MaxLengthWatcher 设置50个字节（一个汉字2个字

import android.text.Editable;
import android.text.Selection;
import android.text.TextWatcher;
import android.widget.EditText;

import java.io.UnsupportedEncodingException;

/*
 * 监听输入内容是否超出最大长度，并设置光标位置
 * */
public class MaxLengthWatcher implements TextWatcher {

    private int maxLen = 0;
    private EditText editText = null;


    public MaxLengthWatcher(int maxLen, EditText editText) {
        this.maxLen = maxLen;
        this.editText = editText;
    }

    @Override
    public void afterTextChanged(Editable arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void beforeTextChanged(CharSequence arg0, int arg1, int arg2,
                                  int arg3) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onTextChanged(CharSequence arg0, int arg1, int arg2, int arg3) {
        // TODO Auto-generated method stub
        if(maxLen<=0){
           return;
        }

    //字符长度 一个中文两个字符
        Editable editable = editText.getText();
        int len = editable.length();
        if(len > maxLen>>2){//大于一半的时候判断
            try {
                String str = editable.toString();
                byte[] bytes = str.getBytes("GBK");
                if (bytes.length >= maxLen) {
                    int selEndIndex = Selection.getSelectionEnd(editable);
                    //创建新byte数组接受值
                    byte[] b1 = new byte[maxLen];
                    System.arraycopy(bytes, 0, b1, 0, maxLen);
                    //获取新字符串
                    String newStr = new String(b1,"GBK");
                    editText.setText(newStr);
                    editable = editText.getText();

                    //新字符串的长度
                    int newLen = editable.length();
                    //旧光标位置超过字符串长度
                    if(selEndIndex > newLen)
                    {
                        selEndIndex = editable.length();
                    }
                    //设置新光标所在的位置
                    Selection.setSelection(editable, selEndIndex);
                }
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }
        }

        //字符的个数
//        Editable editable = editText.getText();
//        int len = editable.length();
//        if(len > maxLen)
//        {
//            int selEndIndex = Selection.getSelectionEnd(editable);
//            String str = editable.toString();
//            //截取新字符串
//            String newStr = str.substring(0,maxLen);
//            editText.setText(newStr);
//            editable = editText.getText();
//
//            //新字符串的长度
//            int newLen = editable.length();
//            //旧光标位置超过字符串长度
//            if(selEndIndex > newLen)
//            {
//                selEndIndex = editable.length();
//            }
//            //设置新光标所在的位置
//            Selection.setSelection(editable, selEndIndex);
//
//        }
    }

}