ตัวอย่างการใช้งาน TextWatcher บน Android

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

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editText.addTextChangedListener(myTextWatcher);
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public TextWatcher myTextWatcher = new TextWatcher() {
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    @Override
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    public void onTextChanged(CharSequence s, int start, int before, int count) {
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    }
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    @Override
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    public void beforeTextChanged(CharSequence s, int start, int count,
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            int after) {
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    }
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    @Override
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    public void afterTextChanged(Editable s) {
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    }
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};

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<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
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    xmlns:tools="http://schemas.android.com/tools"
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    android:layout_width="match_parent"
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    android:layout_height="match_parent"
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    android:paddingBottom="@dimen/activity_vertical_margin"
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    android:paddingLeft="@dimen/activity_horizontal_margin"
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    android:paddingRight="@dimen/activity_horizontal_margin"
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    android:paddingTop="@dimen/activity_vertical_margin">
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    <EditText
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        android:id="@+id/editMessage"
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        android:layout_width="match_parent"
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        android:layout_height="wrap_content"
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        android:layout_alignParentLeft="true"
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        android:layout_alignParentTop="true"
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        android:layout_marginTop="21dp"
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        android:ems="10"
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        android:gravity="top"
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        android:maxLength="120"
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        android:hint="put some word.."
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        android:inputType="textMultiLine" />
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    <TextView
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        android:id="@+id/textCount"
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        android:layout_width="wrap_content"
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        android:layout_height="wrap_content"
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        android:layout_alignRight="@+id/editMessage"
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        android:textColor="#00ff00"
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        android:layout_below="@+id/editMessage"
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        android:layout_marginTop="26dp"
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        android:text="120" />
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</RelativeLayout>

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package com.devahoy.tutorial;
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import android.app.Activity;
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import android.graphics.Color;
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import android.os.Bundle;
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import android.text.Editable;
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import android.text.TextWatcher;
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import android.widget.EditText;
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import android.widget.TextView;
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public class MainActivity extends Activity {
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    private EditText mEditMessage;
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    private TextView mTextCount;
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    @Override
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    public void onCreate(Bundle savedInstanceState) {
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        super.onCreate(savedInstanceState);
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        setContentView(R.layout.activity_main);
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        mEditMessage  = (EditText) findViewById(R.id.editMessage);
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        mTextCount = (TextView) findViewById(R.id.textCount);
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        mEditMessage.addTextChangedListener(myTextWatcher);
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    }
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    public TextWatcher myTextWatcher = new TextWatcher() {
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        @Override
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        public void onTextChanged(CharSequence s, int start, int before, int count) {
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        }
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        @Override
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        public void beforeTextChanged(CharSequence s, int start, int count,
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                int after) {
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        }
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        @Override
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        public void afterTextChanged(Editable s) {
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            int count = 120 - mEditMessage.length();
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            mTextCount.setText(String.valueOf(count));
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            mTextCount.setTextColor(Color.GREEN);
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            if (count < 20) {
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                mTextCount.setTextColor(Color.RED);
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            }
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        }
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    };
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}