How to get result of HttpRequest? - asynchronous

Question

I'm writing the game for mobile using dart. My level-config stored as json. Via httpRequest I get JSON from server and then parse my json. It should create a Level-Object. I want to use it in my model. But I can't assign it to the level-reference in my model (model.level). After HttpRequest it's value is still the same (null). My idea was also to use some art of callback, but I don't know how...
static void load(final int levelNr, Game model) {
final String path = "../json/$levelNr.json";
HttpRequest.getString(path).then((lvlJson) {
Map data = JSON.decode(lvlJson);
Level level = _levelFromMap(data);
model.level = level;
});
}

Answer 1

You can use callback or async await
1. Callback
static void load(final int levelNr, void callbackFunction(Level level)) {
final String path = "../json/$levelNr.json";
HttpRequest.getString(path).then((lvlJson) {
Map data = JSON.decode(lvlJson);
Level level = _levelFromMap(data);
callbackFunction(level);
});
}
Usage:
load(levelNrValue, (level) {
model.level = level;
//refresh view or data to reflect the change in model.
})
2. async and await
static Future<Level> load(final int levelNr) async {
final String path = "../json/$levelNr.json";
final response = await HttpRequest.getString(path);
Map data = JSON.decode(lvlJson);
return _levelFromMap(data);
}
Usage:
1) Using callback at calling place:
load(levelNrValue).then((level) {
model.level = level;
//refresh view or data to reflect the change in model.
});
2) Using async in calling place also:
final level = await load(levelNrValue);
model.level = level;
Note: the above two lines should be inside some async method.
void loadDate(int levelNrValue, Game model) async {
final level = await load(levelNrValue);
model.level = level;
//refresh view or data to reflect the change in model.
}

Answer 2

Maybe the request is finishing in an error which might not be noticed.
Try adding a catchError:
static void load(final int levelNr, Game model) {
final String path = "../json/$levelNr.json";
HttpRequest.getString(path).then((lvlJson) {
Map data = JSON.decode(lvlJson);
Level level = _levelFromMap(data);
model.level = level;
})
.catchError((Error error) {
print(error.toString());
});;
}