I have this function, want to call it synchronously, because if I call it async then I should use FutureBuilder, which I don't prefer because it has extra flicker if user scrolls too fast:
Future<String> getRealHTML(int chapter) async {
var key = _Html.keys.toList()[chapter];
var val = _Html.values.toList()[chapter];
if (val.Content.startsWith("filename:")) {
EpubContentFileRef value = contentRefHtml[key];
return await value.readContentAsText();
}
return null;
}
readContentAsText() returns a Future, you cannot call it synchronously since it is an I/O request which means it will take time to finish the request, that is why it is called asychronously.
Related
I subscribe to a stream with the listen method so I can process the data as it comes in. If I found what I am looking for, I want to cancel the stream and return the data from the stream subscription block.
I have a working implementation but I am using a Completer to block the function from returning immediate and it feels messy, so I want to know if there's a better way to achieve this. Here is my code snippet:
Future<String> _extractInfo(String url) async {
var data = <int>[];
var client = http.Client();
var request = http.Request('GET', Uri.parse(url));
var response = await client.send(request);
var process = Completer();
var interestingData = '';
StreamSubscription watch;
watch = response.stream.listen((value) async {
data.clear(); //clear the previous stale data from the list so searching can be faster
data.addAll(value);
if (
hasInterestingData(data) //search the bytelist for the data I'm looking for
){
interestingData = extractInterestingData(data) //extract the data if it's been found. This is what I want my function to return;
await watch.cancel(); //cancel the stream subscription if the data has been found. ie: Cancel the download
process.complete('done'); //complete the future, so the function can return interestingData.
}
});
watch.onDone(() {
//complete the future if the data was not found...so the function will return an empty string
if (!process.isCompleted) {
process.complete('done');
}
});
await process.future; //blocks the sync ops below so the function doesn't return immediately
client.close();
return interestingData;
}
You should use await for for things like this when you are in an async function.
Future<String> _extractInfo(String url) async {
var client = http.Client();
var request = http.Request('GET', Uri.parse(url));
var response = await client.send(request);
await for (var data in response.stream) {
if (hasInterestingData(data)) {
return extractInterestingData(data); // Exiting loop cancels the subscription.
}
}
return "done";
}
However, while this approach is equivalent to your code, it's probably equally flawed.
Unless you are looking for a single byte, you risk the thing you are looking for being split between consecutive data events. You like do need to retain some part of the previous data array, or some state summarizing what you have already seen, but how much depends on what you are actually looking for.
Dart newbie here, I'm currently learning about asynchronous execution in Dart. I'm a bit irritated about how concurrency works in Dart, take the following scenario from their codelab:
void printOrderMessage () async {
try {
var order = await fetchUserOrder();
print('Awaiting user order...');
print(order);
} catch (err) {
print('Caught error: $err');
}
}
Future<String> fetchUserOrder() {
// Imagine that this function is more complex.
var str = Future.delayed(Duration(seconds: 4), () => throw 'Cannot locate user order');
return str;
}
Future<void> main() async {
await printOrderMessage();
}
In this case, the asynchronous operation is fetching the the user order from say a DB. Now because of Dart's await / async mechanism, every function that is related to the async operation is required to have a Future<> return type and must be tagged with async.
This feels clunky ... Imagine if some value deep in my function chain is being calculated asynchronously, would I really need to always return a future? Is there some other construct to synchronize code in Dart than await? Or have I misunderstood the concept?
If callers need to be able to wait for your asynchronous operation to finish, then your asynchronous function must return a Future that can be awaited. This is contagious; if callers of those callers need to be able to wait, then they too need to have Futures to wait upon.
If callers should not wait, then you can have a "fire-and-forget" function that does not need to return a Future:
Future<void> foo() {
// ...
}
// Does not need to return a Future. Consequently, callers cannot
// directly determine when `foo` completes.
void bar() {
foo();
}
I am building a screen in an app using an existing model class which I can't modify. The model has a method called refreshList() which fetches some data from an API, processes it, and finally it updates a List member in a model with the fetched data.
I want to know the state of the list - i.e. is it uninitialized, being fetched or already fetched - so that I can update the UI accordingly.
The problem is that refreshList() is not async, but uses an async method internally. Additionally, the internal method doesn't return a Future.
void refreshList(){
if( someCondition ){
_doRefreshList();
}
}
void _doRefreshList() async{
// ... do stuff
http.Response response = await http.get(url);
// ... do some more stuff
}
I would like to do something like
setState((){ _isLoading = true; });
await model.refreshList();
setState((){ _isLoading = false; });
but unfortunately I can't modify the methods in the model to return a Future.
Is there any way to achieve this without modifying refreshList() to be async and return a Future, and modifying _doRefreshList() to return a Future?
No, there is no way to wait for an asynchronous operation to complete unless it provides some way to get a callback when it's done.
The usual way to do that is to return a future. Having a callback function parameter is another approach, but it's rarely better than just returning a future.
In your example, refreshVerList has no way to report when it is done, and neither does _doRefreshList. You will have to add that somehow, and returning a future is the simplest and most consistent way to solve the problem for both functions.
(Technically, the _doRefreshList function does return a future, it's just typed as void. You can cast that future to Future, but I strongly discourage that kinds of shenanigans. If the function author decides to return something else, say null, at a later point, they are perfectly within their right to do since the return type is void.)
You can use Completer.
Completer<Null> onFetchCompleter;
Future<Null> _doRefreshList() async{
onFetchCompleter = Completer();
// ... do stuff
http.Response response = await http.get(url);
// ... do some more stuff
onFetchCompleter.complete();
return onFetchCompleter.future;
}
In the same way you can user a completer in your refreshVerList method.
Create a Future with the function reference as argument:
Future(model.refreshList);
Now you can use await since the above gives you a Future<void>:
setState(() { _isLoading = true; });
await Future(model.refreshList);
setState(() { _isLoading = false; });
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()?
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;
}
}
somehow my exceptions seem to being caught by method they are executing in. Here is the code to call the method. As you can see I create a cancellation token with a time out. I register a method to call when the cancellation token fires and then I start a new task. The cancellation token appears to be working OK. As does the registered method.
var cancellationToken = new CancellationTokenSource(subscriber.TimeToExpire).Token;
cancellationToken.Register(() =>
{
subscriber.Abort();
});
var task = Task<bool>.Factory.StartNew(() =>
{
subscriber.RunAsync((T)messagePacket.Body, cancellationToken);
return true;
})
.ContinueWith(anticedant =>
{
if (anticedant.IsCanceled)
{
Counter.Increment(12);
Trace.WriteLine("Request was canceled");
}
if (anticedant.IsFaulted)
{
Counter.Increment(13);
Trace.WriteLine("Request was canceled");
}
if (anticedant.IsCompleted)
{
Counter.Increment(14);
}
The next piece of code is the method that seems to be not only throwing the excetion (expected behavior. but also catching the exception.
public async override Task<bool> ProcessAsync(Message input, CancellationToken cancellationToken)
{
Random r = new Random();
Thread.Sleep(r.Next(90, 110));
cancellationToken.ThrowIfCancellationRequested();
return await DoSomethingAsync(input);
}
The exception is being thrown by the cancellation token but according to intellitrace it is being caught at the end of the method. I have tried a number of different options including throwing my own exception, but no matter what the continuewith function always executes the IsComleted or ran to completion code.
Any ideas on what I am doing wrong?
Thanks
I assume that RunAsync is the same as ProcessAsync.
The exception is being thrown by the cancellation token but according to intellitrace it is being caught at the end of the method.
Yup. Any async method will catch its own exceptions and place them on its returned Task. This is by design.
no matter what the continuewith function always executes the IsComleted or ran to completion code.
Well, let's take another look at the code:
var task = Task<bool>.Factory.StartNew(() =>
{
subscriber.RunAsync((T)messagePacket.Body, cancellationToken);
return true;
})
.ContinueWith(
Consider the lambda passed to StartNew: it calls RunAsync, it ignores the Task that it returns, and then it returns true (successfully). So the Task returned by StartNew will always return successfully. This is why the ContinueWith always executes for a successfully-completed task.
What you really want is to await the Task returned by RunAsync. So, something like this:
var task = Task.Run(async () =>
{
await subscriber.RunAsync((T)messagePacket.Body, cancellationToken);
return true;
})
.ContinueWith(
You're still ignoring the return value of RunAsync (the bool it returns is ignored), but you're not ignoring the Task itself (including cancellation/exception information).
P.S. I'm assuming there's a lot of code you're not showing us; using StartNew/Run to just kick off some async work and return a value is very expensive.
P.P.S. Use await Task.Delay instead of Thread.Sleep.