Load Activity Again and Kill Old Activity Android

When you complete the previous lesson, you accept an app that shows an activity that consists of a unmarried 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 bulletin when the user taps the Send button.

Respond to the Send button

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

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

   Kotlin

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

   Java

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

   You might see an error because Android Studio cannot resolve the View course used as the method statement. To clear the mistake, click the View declaration, identify your cursor on it, and then press Alt+Enter, or Pick+Enter on a Mac, to perform a Quick Fix. If a menu appears, select Import grade.

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 property and select sendMessage [MainActivity] from its driblet-downwards 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 system to recognize the method as compatible with the android:onClick aspect. Specifically, the method has the following characteristics:

   • Public access.
   • A void or, in Kotlin, an implicit unit return value.
   • A View as the only parameter. This is the View object you clicked at the end of Stride 1.
3. Side by side, fill in this method to read the contents of the text field and evangelize that text to another activity.

Build an intent

An Intent is an object that provides runtime bounden between split components, such as two activities. The Intent represents an app's intent to do something. You can employ intents for a wide variety of tasks, just in this lesson, your intent starts another activity.

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

Kotlin

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

Java

public class MainActivity extends AppCompatActivity {              public static terminal String EXTRA_MESSAGE = "com.example.myfirstapp.Message";              @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) {              Intent intent = new Intent(this, DisplayMessageActivity.class);         EditText editText = (EditText) findViewById(R.id.editTextTextPersonName);         Cord bulletin = editText.getText().toString();         intent.putExtra(EXTRA_MESSAGE, message);         startActivity(intent);              } }            

Expect Android Studio to encounter Cannot resolve symbol errors again. To articulate the errors, press Alt+Enter, or Option+Return on a Mac. You should terminate upwardly with the following imports:

Kotlin

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

Java

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

An error withal remains for DisplayMessageActivity, but that's okay. You ready information technology in the next department.

Hither's what'southward going on in sendMessage():

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

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

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

• The putExtra() method adds the value of EditText to the intent. An Intent can conduct data types as cardinal-value pairs called extras.

  Your central is a public constant EXTRA_MESSAGE considering the adjacent activity uses the key to call back the text value. Information technology's a practiced practice to ascertain keys for intent extras with your app'south package name as a prefix. This ensures that the keys are unique, in case your app interacts with other apps.

• The startActivity() method starts an instance of the DisplayMessageActivity that's specified by the Intent. Next, yous demand to create that class.

Create the second activeness

To create the 2nd activity, follow these steps:

1. In the Projection window, right-click the app folder and select New > Activity > Empty Activity.
2. In the Configure Activity window, enter "DisplayMessageActivity" for Activity Proper name. Get out all other properties set to their defaults and click Finish.

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 y'all run the app and tap the push on the first activity, the second activity starts but is empty. This is considering the second activeness uses the empty layout provided past the template.

Add together a text view

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

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

1. Open the file app > res > layout > activity_display_message.xml.
2. Click Enable Autoconnection to Parent in the toolbar. This enables Autoconnect. See figure 1.
3. In the Palette panel, click Text, drag a TextView into the layout, and drop it nigh the summit-center of the layout so that it snaps to the vertical line that appears. Autoconnect adds left and right constraints in order to place the view in the horizontal center.
4. Create one more constraint from the pinnacle of the text view to the summit of the layout, so that it appears every bit shown in figure 1.

Optionally, you can make some adjustments to the text style if yous aggrandize textAppearance in the Common Attributes panel of the Attributes window, and modify attributes such as textSize and textColor.

Display the message

In this pace, yous modify the 2nd activity to display the message that was passed by the first activity.

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

   Kotlin

   override fun onCreate(savedInstanceState: Bundle?) {     super.onCreate(savedInstanceState)     setContentView(R.layout.activity_display_message)                                      // Go the Intent that started this activity and excerpt the string     val bulletin = intent.getStringExtra(EXTRA_MESSAGE)      // Capture the layout's TextView and set the string every bit its text     val textView = findViewById<TextView>(R.id.textView).apply {         text = message     }                  }                

   Java

   @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 cord     Intent intent = getIntent();     String bulletin = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);      // Capture the layout's TextView and set the string as its text     TextView textView = findViewById(R.id.textView);     textView.setText(message);                  }                

2. Printing Alt+Enter, or Option+Render on a Mac, to import these other needed classes:

   Kotlin

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

   Java

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

Add together upward navigation

Each screen in your app that's not the chief entry point, which are all the screens that aren't the home screen, must provide navigation that directs the user to the logical parent screen in the app's hierarchy. To do this, add together an Up button in the app bar.

To add an Up push, 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 <activity> tag for DisplayMessageActivity, and replace information technology with the following:

<activeness android:name=".DisplayMessageActivity"           android:parentActivityName=".MainActivity">     <!-- The meta-data tag is required if you back up API level 15 and lower -->     <meta-data         android:proper noun="android.support.PARENT_ACTIVITY"         android:value=".MainActivity" /> </activeness>        

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

Run the app

Click Apply Changes in the toolbar to run the app. When it opens, type a message in the text field and tap Ship to see the message appear in the second activity.

Figure 2. App opened, with text entered on the left screen and displayed on the right.

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

To continue to learn the nuts about Android app evolution, get dorsum to Build your first app and follow the other links provided at that place.

tejadawaithe.blogspot.com

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

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