TextWatcher详解，限制输入字符，实现文本框输入框同步，TextView与EditText同步显示

（1）使用TextWathcer限制输入字符个数
布局中EditText在android布局中经常用到，对EditText中输入的内容也经常需要进行限制，我们可以通过TextWatcher去观察输入框中输入的内容来限制输入字符个数。

主布局：

<? xml version="1.0" encoding="utf-8" ?>
< LinearLayout  xmlns:android ="http://schemas.android.com/apk/res/android"
android:orientation ="vertical"
android:layout_width ="fill_parent"
android:layout_height ="fill_parent"
>
< TextView  android:id ="@+id/tv"
android:layout_width ="fill_parent" 
android:layout_height ="wrap_content" 
android:textColor ="@android:color/white" 
android:ellipsize ="marquee" 
android:focusable ="true" 
android:marqueeRepeatLimit ="marquee_forever" 
android:focusableInTouchMode ="true" 
android:scrollHorizontally ="true" 
android:text ="Please input the text:"
/>
< EditText  android:id ="@+id/ET" 
android:layout_width ="match_parent" 
android:layout_height ="wrap_content"
android:inputType ="number" />
</ LinearLayout >

Java代码：
package com.android.text;
import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.Log;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class TextWatcherDemo extends Activity {
    private TextView mTextView;
    private EditText mEditText;
   
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mTextView = (TextView)findViewById(R.id.tv);
        mEditText = (EditText)findViewById(R.id.ET);
        mEditText.addTextChangedListener(mTextWatcher);
    }
    TextWatcher mTextWatcher = new TextWatcher() {
        private CharSequence temp;
        private int editStart ;
        private int editEnd ;
        @Override
        public void beforeTextChanged(CharSequence s, int arg1, int arg2,
                int arg3) {
            temp = s;
        }
      
        @Override
        public void onTextChanged(CharSequence s, int arg1, int arg2,
                int arg3) {
            mTextView.setText(s);
        }
      
        @Override
        public void afterTextChanged(Editable s) {
            editStart = mEditText.getSelectionStart();
            editEnd = mEditText.getSelectionEnd();
            if (temp.length() > 10) {
                Toast.makeText(TextWatcherDemo.this,
                        "你输入的字数已经超过了限制！", Toast.LENGTH_SHORT)
                        .show();
                s.delete(editStart-1, editEnd);
                int tempSelection = editStart;
                mEditText.setText(s);
                mEditText.setSelection(tempSelection);
            }
        }
    };
}
（2）使用TextWathcer实现EditeText和TextView同步
TextWatcher自身是一个接口，首先需要实现这个接口并覆盖其三个方法，分别为Text改变之前，改变之后以及改变的过程中各自发生的动作相应，这里我们只需要实现EditText在文本发生改变时候让TextView的内容跟着发生变化。

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

可以看出TextWatcher是专门用来监听文本变化的，正因为它的这个技能，正是我们实现同步的功能所需要的。