Dart Future functions with callbacks on requests - asynchronous

Question

I have an issue with understanding Future functions in dart and async programming. The examples I've found in web do not answer my question, so I would be grateful for community help.
I have a function that asks for players messages list:
Future getMessagesInfo(response) async{
dialogs = []; // I need this list to be filled to proceed
messagesList = response["data"];
await getDialogsFunc(messagesList);
renderMessages();
}
It is a callback for a previous request, which doesn't really matters here.
So, here is getDialogsFunc, which purpose is to runs through the list and ask playerInfo for each ID in the response:
Future getDialogsFunc(messagesList) async{
for(var i=0;i<messagesList.length;i++){
if(messagesList[i]["player2"] != player.ID) await rm.getDialogInfo(messagesList[i]["player2"]);
if(messagesList[i]["player1"] != player.ID) await rm.getDialogInfo(messagesList[i]["player1"]);
}
}
Here is getDialogInfo, which actually sends a request for playerInfo and has a callback function that handles received info:
Future getDialogInfo(int id) async{
messages = querySelector("account-messages-tab");
var request = new Request();
Object data = {"id": ServerAPI.PLAYER_INFO, "data":{"id": id}};
await request.sendRequest(data,false, messages.onGotDialogInfo);
}
The request is a simple class, that handles the requests:
class Request{
Future sendRequest(Object data, bool action,Function callback) async{
HttpRequest request = new HttpRequest();
String url = "http://example.com";
await request
..open('POST', url)
..onLoadEnd.listen((e)=> callback(JSON.decode(request.responseText)))
..send(JSON.encode(data));
}
}
And finally here is a callback for the request:
Future onGotDialogInfo(response) async{
List dialog = new List();
dialog.add(response["data"]["id"]);
dialog.add(response["data"]["login"]);
dialogs.add(dialog);
}
In the first function I wanted to run renderMessages() after I have received information about all messages, so that dialogs List should contain relevant information. In my realisation which I tested with breakpoints the renderMessages() functions runs BEFORE onGotDialogInfo callback.
What should I do to wait for the whole cycle getDialogsFunc functions and only then go to renderMessages()?

Answer 1

whenComplete is like finally, it's called no matter whether the request returned normally or with an error. Normally then is used.
getDialogsFunc uses async but doesn't use await which is a bit uncommon. This might be your intention, but I'm not sure
Future getDialogsFunc(messagesList) async {
for(var i=0;i<messagesList.length;i++){
if(messagesList[i]["player2"] != player.ID)
await rm.getDialogInfo(messagesList[i]["player2"]);
if(messagesList[i]["player1"] != player.ID)
await rm.getDialogInfo(messagesList[i]["player1"]);
}
}
getMessagesInfo could then look like:
void getMessagesInfo(response) await {
dialogs = []; // I need this list to be filled to proceed
messagesList = response["data"];
await getDialogsFunc(messagesList)
renderMessages(messagesList);
}
I don't know where Request comes from, therefore hard to tell how that code should look like. It should at least use await for other awaits to work as expected.
Future getDialogInfo(int id) async {
messages = querySelector("account-messages-tab");
var request = new Request();
Object data = {"id": ServerAPI.PLAYER_INFO, "data":{"id": id}};
final response = await request.sendRequest(data,false);
messages.onGotDialogInfo(response)
}
update
class Request{
Future sendRequest(Object data, bool action) async{
Completer completer = new Completer();
HttpRequest request = new HttpRequest();
String url = "http://example.com";
await request
..open('POST', url)
..onLoadEnd.listen((_) {
if (request.readyState == HttpRequest.DONE) {
if (request.status == 200) {
// data saved OK.
completer.complete(JSON.decode(request.responseText)); // output the response from the server
} else {
completer.completeError(request.status);
}
}
})
..send(JSON.encode(data));
return completer.future;
}
}