How to pass a `where` clause to a function in Dart? - flutter

Question

There is a function, getItems, and I would like to be able to have multiple where to modify the resulting list. I am new to Dart and cannot find the syntax for passing in a where.
I tried creating functions with custom where to call getItems, but cannot due to the async nature of getItems.
Future<List<IioMenuItem>> getItems() async {
// ...
final db = await openDatabase(path, readOnly: true);
final List<Map<String, dynamic>> maps = await db.query('menu_items');
final dbFilteredItems = maps.map((item) => IioMenuItem(
// assign values code removed
)).where((element) { // <-- make 'where' replaceable
if (filterState == FilterState.all) {
return true;
} else {
return element.type.name == filterState.name;
}
}).toList(growable: false);
return List.generate(dbFilteredItems.length, (i) {
return dbFilteredItems[i];
});
}
The failed attempt
Future<List<IioMenuItem>> menuItems(FilterState filterState) async {
final dbFilteredItems = getItems().where((element) { // The method 'where' isn't defined for the type 'Future'.
if (filterState == FilterState.all) {
return true;
} else {
return element.type.name == filterState.name;
}
}).toList(growable: false);
return List.generate(dbFilteredItems.length, (i) {
return dbFilteredItems[i];
});
}
Can I please get help?

Answer 1

The term you're looking for is a "closure" or "first class function".
See Functions as first-class objects on the Language guide.
"A where" isn't a thing. It's not a noun. Iterable.where is just the name of a function, and that function happens to take a function as a parameter, and uses it to determine what things to keep.
In this specific case, you want a function that takes a IioMenuItem, and returns a boolean that determins where or not to keep it. The type of that is a bool Function(IioMenuItem) (see Function).
I called it "predicate":
Future<List<IioMenuItem>> menuItems(
FilterState filterState,
bool Function(IioMenuItem) predicate // <- Take it in as a parameter
) async {
return (await getItems())
.where(predicate) // <- pass it along as an argument to `where`
.toList(growable: false);
}

Answer 2

You can pass any test inside a where.
filterItems(Map<String,dynamic> element) {
if (filterState == FilterState.all) {
return true;
} else {
return element.type.name == filterState.name;
}
}
final dbFilteredItems = maps.map((item) => IioMenuItem(
// assign values code removed
)).where(filterItems).toList(growable: false);
Use then in future
getItems().then((value) => value.where((element ...

Answer 3

Use await to call async functions.
Future<List<IioMenuItem>> menuItems(FilterState filterState) async {
final dbFilteredItems = (await getItems()).where((element) { // await has to be used here.
if (filterState == FilterState.all) {
return true;
} else {
return element.type.name == filterState.name;
}
}).toList(growable: false);
return dbFilteredItems;
}