XXX_Fragment
@Override
public void onResume() {
    super.onResume();
    getFocus();
}
//主界面获取焦点
private void getFocus() {
    getView().setFocusableInTouchMode(true);
    getView().requestFocus();
    getView().setOnKeyListener(new View.OnKeyListener() {
      @Override
      public boolean onKey(View v, int keyCode, KeyEvent event) {
        if (event.getAction() == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK) {
          // 监听到返回按钮点击事件
          ......
          return true;
        }
        return false;
      }
    });
}

//private EditText nickname;
nickname.setOnKeyListener(new View.OnKeyListener() {
  @Override
  public boolean onKey(View v, int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK
        && event.getAction() == KeyEvent.ACTION_UP) {
      //关闭软键盘
      InputMethodManager imm = (InputMethodManager) getActivity().getSystemService(Context.INPUT_METHOD_SERVICE);
      imm.hideSoftInputFromWindow(nickname.getWindowToken(), 0);
      //使得根View重新获取焦点，以监听返回键
      getFocus();
    }
    return false;
  }
});