wiring up the callback functions to widgets in search bar

search_bar.dart

// register that SearchBar will need these callbacks
SearchBar({
  Key key,
  @required this.onCancelSearch,
  @required this.onSearchQueryChanged,
}) : super(key: key);

final VoidCallback onCancelSearch;
final Function(String) onSearchQueryChanged;

// callback function when suffix button is tapped
clearSearchQuery() {
  // clear the input field
  _searchFieldController.clear();
  
  // notify DefaultAppBar by calling onSearchQueryChanged with ''
  widget.onSearchQueryChanged('');
}

// wire onChange event of search input with onSearchQueryChanged
// and also wire up clear button
TextField(
  controller: _searchFieldController,
  cursorColor: Colors.white,
  style: TextStyle(color: Colors.white),
  decoration: InputDecoration(
    border: InputBorder.none,
    prefixIcon: Icon(Icons.search, color: Colors.white),
    hintText: "Search...",
    hintStyle: TextStyle(color: Colors.white),
    suffixIcon: InkWell(
      child: Icon(Icons.close, color: Colors.white),
      
      // map clear button with it's callback function
      onTap: clearSearchQuery,
    ),
  ),
  
  // wiring up the callback function
  onChanged: widget.onSearchQueryChanged,
)

// finally notify DefaultAppBar to stop search mode by calling onCancelSearch
// when user presses on back button.
IconButton(
  icon: Icon(Icons.arrow_back, color: Colors.white),
  onPressed: widget.onCancelSearch,
),