Callbacks in Android

Question

I am new to Android and programming as a whole and I need a little help with callbacks. I understand the gist of callbacks but I am unsure of how to go about implementing.

Context: I am writing a simple notetaking app that allows the user to write text and saving it to the app. The user can then request to read the file with a button. The text is then displayed on a textview in the main activity. There is an option to wipe this file and this is done with a confirmation pop up, which is another activity. This pop up contains 2 buttons, one to cancel and one to wipe. If the file is not empty it will wipe and does nothing if empty. I am not sure if this is the best way to implement it but I want to use the wipe button to callback to the main activity to clear the textview. The way I was thinking of was by using the callback to send a boolean value back. The main activity will check if the boolean is true and clear the textview if it is. I am unsure of how to implement the callback in my popup display to send this boolean value back to the main activity.

Code for main activity

public class MainActivity extends AppCompatActivity implements Popout.ClearTextView {

    Button bnRead,bnWrite,bnClear;
    TextView tvFileOP;
    EditText etInput;
    //    private static final String INPUT_CONTENT = "inputContent";
    public static final String  TV_CONTENT = "textViewContent";

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

        bnRead = (Button) findViewById(R.id.bnRead);
        bnWrite = (Button) findViewById(R.id.bnWrite);
        bnClear = (Button) findViewById(R.id.bnClear);

        tvFileOP = (TextView) findViewById(R.id.tvFileOP);
        etInput = (EditText) findViewById(R.id.etInput);
        tvFileOP.setMovementMethod(new ScrollingMovementMethod());

        final String fileName = "test_file";
        String data;

        bnRead.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                try {
                    FileInputStream fIn = openFileInput(fileName);
                    int c;
                    String temp = "";
                    while ( (c=fIn.read()) != -1){
                        temp = temp + Character.toString((char) c);
                    }
                    tvFileOP.setText(temp);
                    Toast.makeText(getBaseContext(),"file successfully read", Toast.LENGTH_LONG).show();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        bnWrite.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String data = etInput.getText().toString();
                try {
                    FileOutputStream fOut = openFileOutput(fileName,MODE_APPEND);
                    fOut.write(data.getBytes());
                    fOut.close();
                    etInput.setText("");
                    Toast.makeText(getBaseContext(),"file successfully written", Toast.LENGTH_LONG).show();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        bnClear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this,Popout.class));
            }
        });

    }

    @Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
        outState.putString(TV_CONTENT,tvFileOP.getText().toString());
        super.onSaveInstanceState(outState);
    }

    @Override
    protected void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        tvFileOP.setText(savedInstanceState.getString(TV_CONTENT));
    }


    @Override
    public void clearTextView(Boolean clear) {
        if (clear){
            tvFileOP.setText("");
        }
    }
}

Code for popup confirmation menu

public class Popout extends AppCompatActivity {

    Button bnClosepopup,bnWipe;
    TextView tvConfirmation;
    String fileName = "test_file";
    TextView tvFileOP;

    public interface ClearTextView {
        public void clearTextView(Boolean clear);
    }

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

        bnClosepopup = (Button) findViewById(R.id.bnClosepopup);
        bnWipe = (Button) findViewById(R.id.bnWipe);
        tvConfirmation = (TextView) findViewById(R.id.tvConfirmation);


        //HIDING THE TOOL BAR AT THE TOP OF THE SCREEN
        this.getSupportActionBar().hide();

        //GETTING THE SIZE OF THE SCREEN
        DisplayMetrics displayMetrics = new DisplayMetrics();
        getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
        int height = displayMetrics.heightPixels;
        int width = displayMetrics.widthPixels;
        getWindow().setLayout((int) (width*0.8) , (int) (0.8*height));

        bnClosepopup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });

        bnWipe.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    File dir = getFilesDir();
                    File file = new File(dir, fileName);
                    boolean deleted = file.delete();
                    Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();

                } catch (Exception e) {
                    Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
                }
                finish();

            }
        });
    }
}

I am very new to android development and any tips on how to improve my code would be greatly appreciated :)

user_8873644 · Answer

In this case there is no way to pass the interface to the other activity, because this is an activity to activity communication.

You have to use some other method, there is multiple ways to approach, the best way I can think of is to use startActivityForResult() to start the activity and then wait for a response to come back, and then query this response in the MainActivity by overriding the onActivityResult() method:

Example

In the MainActivity:

    //on click of this button
    bnClear.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            Intent intent = new Intent(MainActivity.this,Popout.class);
            int requestCode = 12; //it could be whatever you want
            startActivityForResult(intent , requestCode);

        }
    });


//override this method


@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);

//this is triggered when you finish the Popout Activity
if(requestCode == 12 && resultCode == Activity.RESULT_OK){

// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false); //false is default if no value exists

}


}

In the Popout activity:

    bnWipe.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            try {
                File dir = getFilesDir();
                File file = new File(dir, fileName);
                boolean deleted = file.delete();

                //send the result to onActivtyResult() in MainActivity
                Intent result = new Intent();
                result.putExtra("deleted_state", deleted ); 
                setResult(Activity.RESULT_OK, result);

                Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();

            } catch (Exception e) {
                Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
            }
            finish();

        }
    });

UPDATE:

It will be like this:

// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false);
    if (deleted){
        tvFileOP.setText("");
    }
..........

user_10097879 · Answer

As far as what if Understood your problem correctly: You want to control your 'Wipe' button click event from your activity. Here is the solution which may help you.

1: Make an overridden constructor of your dialog class.

2: Create one abstract method in the dialog class. (say - onWipeButtonClick) You need to make your dialog class abstract as well.

3: Inside on Click Listener of 'Wipe' button, call onWipeButtonClick abstract method.

4: Create the instance of dialog in the main activity where ever you want. The compiler will give you an error because you haven't implemented the call back method. do implement your onWipeButtonClick method and do needful for wipe data inside the method.

public abstract class WipeDialog extends Dialog{

private Context context;

  public WipeDialog(Context context){
   this.context = context;
  }

 public abstract void onWipeButtonClick(boolean isTextEmpty); 

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

    

    btnWipe.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            onWipeButtonClick();
        }
    });
 }
}

And now in Activity:

WipeDialog dialog = new WipeDialog(MainActivity.this) {
  @Override
  public void onWipeButtonClick(boolean isTextEmpty) {
      //Do Need full with respected to your requirement on click of button 'WIPE'
  }
};

Hope this will help. Thanks!

Callbacks in Android

2 Answers

Example