Android Studio: Memulai Aktifitas Lain

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

Setelah menyelesaikan bagian sebelumnya, Anda memiliki sebuah aplikasi yang menunjukkan suatu kegiatan (satu layar) dengan field text dan tombol. Dalam bagian ini, anda akan menambahkan beberapa kode untuk MyActivity yang memulai activity baru ketika pengguna mengklik tombol Send.

Respond ke tombol Send

• Di Android Studio, dari res/layout directory, edit file activity_my.xml.
• Ke elemen <Button> , tambahkan atribut android:onClick.

res/layout/activity_my.xml

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage" />

• Isi atribut android:onClick , "sendMessage", adalah nama dari method dalam activity kita yang akan di panggil oleh sistem saat user klik tombol.
• Dalam directory java/com.mycompany.myfirstapp , buka file MyActivity.java.
• Dalam class MyActivity, tambahkan method sendMessage() stub seperti tampak di bawah ini,

java/com.mycompany.myfirstapp/MyActivity.java

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    // Do something in response to button
}

Agar sistem dapat mencocokan method di atas dengan nama method yang di berikan di android:onClick, signature harus persis seperti yang di perlihatkan. Terutama, method harus :

• public
• nilai return yang berisi void
• mempunyai View sebagai satu-satunya parameter (ini karena View yang akan di klik)

Selanjutnya, kita akan coba method untuk membaca isi dari field text dan mengirimnya ke activity lainnya.

Build sebuah Intent

Sebuah Intent adalah obyek yang menyediakan runtime mengikat antara komponen yang terpisah (seperti dua activity). Intent merupakan "niat untuk melakukan sesuatu." Anda dapat menggunakan Intent untuk berbagai macam tugas, tetapi paling sering mereka digunakan untuk memulai activity lain. Untuk informasi lebih lanjut, lihat Intent and Intent Filter.

Di MyActivity.java, dalam method sendMessage() , buat sebuah Intent untuk memulai sebuah activity denga nama DisplayMessageActivity menggunakan code berikut :

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
}

Catatan: referensi ke DisplayMessageActivity akan menyebabkan error jika kita menggunakan IDE seperti Android Studio karena class tersebut tidak ada. Biarkan error tersebut untuk sementara waktu, kita akan membuat class tersebut secepatnya.

Constructor yang kita gunakan disini akan mengambil dua parameter:

• Sebuah Context sebagai parameter pertamanya (ini digunakan karena class Activity adalah subclass dari Context)
• Class dari komponen app yang oleh sistem harus menjalankan Intent (dalam hal ini, activity yang harus di jalankan)

Android Studio akan mengindikasikan bahwa kita harus mengimport class Intent.

• Di paling atas file, import Intent class:

java/com.mycompany.myfirstapp/MyActivity.java

import android.content.Intent;

Tip: di Android Studio, press Alt + Enter untuk import class yang hilang / tidak ada.

• Di dalam method sendMessage() , gunakan findViewById() untuk memperoleh elemen EditText .

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
  EditText editText = (EditText) findViewById(R.id.edit_message);
}

• Di ujung atas file, import class EditText . Di Android Studio, tekan Alt + Enter untuk import class yang tidak ada.

• Assign the text to a local message variable, and use the putExtra() method to add its text value to the intent.

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
  EditText editText = (EditText) findViewById(R.id.edit_message);
  String message = editText.getText().toString();
  intent.putExtra(EXTRA_MESSAGE, message);
}

An Intent can carry data types as key-value pairs called extras. The putExtra() method takes the key name in the first parameter and the value in the second parameter.

• At the top of the MyActivity class, add the EXTRA_MESSAGE definition as follows:

java/com.mycompany.myfirstapp/MyActivity.java

public class MyActivity extends ActionBarActivity {
    public final static String EXTRA_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";
    ...
}

For the next activity to query the extra data, you should define the key for your intent's extra using a public constant. It's generally 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.

• In the sendMessage() method, to finish the intent, call the startActivity() method, passing it the Intent object created in step 1.

With this new code, the complete sendMessage() method that's invoked by the Send button now looks like this:

java/com.mycompany.myfirstapp/MyActivity.java

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}

The system receives this call and starts an instance of the Activity specified by the Intent. Now you need to create the DisplayMessageActivity class in order for this to work.

Create the Second Activity

All subclasses of Activity must implement the onCreate() method. This method is where the activity receives the intent with the message, then renders the message. Also, the onCreate() method must define the activity layout with the setContentView() method. This is where the activity performs the initial setup of the activity components.

Create a new activity using Android Studio

Figure 1. The new activity wizard in Android Studio.

Android Studio includes a stub for the onCreate() method when you create a new activity.

   In Android Studio, in the java directory, select the package, com.mycompany.myfirstapp, right-click, and select New > Activity > Blank Activity.
   In the Choose options window, fill in the activity details:
       Activity Name: DisplayMessageActivity
       Layout Name: activity_display_message
       Title: My Message
       Hierarchical Parent: com.mycompany.myfirstapp.MyActivity
       Package name: com.mycompany.myfirstapp

   Click Finish.
   Open the DisplayMessageActivity.java file.

   The class already includes an implementation of the required onCreate() method. You will update the implementation of this method later. It also includes an implementation of onOptionsItemSelected(), which handles the action bar's Up behavior. Keep these two methods as they are for now.
   Remove the onCreateOptionsMenu() method.

   You won't need it for this app.

If you're developing with Android Studio, you can run the app now, but not much happens. Clicking the Send button starts the second activity, but it uses a default "Hello world" layout provided by the template. You'll soon update the activity to instead display a custom text view.

Create the activity without Android Studio

If you're using a different IDE or the command line tools, do the following:

   Create a new file named DisplayMessageActivity.java in the project's src/ directory, next to the original MyActivity.java file.
   Add the following code to the file:

   public class DisplayMessageActivity extends ActionBarActivity {

       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_display_message);

           if (savedInstanceState == null) {
               getSupportFragmentManager().beginTransaction()
                   .add(R.id.container, new PlaceholderFragment()).commit();
           }
       }

       @Override
       public boolean onOptionsItemSelected(MenuItem item) {
           // Handle action bar item clicks here. The action bar will
           // automatically handle clicks on the Home/Up button, so long
           // as you specify a parent activity in AndroidManifest.xml.
           int id = item.getItemId();
           if (id == R.id.action_settings) {
               return true;
           }
           return super.onOptionsItemSelected(item);
       }

       /**
        * A placeholder fragment containing a simple view.
        */
       public static class PlaceholderFragment extends Fragment {

           public PlaceholderFragment() { }

           @Override
           public View onCreateView(LayoutInflater inflater, ViewGroup container,
                     Bundle savedInstanceState) {
                 View rootView = inflater.inflate(R.layout.fragment_display_message,
                         container, false);
                 return rootView;
           }
       }
   }

   Note: If you are using an IDE other than Android Studio, your project does not contain the activity_display_message layout that's requested by setContentView(). That's OK because you will update this method later and won't be using that layout.
   To your strings.xml file, add the new activity's title as follows:

   <resources>
       ...
       <string name="title_activity_display_message">My Message</string>
   </resources>

   In your manifest file, AndroidManifest.xml, within the Application element, add the <activity> element for your DisplayMessageActivity class, as follows:

   <application ... >
       ...
       <activity
           android:name="com.mycompany.myfirstapp.DisplayMessageActivity"
           android:label="@string/title_activity_display_message"
           android:parentActivityName="com.mycompany.myfirstapp.MyActivity" >
           <meta-data
               android:name="android.support.PARENT_ACTIVITY"
               android:value="com.mycompany.myfirstapp.MyActivity" />
       </activity>
   </application>

The android:parentActivityName attribute declares the name of this activity's parent activity within the app's logical hierarchy. The system uses this value to implement default navigation behaviors, such as Up navigation on Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for older versions of Android by using the Support Library and adding the <meta-data> element as shown here.

Note: Your Android SDK should already include the latest Android Support Library, which you installed during the Adding SDK Packages step. When using the templates in Android Studio, the Support Library is automatically added to your app project (you can see the library's JAR file listed under Android Dependencies). If you're not using Android Studio, you need to manually add the library to your project—follow the guide for setting up the Support Library then return here.

If you're using a different IDE than Android Studio, don't worry that the app won't yet compile. You'll soon update the activity to display a custom text view.

Receive the Intent

Every Activity is invoked by an Intent, regardless of how the user navigated there. You can get the Intent that started your activity by calling getIntent() and retrieve the data contained within the intent.

   In the java/com.mycompany.myfirstapp directory, edit the DisplayMessageActivity.java file.
   In the onCreate() method, remove the following line:

     setContentView(R.layout.activity_display_message);

   Get the intent and assign it to a local variable.

   Intent intent = getIntent();

   At the top of the file, import the Intent class.

   In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
   Extract the message delivered by MyActivity with the getStringExtra() method.

   String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);

Display the Message

In the onCreate() method, create a TextView object.

TextView textView = new TextView(this);

Set the text size and message with setText().

textView.setTextSize(40);
textView.setText(message);

Then add the TextView as the root view of the activity’s layout by passing it to setContentView().

setContentView(textView);

   At the top of the file, import the TextView class.

   In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

The complete onCreate() method for DisplayMessageActivity now looks like this:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Get the message from the intent
    Intent intent = getIntent();
    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);

    // Create the text view
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);

    // Set the text view as the activity layout
    setContentView(textView);
}

You can now run the app. When it opens, type a message in the text field, click Send, and the message appears on the second activity.

Figure 2. Both activities in the final app, running on Android 4.4.

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

To learn more, follow the link below to the next class.

Referensi

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