Mobile Application Development

• These notes don’t include Android Studio tutorials as:
  1. Graphical tutorials are a lot of effort
  2. Android Studio keeps evolving so it’s going to be different when you’re reading this.
• Unlike most of my other notes, these should be used as complementary material. I really don’t think these notes can replace lectures or a course itself.

• This code follows from this video.

• First declare private variables in the MainActivity class:

  private EditText num1, num2;
  private Button add;
  private TextView result;
  

  Make sure you do alt + enter when your mouse is over the EditText, Button or TextView datatype, to resolve all imports.

• Next, in the end of onCreate() function, get all of views using their IDs R.id.idName, and then assign them to the private variables you created:

  num1 = (EditText) findViewById(R.id.no1);
  num2 = (EditText) findViewById(R.id.no2);
  result = (TextView) findViewById(R.id.result);
  add = (Button) findViewById(R.id.addButton);
  

  Before assigning, make sure you appropriately typecast the View.

• To the button object add, use the setOnClickListener() method as add.setOnClickListener();. Inside, add a parameter that starts from new View and just pick the first suggestion (it’s most likely what we need). So the function will look like this:

  add.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          // here is where we do the logic
      }
  });
  
  

• Use the setText method on the result TextView. It can only take a String, so typecast whatever you want to show using String.valueOf().

  add.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          result.setText(String.valueOf(
                  Integer.parseInt(num1.getText().toString())
                  + Integer.parseInt(num2.getText().toString())
          ));
      }
  });
  
  

  Just try typing whatever is inside, all you need to do is tab your way through.

• The complete app looks like this:

  public class MainActivity extends AppCompatActivity {
      // declare these variables
      private EditText num1, num2;
      private Button add;
      private TextView result;
   
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          // some code already here, just add the below code
       
          num1 = (EditText) findViewById(R.id.no1);
          num2 = (EditText) findViewById(R.id.no2);
          result = (TextView) findViewById(R.id.result);
          add = (Button) findViewById(R.id.addButton);
          add.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  result.setText(String.valueOf(
                          Integer.parseInt(num1.getText().toString())
                          + Integer.parseInt(num2.getText().toString())
                  ));
              }
          });
       
      }
  }
  

• An Intent is an object used to request an action from another app component via the Android system.
• It’s a message to the Android runtime to run an Activity.

• ScrollView makes a single child under it, vertically scrollable. For horizontal scrolling you’ll have to use HorizontalScrollView.
• When you use scroll view, drag and drop a linear layout into the component tree, and then put whatever you want in there.
• Drag and drop only into the component tree (bottom left corner, basically under palette), because all Views should be under the linear layout.

• Just add Spinner into the XML, and then in the onCreate() function:

  List<String> spinnerList = new ArrayList<>();
  spinnerList.add("Item 1");
  spinnerList.add("Item 2");
  spinnerList.add("Item 3");
  spinnerList.add("Item 4");
  
  Spinner spinner = findViewById(R.id.spinner);
  
  ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, spinnerList);
  adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
  spinner.setAdapter(adapter);
  

• Make scroll down/ context menu item
• Make a ’check balance’ option

• A Service is an Android component that runs in the background and has no user interface. It’s designed for long-running operations.
• A Service runs on the main thread.

• A Service is started using an Intent:

  Intent intent = new Intent(this, MyService.class);
  startService(intent);  // starts the service
  stopService(intent);         // from outside the service
  // OR
  stopSelf();                  // from inside the service itself
  

• A BroadcastReceiver is a component in your Android app that “listens” for broadcasts and reacts to them. It’s a class that you create by extending Android’s BroadcastReceiver base class and overriding one method: onReceive().

• There are two types:

  Static	Dynamic
  Declared inside AndroidManifest.xml	Declared programmatically
  Works even when the app is completely closed (not even running in the background)	Works only when the app is open or minimized

• Here’s an example: The receiver:

  public class BatteryLowReceiver extends BroadcastReceiver {
      @Override
      public void onReceive(Context context, Intent intent) { 
          if (intent.getAction().equals(Intent.ACTION_BATTERY_LOW)) {
              Toast.makeText(context, "Battery is Low", Toast.LENGTH_SHORT).show();
          }
      }
  }
  

  In MainActivity:

  package com.example.broadcastreceiver_airplanemode;
  
  import androidx.appcompat.app.AppCompatActivity;
  import android.content.Intent;
  import android.content.IntentFilter;
  import android.os.Bundle;
  
  public class MainActivity extends AppCompatActivity {
      BatteryLowReceiver batteryLowReceiver = new BatteryLowReceiver(); // Make receiver object
   
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
      }
   
      @Override
      protected void onStart() {
          super.onStart();
          IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_LOW); // listen to only these intents
          registerReceiver(batteryLowReceiver, filter);                      // pass receiver object and the filter object
      }
   
      @Override
      protected void onStop() {
          super.onStop();
          unregisterReceiver(batteryLowReceiver);
      }
  }
  

You can make it listen to custom Intents from within the app too. Instead of Intent.ACTION_BATTERY_LOW, you’d have com.packageName.broadcastreceiever.someaction.

• sendBroadcast(intent) is asynchronous and random, while sendOrderedBroadcast(intent, receiverPermission) is synchronous and ordered. In sendOrderedBroadcast(), you can abort by abortBroadcast() and control the order by android:priority in the manifest.

In AndroidManifest.xml:

<!-- Required to make any network request -->
<uses-permission android:name="android.permission.INTERNET" />

<!-- Required to check whether you're currently connected before making a call -->
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />