Dart / Flutter : Waiting for a loop to be completed before continuing... (Async Await?) - flutter

Question

I have a function which creates a sublist from a large(very large list). After creating this list, the function goes on treating it (deleting duplicates, sorting...).
As long as the list was not too big, it worked fine. But now, I get "The Getter length was called on null". I suppose, it's because the second part of the function (after the loop) starts before the sublist is completed... so it doesn't work...
How can we force the function to wait for the loop to be over to continue the rest of the treatment ?
Is it with Async /Await ? Or can we do something like "While... something is not over...", or "As soon as something is done... do that" ? (My suggestions might be naive, but I am a beginner...)
Here is the code :
List themeBankFr() {
List<Map> themeBankFr = [];
for (Word word in wordBank) {
for (Thematique wordTheme in word.theme) {
themeBankFr.add({
'themeFr': wordTheme.themeFr,
'image': wordTheme.image,
});
}
}
// convert each item to a string by using JSON encoding
final jsonList = themeBankFr.map((item) => jsonEncode(item)).toList();
// using toSet - toList strategy
final uniqueJsonList = jsonList.toSet().toList();
// convert each item back to the original form using JSON decoding
final result = uniqueJsonList.map((item) => jsonDecode(item)).toList();
// sort the list of map in alphabetical order
result.sort((m1, m2) {
var r = m1['themeFr'].compareTo(m2['themeFr']);
if (r != 0) return r;
return m1['image'].compareTo(m2['image']);
});
return result;
}

Answer 1

i think i have a good answer that may helps you and it will as following
first create another function to do the work of for loops and this function returns a future of list that you need like below
Future<List<Map>> futureList(List wordBank){
List<Map> themeBankFr = [];
for (Word word in wordBank) {
for (Thematique wordTheme in word.theme) {
themeBankFr.add({
'themeFr': wordTheme.themeFr,
'image': wordTheme.image,
});
}
}
return Future.value(themeBankFr);
}
after that you can use this function inside your code and use it as async await and now you will never run the below lines before you return this array like below
List themeBankFr() async {
List<Map> themeBankFr = await futureList(wordBank);
// convert each item to a string by using JSON encoding
final jsonList = themeBankFr.map((item) => jsonEncode(item)).toList();
// using toSet - toList strategy
final uniqueJsonList = jsonList.toSet().toList();
// convert each item back to the original form using JSON decoding
final result = uniqueJsonList.map((item) => jsonDecode(item)).toList();
// sort the list of map in alphabetical order
result.sort((m1, m2) {
var r = m1['themeFr'].compareTo(m2['themeFr']);
if (r != 0) return r;
return m1['image'].compareTo(m2['image']);
});
return result;
}
i think this will solve your problem and i hope this useful for you