问题
I'm new to Dart and therefore having trouble with asynchronous programming. I'm trying to loop through a list of elements (let's call them ingredients for now) and query the database for recipes which contain the ingredient. To achieve this, I have a list 'ingredientsSelectedList' and pass it over to a future which is supposed to query the Firestore Database and add the result to the 'possibleRecipes' List. The problem is, that I can't figure out how to 'await' the for loop to finish, before returning the 'possibleRecipes' List. Everytime I run it, it returns an empty list. Hope I didn't make it too complicated and Thanks in advance for everyone that's taking the time to read this :)
PS: I have spent hours to find a solution to this online, but couldn't find anything.
Future searchRecipe(ingredients) async {
var possibleRecipes = []; //List to store results
for (int i = 0; i < ingredients.length; ++i) {
var currentIngredient = ingredients[i];
//now query database for recipes with current ingredient
var fittingRecipes = Firestore.instance
.collection('recipes-01')
.where('ingr.$currentIngredient', isEqualTo: true);
fittingRecipes.snapshots().listen((data) => data.documents.forEach((doc) {
possibleRecipes.add(doc['name']); //add names of results to the list
}));
}
return possibleRecipes; //this returns an empty list
}
回答1:
Yes you can
Simply use this code
Future searchRecipe( List ingredients) async {
var possibleRecipes = []; //List to store results
ingredients.forEach((currentIngredient) async{
//you can await anything here. e.g await Navigator.push(context, something);
//now query database for recipes with current ingredient
var fittingRecipes = await Firestore.instance
.collection('recipes-01')
.where('ingr.$currentIngredient', isEqualTo: true);
fittingRecipes.snapshots().listen((data) => data.documents.forEach((doc) {
possibleRecipes.add(doc['name']); //add names of results to the list
}));
});
return possibleRecipes; //this returns an empty list
}
来源：https://stackoverflow.com/questions/58560570/is-it-possible-to-await-a-for-loop-in-dart