安卓IMEI智能格式化

今天朋友让我帮忙搞个Imei的格式，我想这还不简单，一顿猛敲后，发现丫的光标位置好坑，现在我记录下我的思路，如果有不好的地方或有待改进的地方，还请各位大佬下面留言，先来个效果图

下面是代码

package com.gongwenbo.demo;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;
/**
 * Imei智能格式填补 12:34:56
 */
public class ImeiActivity extends AppCompatActivity {

    private EditText mEt;
    private int      preLength;
    private boolean flagAfter = true;   
    private boolean flag      = true;
    int index = 0;
    private static final String TAG = "ImeiActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_imei);
        mEt = (EditText) findViewById(R.id.et);
        mEt.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
                // 获取光标的位置
                index = mEt.getSelectionStart();
            }
            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
            }
            @Override
            public void afterTextChanged(Editable editable) {
                String content = editable.toString();
                // 判断是添加还是删除，此处要获取第一次的值，因为重新赋值了，他还会走一次
                if (flagAfter) {
                    flag = preLength > content.length() ? true : false;
                }
                flagAfter = !flagAfter;
                // 判断值是否一样，防止死循环
                if (preLength == content.length()) {
                    return;
                }
                // 添加格式，我的是 12:34:45 自行添加
                content = content.replace(":", "");
                char[] chars = content.toCharArray();
                StringBuilder stringBuilder = new StringBuilder();
                for (int i = 0; i < chars.length; i++) {
                    stringBuilder.append(chars[i]);
                    if (i % 2 == 1 && i != chars.length - 1) {
                        stringBuilder.append(":");
                    }
                }
                preLength = stringBuilder.length();
                String msg = stringBuilder.toString();
                mEt.setText(msg);
                // 防止数组越界
                if (index > preLength) {
                    index = preLength;
                }
                // 如果是添加的话，光标要往后移1
                if (!flag && index >= 1 && msg.toCharArray()[index - 1] == ':') {
                    index = index + 1;
                }
                // 设置光标的位置
                mEt.setSelection(index);
            }
        });
    }
}