ตัวอย่างการใช้งาน TextWatcher บน Android

editText.addTextChangedListener(new TextWatcher() {...});

editText.addTextChangedListener(myTextWatcher);

public TextWatcher myTextWatcher = new TextWatcher() {

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count,
            int after) {

    }

    @Override
    public void afterTextChanged(Editable s) {

    }
};

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin">

    <EditText
        android:id="@+id/editMessage"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="21dp"
        android:ems="10"
        android:gravity="top"
        android:maxLength="120"
        android:hint="put some word.."
        android:inputType="textMultiLine" />

    <TextView
        android:id="@+id/textCount"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignRight="@+id/editMessage"
        android:textColor="#00ff00"
        android:layout_below="@+id/editMessage"
        android:layout_marginTop="26dp"
        android:text="120" />

</RelativeLayout>

package com.devahoy.tutorial;

import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity {

    private EditText mEditMessage;
    private TextView mTextCount;

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

        mEditMessage  = (EditText) findViewById(R.id.editMessage);
        mTextCount = (TextView) findViewById(R.id.textCount);

        mEditMessage.addTextChangedListener(myTextWatcher);

    }

    public TextWatcher myTextWatcher = new TextWatcher() {

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {

        }

        @Override
        public void beforeTextChanged(CharSequence s, int start, int count,
                int after) {

        }

        @Override
        public void afterTextChanged(Editable s) {
            int count = 120 - mEditMessage.length();

            mTextCount.setText(String.valueOf(count));

            mTextCount.setTextColor(Color.GREEN);

            if (count < 20) {
                mTextCount.setTextColor(Color.RED);
            }
        }
    };

}