How to Monitor TextView Changes in Android

Some form-based applications need to continuously monitor for text changes to perform the front-end form validations. This can be achieved by implementing the android.text.TextWatcher interface. The TextWatcher interface provides the following three callback methods, that are called while the text view is being updated.

public void beforeTextChanged(CharSequence s, int start, int count, int after);
public void onTextChanged(CharSequence s, int start, int before, int count);
public void afterTextChanged(Editable s);

The beforeTextChanged() and onTextChanged() methods are provided mainly as notifications, as shouldn’t make changes to the CharSequence in either of these methods. If you are attempting to intercept the text entered into the view, changes may be made when afterTextChanged() is called.

First, you need to register a TextWatcher instance with a TextView and call the textView.addTextChangedListener() method. Notice from the syntax that more than one TextWatcher can be registered with a TextView. Now that we understand the methods to be used, let us examine the sample for usage.

Count Characters on TextView Changes

A simple use of TextWatcher is to create a live character counter that follows an EditText as the user types or deletes information.

public class MyActivity extends Activity implements TextWatcher {
    private EditText text;
    private int textCount;
    private static final String TAG = "MyActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //Create an EditText widget and add the watcher
        editText = new EditText(this);
        editText.addTextChangedListener(this);
        setContentView(editText);
    }

    /* TextWatcher Implementation Methods */
    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {
        textCount = editText.getText().length();
        setTitle(String.valueOf(textCount));
        Log.e(TAG, String.valueOf(textCount))
    }

    @Override
    public void afterTextChanged(Editable s) {

    }
}

Because our needs do not include modifying the text being inserted, we can read the count, which happens as soon as the text change occurs. The other methods are unused and left empty.