TextWatcher提供了3个回调方法：

1.文本改变前：beforeTextChanged
2.文本改变：onTextChanged
3.文本改变之后：afterTextChanged
布局文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">

    <TextView
        android:id="@+id/tv"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:ellipsize="marquee"
        android:focusable="true"
        android:focusableInTouchMode="true"
        android:marqueeRepeatLimit="marquee_forever"
        android:scrollHorizontally="true"
        android:text="输入的结果为:" />

    <EditText
        android:id="@+id/et"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入:"/>
</LinearLayout>

主类：

public class MainActivity extends AppCompatActivity {
    private TextView mTextView;
    private EditText mEditText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView) findViewById(R.id.tv);
        mEditText = (EditText) findViewById(R.id.et);
        /**
         * 监听EditText框中的变化
         */
        mEditText.addTextChangedListener(new TextWatcher() {
            private CharSequence temp;
            private int editStart;
            private int editEnd;

            /**
             * 文本变化之前
             * @param s
             * @param start
             * @param count
             * @param after
             */
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
                temp = s;
            }

            /**
             * 文本变化中
             * @param s
             * @param start
             * @param before
             * @param count
             */
            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                mTextView.setText("输入的结果为:"+s);
            }

            /**
             * 文本变化之后
             * @param s
             */
            @Override
            public void afterTextChanged(Editable s) {
                editStart = mEditText.getSelectionStart();
                editEnd = mEditText.getSelectionEnd();
                if (temp.length() > 10) {//限制长度
                    Toast.makeText(MainActivity.this,
                            "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);
                }

            }
        });

    }
}

结果：