault withal remains for DisplayMessageActivity, but that'south okay. Y'all gear up information technology in the next section.

Here's what's going on in sendMessage():

• The Intent constructor takes two parameters, a Context and a Course.

  The Context parameter is used first because the Activeness class is a subclass of Context.

  The Class parameter of the app component, to which the system delivers the Intent, is, in this case, the activity to start.

• The putExtra() method adds the value of EditText to the intent. An Intent tin bear data types every bit primal-value pairs called extras.

  Your key is a public abiding EXTRA_MESSAGE considering the next activeness uses the key to call up the text value. It'southward a adept practice to ascertain keys for intent extras with your app's parcel name as a prefix. This ensures that the keys are unique, in example your app interacts with other apps.

• The startActivity() method starts an example of the DisplayMessageActivity that's specified past the Intent. Side by side, yous need to create that grade.

Create the second activity

To create the second activity, follow these steps:

1. In the Project window, correct-click the app binder and select New > Activity > Empty Activity.
2. In the Configure Action window, enter "DisplayMessageActivity" for Activity Proper noun. Go out all other properties set to their defaults and click Stop.

Android Studio automatically does iii things:

• Creates the DisplayMessageActivity file.
• Creates the layout file activity_display_message.xml, which corresponds with the DisplayMessageActivity file.
• Adds the required <activity> element in AndroidManifest.xml.

If you run the app and tap the push button on the starting time activity, the 2d activity starts merely is empty. This is because the second activity uses the empty layout provided by the template.

Add together a text view

Figure ane. The text view centered at the top of the layout.

The new activeness includes a blank layout file. Follow these steps to add a text view to where the message appears:

1. Open up the file app > res > layout > activity_display_message.xml.
2. Click Enable Autoconnection to Parent in the toolbar. This enables Autoconnect. See figure ane.
3. In the Palette panel, click Text, drag a TextView into the layout, and drib it about the top-center of the layout so that it snaps to the vertical line that appears. Autoconnect adds left and right constraints in order to identify the view in the horizontal center.
4. Create 1 more constraint from the top of the text view to the top of the layout, so that it appears as shown in effigy ane.

Optionally, you can make some adjustments to the text style if you expand textAppearance in the Mutual Attributes panel of the Attributes window, and change attributes such as textSize and textColor.

Display the message

In this stride, you modify the second activity to brandish the message that was passed by the start activity.

1. In DisplayMessageActivity, add the following code to the onCreate() method:

   Kotlin

   override fun onCreate(savedInstanceState: Bundle?) {     super.onCreate(savedInstanceState)     setContentView(R.layout.activity_display_message)                                      // Get the Intent that started this activeness and extract the string     val bulletin = intent.getStringExtra(EXTRA_MESSAGE)      // Capture the layout's TextView and set up the cord as its text     val textView = findViewById<TextView>(R.id.textView).utilize {         text = bulletin     }                  }                

   Java

   @Override protected void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);     setContentView(R.layout.activity_display_message);                                      // Go the Intent that started this activity and extract the string     Intent intent = getIntent();     String bulletin = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);      // Capture the layout's TextView and set the string equally its text     TextView textView = findViewById(R.id.textView);     textView.setText(message);                  }                

2. Press Alt+Enter, or Choice+Return on a Mac, to import these other needed classes:

   Kotlin

   import androidx.appcompat.app.AppCompatActivity import android.content.Intent import android.os.Packet import android.widget.TextView                

   Coffee

   import androidx.appcompat.app.AppCompatActivity; import android.content.Intent; import android.os.Bundle; import android.widget.TextView;                

Add upward navigation

Each screen in your app that's non the master entry betoken, which are all the screens that aren't the dwelling screen, must provide navigation that directs the user to the logical parent screen in the app'due south hierarchy. To practice this, add an Up button in the app bar.

To add together an Up button, you need to declare which activity is the logical parent in the AndroidManifest.xml file. Open the file at app > manifests > AndroidManifest.xml, locate the <action> tag for DisplayMessageActivity, and replace it with the following:

<activity android:name=".DisplayMessageActivity"           android:parentActivityName=".MainActivity">     <!-- The meta-information tag is required if you support API level fifteen and lower -->     <meta-information         android:name="android.back up.PARENT_ACTIVITY"         android:value=".MainActivity" /> </action>        

The Android system now automatically adds the Upward push to the app bar.

Run the app

Click Utilize Changes in the toolbar to run the app. When information technology opens, blazon a bulletin in the text field and tap Send to see the message announced in the second activity.

Figure ii. App opened, with text entered on the left screen and displayed on the right.

That'south it, you've built your first Android app!

To continue to learn the basics nigh Android app development, go back to Build your first app and follow the other links provided at that place.

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Source: https://developer.android.com/training/basics/firstapp/starting-activity

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