How to show progress indicator when fetching data from API?

Question

The problem is very simple but I am unable to find a proper solution. I have a search screen and here is what I want to do.

• When screen is started for first time, I want to show a message in Text widget 'Your results will appear here'.
• If user enters something in search box and press search, I want to show CircularProgressIndicator.
• And then, when API response is received, I want to either show a ListView or Text saying 'Found nothing'.
• If user searches something again, I want to show CircularProgressIndicator again.

How can I achieve this behavior using Stream? Following is my code so far:

Widgets

StreamBuilder<List<Model>>(
  stream: bloc.searchStream,
  builder: (context, snapshot) {

    if (snapshot.hasError) {
      return Center(
        child: Text(
          'Error occurred'
        ),
      );
    }

    if (!snapshot.hasData) {
      return Center(
        child: Text(
          'Your results will appear here'
        ),
      );
    }

    if (snapshot.hasData && snapshot.data.length < 1) {
      return Center(
        child: Text(
          'Found nothing'
        ),
      );
    }

    // Where should I put my CircularProgressIndicator?

    return ListView.separated(
      itemCount: snapshot.data.length,
      separatorBuilder: (context, index) => Divider(height: 1),
      itemBuilder: (context, index) {
        final item = snapshot.data[index];
        return ItemWidget(item);
      },
    );
  },
)

BLoC

final _searchStreamController = StreamController<List<Model>>();
Stream<List<Model>> get searchStream => _searchStreamController.stream;

void search(String searchTerm) async {

  if (searchTerm.isEmpty) {
    _searchStreamController.add(null);
    return;
  }

  final client = HttpClient();
  client.method = HttpMethod.POST;
  client.endPoint = 'search';
  client.addData('query', searchTerm);

  try {
    final responseStr = await client.execute();
    final response = SearchResponse.fromJson(responseStr);
    _searchStreamController.add(response.data);
  } catch (e) {
    _searchStreamController.addError(e);
  }
}

I want to display CircularProgressIndicator every time search(String searchedTerm) function is called.

Thank you.

Answer 1

Here is what hack I had to do to achieve my requirements. If there is a better way to achieve this, please post an answer and I will mark it as correct.

Hack

class JugarModel<T> {
  bool isProcessing;
  T data;

  JugarModel({this.isProcessing, this.data});
}

And then use this isProcessing property to check whether to show CircularProgressIndicator or ListView. The rest of the code became:

Widget

StreamBuilder<JugarModel>(
  stream: bloc.searchStream,
  builder: (context, snapshot) {

    if (snapshot.hasError) {
      return Center(
        child: Text(
          'Error occurred'
        ),
      );
    }

    if (!snapshot.hasData) {
      return Center(
        child: Text(
          'Your results will appear here'
        ),
      );
    }

    if (snapshot.hasData && snapshot.data.data.isEmpty) {

      if (snapshot.data.isProcessing) {
        return Center(
          child: CircularProgressIndicator(),
        );
      } else {
        return Center(
          child: Text(
            'Found nothing'
          ),
        );
      }
    }

    return ListView.separated(
      itemCount: snapshot.data.data.length,
      separatorBuilder: (context, index) => Divider(height: 1),
      itemBuilder: (context, index) {
        final item = snapshot.data.data[index];
        return ItemWidget(item);
      },
    );
  },
)

BLoC

final _searchStreamController = StreamController<JugarModel<List<Data>>>();
Stream<JugarModel<List<Data>>> get searchStream => _searchStreamController.stream;

void search(String searchTerm) async {

  if (searchTerm.isEmpty) {
    _searchStreamController.add(null);
    return;
  }

  final client = HttpClient();
  client.method = HttpMethod.POST;
  client.endPoint = 'search';
  client.addData('query', searchTerm);

  // This MAGIC line will call StreamBuilder callback with isProcessing set to true.
  _searchStreamController.add(JugarModel<List<Data>>(isProcessing: true, data: List()));
  try {
    final responseStr = await client.execute();
    final response = SearchResponse.fromJson(responseStr);

    // And after we've received response from API and parsed it, we're calling StreamBuilder
    // callback again with isProcessing set to false.
    _searchStreamController.add(JugarModel<List<Data>>(isProcessing: false, data: response.data));
  } catch (e) {
    _searchStreamController.addError(e);
  }
}

Answer 2

    StreamBuilder<List<Model>>(
      stream: bloc.searchStream,
      builder: (context, snapshot) {
 
        if (snapshot.hasData) {
         if (snapshot.data.length ==0){ 
              return  Center(
                 child: Text(
                      'Found nothing'
                    ),
              );
    
            } 
        return ListView.separated(
          itemCount: snapshot.data.length,
          separatorBuilder: (context, index) => Divider(height: 1),
          itemBuilder: (context, index) {
            final item = snapshot.data[index];
            return ItemWidget(item);
          },
        );
        }else if (snapshot.hasError) {
          return Center(
            child: Text(
              'Error occurred'
            ),
          );
      }else {
         return Center(
            child:CircularProgressIndicator()
          );
       }
      },
    )

give an initial Text() widget with value as "Results will appear here", then onClick of Button, replace that initial Text() widget with the StreamBuilder

Answer 3

You can try :

StreamBuilder<List<Model>>(
          builder: (context, snapshot) {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
                return Center(child: Text('Your results will appear here'));

              case ConnectionState.active:
              case ConnectionState.waiting:
                return Center(child: CircularProgressIndicator());

              case ConnectionState.done:
                if (snapshot.hasError)
                  return Center(child: Text('Error occurred'));

                if (snapshot.data.isEmpty) {
                  return Container(child: Center(child: Text('Found nothing')));
                } else {
                  return ListView.separated(
                    itemCount: snapshot.data.length,
                    separatorBuilder: (context, index) => Divider(height: 1),
                    itemBuilder: (context, index) {
                      final item = snapshot.data[index];
                      return ItemWidget(item);
                    },
                  );
                }
            }
            return Center(child: Text('Your results will appear here'));
          },
        )