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Start Another Activity
02.04.2017
08:41
Текстовые метки: Activity
Раздел: Android - Ресурсы - developer.android.com - Training

Start Another Activity

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This lesson teaches you to

  1. Respond to the send button
  2. Build an Intent
  3. Create the second activity
  4. Add a text view
  5. Display the message
  6. Add up navigation

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:

  1. In the file app > java > com.example.myfirstapp > MainActivity.java, add thesendMessage() method stub as shown below:
  2. 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 button
       
    }
    }

    You may see an error because Android Studio cannot resolve the View class used as the method argument. So click to place your cursor on the View declaration, and then perform a Quick Fix by pressing Alt + Enter (or Option + Enter on Mac). (If a menu appears, select Import class.)

  3. Now return to the activity_main.xml file to call this method from the button:
  1. Click to select the button in the Layout Editor.
  2. In the Properties window, locate the onClick property and select sendMessage [MainActivity] from the drop-down list.

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

Take note of the details in this method that are required in order for the system to recognize it as compatible with theandroid:onClick attribute. Specifically, the method must declare the following:

  • Public access
  • A void return value
  • View as the only parameter (it is the View object that was clicked)

Next, you’ll fill in this method to read the contents of the text field and deliver that text to another activity.

Build an Intent


An Intent is an object that provides runtime binding between separate components, such as two activities. The Intent represents an app’s "intent to do something." You can use intents for a wide variety of tasks, but in this lesson, your intent starts another activity.

In MainActivity.java, add the EXTRA_MESSAGE constant and the sendMessage() code, as shown here:

public class MainActivity extends AppCompatActivity {
   
public static final 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 button */
   
public void sendMessage(View view) {
       
Intent intent = new Intent(this, DisplayMessageActivity.class);
       
EditText editText = (EditText) findViewById(R.id.editText);
       
String message = editText.getText().toString();
        intent
.putExtra(EXTRA_MESSAGE, message);
        startActivity
(intent);
   
}
}

Android Studio will again encounter Cannot resolve symbol errors, so press Alt + Enter (or Option + Return on Mac). Your imports should end up as the following:

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

An error remains for DisplayMessageActivity, but that's okay; you'll fix that in the next section.

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

  • The Intent constructor takes two parameters:
    • Context as its first parameter (this is used because the Activity class is a subclass of Context)
    • The Class of the app component to which the system should deliver the Intent (in this case, the activity that should be started).
  • The putExtra() method adds the EditText's value to the intent. An Intent can carry data types as key-value pairs called extras. Your key is a public constant EXTRA_MESSAGE because the next activity uses the key to retrieve the text value. It's a good practice to define keys for intent extras using your app's package name as a prefix. This ensures the keys are unique, in case your app interacts with other apps.
  • The startActivity() method starts an instance of the DisplayMessageActivity specified by the Intent. Now you need to create that class.

Create the second activity


  1. In the Project window, right-click the app folder and select New > Activity > Empty Activity.
  2. 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.

  1. Open the file app > res > layout > activity_display_message.xml.
  2. Click Turn On Autoconnect  in the toolbar.
  3. From the Pallete window, drag a TextView into the layout and place it near the the top of the layout, near the center so it snaps to the vertical line that appear, and then drop it. Autoconnect adds constraints to place the view in the horizontal center.
  4. Create one more constraint from the top of the text view to the top of the layout, so it appears as shown in figure 1.

Optionally, make some adjustments to the text style by expanding textAppearance in the Properties window and change attributes such as textSize and textColor.

Display the message


Now you will modify the second activity to display the message that was passed by the first activity.

  1. In DisplayMessageActivity.java, add the following code to the onCreate() method:
  2. @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);
    }

  3. 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;
import android.os.Bundle;
import android.view.ViewGroup;
import android.widget.TextView;

Add up navigation


Each screen in your app that is not the main entrance (all screens that are not the "home" screen) should provide navigation so the user can return to the logical parent screen in the app hierarchy by tapping the Up button in the app bar.

All you need to do is declare which activity is the logical parent in the AndroidManifest.xml file. So open the file at app > Manifests > AndroidManifest.xml, locate the <activity> tag for DisplayMessageActivity and replace it with the following:

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

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

Run the app


Now run the app again by clicking Apply Changes  in the toolbar. When it opens, type a message in the text field, and tap Sendto see the message appear in the second activity.

Figure 2. Screenshots of both activities

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

To continue learning the basics about Android app development, follow the link below to the next class.

 
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