How to use Future from Async function?

Question

I'm trying to get the value from an Async function that returns an integer which is then used to set the color of a widget in my UI.

Async function:

// Returns a future<int> instead of an int due to async
Future<int> getLikeStatus(String name) async {
  int likeStatus =
      await getLikeStatusFromPostLikes(name); // this returns an int
  return likeStatus;
}

Post Function:

 Future <List<dynamic>> fetchData() async {
  // Some code to make GET request to private server with data and push as posts
  final response = await posts.get();
  final result = response.map((m) => Post.fromJson(m)).toList();

  return result;
}

Downstream usage - uses the above post function:

// snapshot is populated by post future in FutureBuilder
child: FutureBuilder<List<dynamic>>(
              future: post,
              builder: (context, snapshot) {
                if (snapshot.hasData) { 
                   ...
                   Row(
                      mainAxisSize: MainAxisSize.min,
                      children: <Widget>[
                         InkWell(
                            child: IconButton(
                            icon: Icon(Icons.thumb_up),
                            color: getLikeStatus(snapshot.data[index].name) == 1
                ? Colors.green
                : Colors.blue) // cannot use future here
                      ),
  ],
);

How can I return the likeStatus variable to use for the color attribute in my widget?

EDIT: Added code showing use of FutureBuilder

use FutureBuilder (api.flutter.dev/flutter/widgets/FutureBuilder-class.html) widget — dlohani
– dlohani, Commented Jan 20, 2020 at 15:38
@Black but you can still use the solution that creativecreatorormaybenot gave. — Crazy Lazy Cat
– Crazy Lazy Cat, Commented Jan 20, 2020 at 16:57

Crazy Lazy Cat · Accepted Answer · 2020-01-21 05:22:16Z

1

If you don't want to use nested FutureBuilder then you can make them as single future function(fetchAll) like this,

post = fetchAll();

....

Future<Map<Post, int>> fetchAll() async {
  final posts = await fetchData();
  final result = <Post, int>{};
  for(final x in posts){
    final status = await getLikeStatus(x.name);
    result[x] = status;
  }
  return result;
}

Future <List<Post>> fetchData() async {
  // Some code to make GET request to private server with data and push as posts
  final response = await posts.get();
  final result = response.map((m) => Post.fromJson(m)).toList();
  return result;
}

// Returns a future<int> instead of an int due to async
Future<int> getLikeStatus(String name) async {
  int likeStatus =
  await getLikeStatusFromPostLikes(name); // this returns an int
  return likeStatus;
}

...

    child: FutureBuilder<Map<Post, int>>(
      future: post,  //TODO: `fetchAll` instead of `fetchData`
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          //Since snapshot is a map you can get `Post` data using `snapshot.data.entries.elementAt(index)`
          ...
          Row(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              InkWell(
                  child: IconButton(
                      icon: Icon(Icons.thumb_up),
                      color: snapshot.data.entries.elementAt(index).value
                          ? Colors.green
                          : Colors.blue) // cannot use future here
              ),
            ],
          ),
          //...
        }
      },
    ),

Above the fetch status request send one after another. So it may take long if the name list is large. You can make it parallel like this,

Future<Map<Post, int>> fetchAll() async {
  final posts = await fetchData();
  final result = <Post, int>{};
  final statusList =
      await Future.wait<int>(posts.map((x) => getLikeStatus(x.name)));
  for (int i = 0; i < posts.length; i++) {
    result[posts[i]] = statusList[i];
  }
  return result;
}

edited Jan 21, 2020 at 5:22

answered Jan 20, 2020 at 17:28

Crazy Lazy Cat

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3 Comments

Black Over a year ago

I didn't know multiple futures could be added into one super future. Thank you

Crazy Lazy Cat Over a year ago

@Black Don't use .forEach in fetchAll method. I updated the code

Black Over a year ago

Changed this, optimisation makes a lot of sense. Thank you

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