监听EditText内容变化及字数限制

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="wrap_content">

<EditText
android:id="@+id/et_content"
android:layout_width="match_parent"
android:layout_height="200dp"
android:hint="请输入不超过10的内容"
android:gravity="top"/>

<TextView
android:id="@+id/tv_left_num"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|right"
android:layout_margin="10dp"/>

</FrameLayout>

public class MainActivity extends Activity {
private static final int MAX_NUM = 10;
private EditText etContent;
private TextView tvLeftNum;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

etContent = (EditText)findViewById(R.id.et_content);
tvLeftNum = (TextView)findViewById(R.id.tv_left_num);
tvLeftNum.setText(String.valueOf(MAX_NUM));

etContent.addTextChangedListener(watcher);
}

TextWatcher watcher = new TextWatcher() {

@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
//只要编辑框内容有变化就会调用该方法，s为编辑框变化后的内容
Log.i("onTextChanged", s.toString());
}

@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
//编辑框内容变化之前会调用该方法，s为编辑框内容变化之前的内容
Log.i("beforeTextChanged", s.toString());
}

@Override
public void afterTextChanged(Editable s) {
//编辑框内容变化之后会调用该方法，s为编辑框内容变化后的内容
Log.i("afterTextChanged", s.toString());
if (s.length() > MAX_NUM) {
s.delete(MAX_NUM, s.length());
}
int num = MAX_NUM - s.length();
tvLeftNum.setText(String.valueOf(num));
}
};
}