I have reason to believe that some ServerValue.increment() commands are not executing.
In my App, when the user submits, two commands are executed:
Future<void> _submit() async {
alimentoBloc.descontarAlimento(foodId, quantity);
salidaAlimentoBloc.crearSalidaAlimento(salidaAlimento);
}
The first command updates the amount of inventory left in the warehouse (using ServerValue.increment)...
Future<bool> descontarAlimento(String foodId, int quantity) async {
try {
dbRef.child('inventory/$foodId/quantity')
.set(ServerValue.increment(-quantity));
} catch (e) {
print(e);
}
return true;
}
The second command makes a food output register, where it records the quantity, type of food and other key data.
Future<bool> crearSalidaAlimento(SalidaAlimentoModel salidaAlimento) async {
try {
dbRef.child('output')
.push().set(salidaAlimento.toJson());
} catch (e) {
print(e);
}
return true;
}
After several reviews, I have noticed that the increase command is not executed sometimes, and then the inventory does not correspond to what it should be.
Then, I would like to do something similar to a transaction, this is: If neither of the two commands is executed, do not execute either of the two.
Is it possible to do a batch of commands in Firebase Realtime without losing the offline functionalities?
You can do a multi-path update to perform both writes transactionally:
var id = dbRef.push().key;
Map<String, dynamic> updates = {
"inventory/$foodId/quantity": ServerValue.increment(-quantity),
"output/$id": salidaAlimento.toJson()
}
dbRef.update(updates);
With the above, either both writes are completed, or neither of them is.
While you're offline, the client will fire local events based on its best guess for the current value of the server (which is gonna be 0 if it never read the value), and it will then send all pending changes to the server when it reconnects. For a quick test, see https://jsbin.com/wuhuyih/2/edit?js,console
You can't use a transaction while the device is offline.
They need to check the current value on the database and that is not possible while offline. If you want to make sure that they succeed you would need to check if a connection is awailable or not.
Related
Hey I need some help here for How to use timeouts in flutter correctly. First of all to explain what the main goal is:
I want to recive data from my Firebase RealTime Database but need to secure this request api call with an time out of 15 sec. So after 15 sec my timeout should throw an exception that will return to the Users frontend the alert for reasons of time out.
So I used the simple way to call timeouts on future functions:
This functions should only check if on some firebase node an ID is existing or not:
Inside this class where I have declared this functions I also have an instance which called : timeoutControl this is a class which contains a duration and some reasons for the exceptions.
Future<bool> isUserCheckedIn(String oid, String maybeCheckedInUserIdentifier, String onGateId) async {
try {
databaseReference = _firebaseDatabase.ref("Boarding").child(oid).child(onGateId);
final snapshot = await databaseReference.get().timeout(Duration(seconds: timeoutControl.durationForTimeOutInSec), onTimeout: () => timeoutControl.onEppTimeoutForTask());
if(snapshot.hasChild(maybeCheckedInUserIdentifier)) {
return true;
}
else {
return false;
}
}
catch (exception) {
return false;
}
}
The TimeOutClass where the instance timeoutControl comes from:
class CustomTimeouts {
int durationForTimeOutInSec = 15; // The seconds for how long to try until we throw an timeout exception
CustomTimeouts();
// TODO: Implement the exception reasons here later ...
onEppTimeoutForUpload() {
throw Exception("Some reason ...");
}
onEppTimeoutForTask() {
throw Exception("Some reason ...");
}
onEppTimeoutForDownload() {
throw Exception("Some reason ...");
}
}
So as you can see for example I tried to use this implementation above. This works fine ... sometimes I need to fight with un explain able things -_-. Let me try to introduce what in somecases are the problem:
Inside the frontend class make this call:
bool isUserCheckedIn = await service.isUserCheckedIn(placeIdentifier, userId, gateId);
Map<String, dynamic> data = {"gateIdActive" : isUserCheckedIn};
/*
The response here is an Custom transaction handler which contains an error or an returned param
etc. so this isn't relevant for you ...
*/
_gateService.updateGate(placeIdentifier, gateId, data).then((response) {
if(response.hasError()) {
setState(() {
EppDialog.showErrorToast(response.getErrorMessage()); // Shows an error message
isSendButtonDiabled = false; /*Reset buttons state*/
});
}
else {
// Create an gate process here ...
createGateEntrys(); // <-- If the closures update was successful we also handle some
// other data inside the RTDB for other reasons here ...
}
});
IMPORTANT to know for you guys is that I am gonna use the returned "boolean" value from this function call to update some other data which will be pushed and uploaded into another RTDB other node location for other reasons. And if this was also successful the application is going on to update some entrys also inside the RTDB -->createGateEntrys()<-- This function is called as the last one and is also marked as an async function and called with its closures context and no await statement.
The Data inside my Firebase RTDB:
"GateCheckIns" / "4mrithabdaofgnL39238nH" (The place identifier) / "NFdxcfadaies45a" (The Gate Identifier)/ "nHz2mhagadzadzgadHjoeua334" : 1 (as top of the key some users id who is checked in)
So on real devices this works always without any problems... But the case of an real device or simulator could not be the reason why I'am faceing with this problem now. Sometimes inside the Simulator this Function returns always false no matter if the currentUsers Identifier is inside the this child nodes or not. Therefore I realized the timeout is always called immediately so right after 1-2 sec because the exception was always one of these I was calling from my CustomTimeouts class and the function which throws the exception inside the .timeout(duration, onTimeout: () => ...) call. I couldn't figure it out because as I said on real devices I was not faceing with this problem.
Hope I was able to explain the problem it's a little bit complicated I know but for me is important that someone could explain me for what should I pay attention to if I am useing timeouts in this style etc.
( This is my first question here on StackOverFlow :) )
I'm on the way to evaluate Dart for a German company by porting various Java programs to Dart and compare and analyze the results. In the browser Dart wins hands down. For server software performance seemed to be a serious isssue (see this question of me) but that got mostly defused.
Now I'm in the area of porting some "simple" command-line tools where I did not expect any serious problems at all but there is at least one. Some of the tools do make HTTP requests to collect some data and the stand-alone Dart virtual machine only supports them in an asynchronous fashion. Looking through all I could find it does not seem to be possible to use any asynchronous call in a mostly synchronous software.
I understand that I could restructure the available synchronous software into an asynchronous one. But this would transform a well-designed piece of software into something less readable and more difficult to debug and maintain. For some software pieces this just does not make sense.
My question: Is there an (overlooked by me) way to embed an asynchronous call into a synchronously called method?
I imagine that it would not be to difficult to provide a system call, usable only from within the main thread, which just transfers the execution to the whole list of queued asynchronous function calls (without having to end the main thread first) and as soon as the last one got executed returns and continues the main thread.
Something which might look like this:
var synchFunction() {
var result;
asyncFunction().then(() { result = ...; });
resync(); // the system call to move to and wait out all async execution
return result;
}
Having such a method would simplify the lib APIs as well. Most "sync" calls could be removed because the re-synchronisation call would do the job. It seems to be such a logical idea that I still think it somehow exists and I have missed it. Or is there a serious reason why that would not work?
After thinking about the received answer from lm (see below) for two days I still do not understand why the encapsulation of an asynchronous Dart call into a synchronous one should not be possible. It is done in the "normal" synchronous programing world all the time. Usually you can wait for a resynchronization by either getting a "Done" from the asynchronous routine or if something fails continue after a timeout.
With that in mind my first proposal could be enhanced like that:
var synchFunction() {
var result;
asyncFunction()
.then(() { result = ...; })
.whenComplete(() { continueResync() }); // the "Done" message
resync(timeout); // waiting with a timeout as maximum limit
// Either we arrive here with the [result] filled in or a with a [TimeoutException].
return result;
}
The resync() does the same that would normally happen after ending the main method of an isolate, it starts executing the queued asynchronous functions (or waits for events to make them executable). As soon as it encounters a continueResync() call a flag is set which stops this asynchronous execution and resync() returns to the main thread. If no continueResync() call is encountered during the given timeout period it too aborts the asynchronous execution and leaves resync() with a TimeoutException.
For some groups of software which benefit from straight synchronous programing (not the client software and not the server software) such a feature would solve lots of problems for the programer who has to deal with asynchrounous-only libraries.
I believe that I have also found a solution for the main argument in lm's argumentation below. Therefore my question still stands with respect to this "enhanced" solution which I proposed: Is there anything which really makes it impossible to implement that in Dart?
The only time that you can wrap an async method in a synchronous one is when you don't need to get a return value.
For example if you want to disable the save button, save results to the server asynchronously and re-enable the save button when the job is done you can write it like this:
Future<bool> save() async {
// save changes async here
return true;
}
void saveClicked() {
saveButton.enabled = false;
save()
.then((success) => window.alert(success ? 'Saved' : 'Failed'))
.catchError((e) => window.alert(e))
.whenComplete(() { saveButton.enabled = true; });
}
Note that the saveClicked method is fully synchronous, but executes the save method asynchronously.
Note that if you make saveClicked async, not only do you have to call it using the async pattern, but the entire method body will run asynchronously so the save button will not be disabled when the function returns.
For completeness the async version of saveClicked looks like this:
Future<Null> saveClicked() async {
saveButton.enabled = false;
try {
bool success = await save();
window.alert(success ? 'Saved' : 'Failed');
}
catch (e) {
window.alert(e);
}
finally {
saveButton.enabled = true;
}
}
Yes, this is way late, but I think this is a cool feature new people should know about.
There is a way, but the Dart docs warn against it (and it's somehow "experimental", although the implications aren't really discussed).
The waitFor command.
You basically pass in an asynchronous function that returns a Future, an optional timeout parameter, and the waitFor function will return the result.
For example:
final int number = waitFor<int>(someAsyncThatReturnsInt);
The resync function cannot be implemented in Dart's current execution model.
Asynchronous execution is contagious. A synchronous function must return before any other asynchronous events can execute, so there is no way to synchronously wait for asynchronous execution.
Execution in Dart is single-threaded and event based. There is no way for the resync function to block without it also blocking all other execution in the same isolate, so the pending async operations will never happen.
To block the synchronous execution, and continue executing something else, you need to preserve the entire call stack up to that point, and reinstate it later when the synchronous operations have completed. If you have that functionality, then there are probably better ways to do things than Future and Stream :)
Also, waiting for "all async execution" isn't well-defined in an event based system. There might be a broadcast Stream emitting events coming in from the network, a periodic timer, or a receive port getting data from another isolate, or some other source of events that you can't wait for because they come from outside the isolate, or event the process. When the current isolate shuts down, it might send a final shut-down message to another isolate, so effectively the "async execution" isn't over until the isolate dies.
Using the async/await syntax, you won't get synchronous operation, but it will be easier to code the similar asynchronous operation:
function() async {
var result = await asyncFunction();
return result;
}
It won't wait for async operations that aren't reflected in the Future returned by asyncFunction, but that's the job of asyncFunction to not complete until its operations are complete.
Dart is inherently async. Trying to avoid asynchronity won't work out.
There are sync versions of some API calls for example in dart:io and in some situations it might seem simpler to use them instead but because there aren't sync versions for all methods/functions you can't avoid async entirely.
With the recent introduction of the async/await feature programming async become much simpler and the code looks almost like sync code (but it isn't).
If a call went async it stays async. As far as I know there is nothing you can do about it.
import 'package:synchronized_lite/synchronized_lite.dart';
import 'dart:async';
// Using Lock as a mixin to further mimic Java-style synchronized blocks
class SomeActivity with Lock {
bool _started = false;
Future<bool> start() async {
// It's correct to return a Future returned by synchronized()
return synchronized(() async {
if(_started)
return false;
// perform the start operation
await Future.delayed(Duration(seconds: 1));
print("Started");
_started = true;
return true;
});
}
Future<void> stop() async {
// It's also correct to await a synchronized() call before returning
// It's incorrect to neither await a synchronized() call nor return its Future.
await synchronized(() async {
if(!_started)
return;
// perform the stop operation`enter code here`
await Future.delayed(Duration(seconds: 1));
print("Stopped");
_started = false;
});
}
}
// Prints:
// Started
// Stopped
main() async {
var a = SomeActivity();
print("Hello");
a.start();
a.start();
a.stop();
await a.stop();
}
/*Since the Await statement can be used in only asynchronous methods. Then we do two methods.I thinking first we call the async method and then we constantly query the null result for the non-async method. Then we get a synchronized model. In this way, we will wait for the answer in the non-async method. Such a method comes to my mind. But as far as I can see, there is no escape from the async working model in flutter dart language. Need to get used to it.It may be unprofessional, but I wanted to share the solution that came to my mind. hope it helps.
Stock resultStockQueryByBarcodeAsync;
bool waitStockQueryByBarcodeAsyncCompleted = false;
Stock WaitStockQueryByBarcodeAsync(String barcode, int timeOut) {
CallStockQueryByBarcodeAsync(barcode);
var startTime = new DateTime.now();
while (!waitStockQueryByBarcodeAsyncCompleted) {
Duration difference = DateTime.now().difference(startTime);
if (difference.inMilliseconds > timeOut) {
throw TimeoutException("Timeout Exceeded");
}
//we must scope time. Because it can be enter endless loop.
}
return resultStockQueryByBarcodeAsync;
}
void CallStockQueryByBarcodeAsync(String barcode) async {
waitStockQueryByBarcodeAsyncCompleted = false;
resultStockQueryByBarcodeAsync = null;
var stock = await StockQueryByBarcodeAsync(barcode);/*your target async method*/
waitStockQueryByBarcodeAsyncCompleted = true;
resultStockQueryByBarcodeAsync = stock;
}
In my case, I had to initialize the database connection from constructor. I am pretty new in Flutter and I don't know what are the best practices right now. But, here is what I did.
class Storage {
late Database database;
Storage() {
getConnection().then((value) => database = value);
}
Future<Database> getConnection() async {
return await openDatabase('ims.db');
}
}
All I have done, is used the callback method to assign the value when the value is available.
Here's a solution based on staggering the start of the async function with start times at least 1 second apart, when calls come in almost simultaneously.
Steps:
Use the lastKnownTime to calculate the delta, where the initial value is 0
Once the delta is not some huge number, you know it's a duplicate call.
class StartConversationState extends State<StartConversationStatefulWidget> {
#override
Widget build(BuildContext context) {
_delayPush(); // this is the call that gets triggered multiple times
}
int lastKnownTime = 0;
int delayMillis = 3000;
_delayPush() async {
delayMillis += 1500;
await new Future.delayed(Duration(milliseconds: delayMillis));
int millisSinceEpoch = new DateTime.now().millisecondsSinceEpoch;
int delta = millisSinceEpoch - lastKnownTime;
// if delta is less than 10 seconds, means it was a subsequent interval
if (delta < 10000) {
print('_delayPush() , SKIPPING DUPLICATE CALL');
return;
}
// here is the logic you don't want to duplicate
// eg, insert DB record and navigate to next screen
}
I coded the next Node/Express/Mongo script:
const { MongoClient } = require("mongodb");
const stream = require("stream");
async function main() {
// CONECTING TO LOCALHOST (REPLICA SET)
const client = new MongoClient("mongodb://localhost:27018");
try{
// CONECTION
await client.connect();
// EXECUTING MY WATCHER
console.log("Watching ...");
await myWatcher(client, 15000);
} catch (e) {
// ERROR MANAGEMENT
console.log(`Error > ${e}`);
} finally {
// CLOSING CLIENT CONECTION ???
await client.close(); << ????
}
}main().catch(console.error);
// MY WATCHER. LISTENING CHANGES FROM MY DATABASE
async function myWatcher(client, timeInMs, pipeline = []) {
// TARGET TO WATCH
const watching = client.db("myDatabase").collection("myCollection").watch(pipeline);
// WATCHING CHANGES ON TARGET
watching.on("change", (next) => {
console.log(JSON.stringify(next));
console.log(`Doing my things...`);
});
// CLOSING THE WATCHER ???
closeChangeStream(timeInMs, watching); << ????
}
// CHANGE STREAM CLOSER
function closeChangeStream(timeInMs = 60000, watching) {
return new Promise((resolve) => {
setTimeout(() => {
console.log("Closing the change stream");
watching.close();
resolve();
}, timeInMs);
});
}
So, the goal is to keep always myWatcher function in an active state, to watch any database changes and for example, send an user notification when is detected some updating. The closeChangeStream function close myWatcher function in X seconds after any database changes. So, to keep the myWatcher always active, do you recomment not to use the closeChangeStream function ??
Another thing. With this goal in mind, to keep always myWatcher function in an active state, if I keep the await client.close();, my code emits an error: Topology is closed, so when I ignore this await client.close(), my code works perfectly. Do you recomment not to use the await client.close() function to keep always myWatcher function in an active state ??
Im a newbee in this topics !
thanks for the advice !
Thanks for help !
MongoDB change streams are implemented in a pub/sub paradigm.
Send your application to a friend in the Sudan. Have both you and your friend run the application (that has the change stream implemented). If you open up mongosh and run db.getCollection('myCollection').updateOne({_id: ObjectId("6220ee09197c13d24a7997b7")}, {FirstName: Bob}); both you and your friend will get the console.log for the change stream.
This is assuming you're not running localhost, but you can simulate this with two copies of the applications locally.
The issue comes from going into production and suddenly you have 200 load bearers, 5 developers, etc. running and your watch fires a ton of writes around the globe.
I believe, the practice is to functionize it. Wrap your watch in a function and fire the function when you're about to do a write (and close after you do your associated writes).
My goal with this function is to return an integer representing all of the habits completed by the user. My databases structure is a collection of habits, each with a sub-collection containing its history. The function below utilizes a list of habits, and using the habit's ID it gets a stream of the history for that habit. My problem is that when I try to loop through the history for each habit it stalls at the await for. Any advice would be appreciated, I am still trying to fully understand streams/asynchronous functions.
Future<int> getCompleteHabits(User user, List<Habit> habits) async {
int completed = 0;
if(habits.isNotEmpty) {
for(Habit habit in habits) {
Stream<List<HabitHistory>> streamHistory = db.streamHabitHistory(user, habit.id);
await for(var h in streamHistory){
print(habit.title);
print(h);
}
print("test");
}
}
return completed;
}
This function will add to the variable completed, however, since I cannot access all the history I have yet to do so.
This function prints the following to the console, it doesn't get to the print("test").
flutter: Get in bed by 11:30 pm
flutter: [Instance of 'HabitHistory', Instance of 'HabitHistory', Instance of 'HabitHistory', Instance of 'HabitHistory', Instance of 'HabitHistory']
From the documentaion on Streams:
Streams are done when there are no more events in them, and the code receiving the events is notified of this just as it is notified that a new event arrives. When reading events using an await for loop, the loops stops when the stream is done.
The reason your loop is hanging is that the stream hasn't yet been closed. It will continue to hang until the stream gets closed or until you execute a break or return statement within the loop.
As Abion47 stated the stream has not been closed so the await for is never complete. It is waiting for new events.
Knowing this I can use the await streamHabitHistory(user, habit.id).first to get the list (so it is not waiting for events). The code is below.
Future<int> getCompleteHabits(User user, List<Habit> habits) async {
int completed = 0;
if(habits.isNotEmpty) {
for(Habit habit in habits) {
List<HabitHistory> history = await db.streamHabitHistory(user, habit.id).first;
for (HabitHistory h in history) {
completed = completed + h.completed.length;
}
}
}
return completed;
}
I'm on the way to evaluate Dart for a German company by porting various Java programs to Dart and compare and analyze the results. In the browser Dart wins hands down. For server software performance seemed to be a serious isssue (see this question of me) but that got mostly defused.
Now I'm in the area of porting some "simple" command-line tools where I did not expect any serious problems at all but there is at least one. Some of the tools do make HTTP requests to collect some data and the stand-alone Dart virtual machine only supports them in an asynchronous fashion. Looking through all I could find it does not seem to be possible to use any asynchronous call in a mostly synchronous software.
I understand that I could restructure the available synchronous software into an asynchronous one. But this would transform a well-designed piece of software into something less readable and more difficult to debug and maintain. For some software pieces this just does not make sense.
My question: Is there an (overlooked by me) way to embed an asynchronous call into a synchronously called method?
I imagine that it would not be to difficult to provide a system call, usable only from within the main thread, which just transfers the execution to the whole list of queued asynchronous function calls (without having to end the main thread first) and as soon as the last one got executed returns and continues the main thread.
Something which might look like this:
var synchFunction() {
var result;
asyncFunction().then(() { result = ...; });
resync(); // the system call to move to and wait out all async execution
return result;
}
Having such a method would simplify the lib APIs as well. Most "sync" calls could be removed because the re-synchronisation call would do the job. It seems to be such a logical idea that I still think it somehow exists and I have missed it. Or is there a serious reason why that would not work?
After thinking about the received answer from lm (see below) for two days I still do not understand why the encapsulation of an asynchronous Dart call into a synchronous one should not be possible. It is done in the "normal" synchronous programing world all the time. Usually you can wait for a resynchronization by either getting a "Done" from the asynchronous routine or if something fails continue after a timeout.
With that in mind my first proposal could be enhanced like that:
var synchFunction() {
var result;
asyncFunction()
.then(() { result = ...; })
.whenComplete(() { continueResync() }); // the "Done" message
resync(timeout); // waiting with a timeout as maximum limit
// Either we arrive here with the [result] filled in or a with a [TimeoutException].
return result;
}
The resync() does the same that would normally happen after ending the main method of an isolate, it starts executing the queued asynchronous functions (or waits for events to make them executable). As soon as it encounters a continueResync() call a flag is set which stops this asynchronous execution and resync() returns to the main thread. If no continueResync() call is encountered during the given timeout period it too aborts the asynchronous execution and leaves resync() with a TimeoutException.
For some groups of software which benefit from straight synchronous programing (not the client software and not the server software) such a feature would solve lots of problems for the programer who has to deal with asynchrounous-only libraries.
I believe that I have also found a solution for the main argument in lm's argumentation below. Therefore my question still stands with respect to this "enhanced" solution which I proposed: Is there anything which really makes it impossible to implement that in Dart?
The only time that you can wrap an async method in a synchronous one is when you don't need to get a return value.
For example if you want to disable the save button, save results to the server asynchronously and re-enable the save button when the job is done you can write it like this:
Future<bool> save() async {
// save changes async here
return true;
}
void saveClicked() {
saveButton.enabled = false;
save()
.then((success) => window.alert(success ? 'Saved' : 'Failed'))
.catchError((e) => window.alert(e))
.whenComplete(() { saveButton.enabled = true; });
}
Note that the saveClicked method is fully synchronous, but executes the save method asynchronously.
Note that if you make saveClicked async, not only do you have to call it using the async pattern, but the entire method body will run asynchronously so the save button will not be disabled when the function returns.
For completeness the async version of saveClicked looks like this:
Future<Null> saveClicked() async {
saveButton.enabled = false;
try {
bool success = await save();
window.alert(success ? 'Saved' : 'Failed');
}
catch (e) {
window.alert(e);
}
finally {
saveButton.enabled = true;
}
}
Yes, this is way late, but I think this is a cool feature new people should know about.
There is a way, but the Dart docs warn against it (and it's somehow "experimental", although the implications aren't really discussed).
The waitFor command.
You basically pass in an asynchronous function that returns a Future, an optional timeout parameter, and the waitFor function will return the result.
For example:
final int number = waitFor<int>(someAsyncThatReturnsInt);
The resync function cannot be implemented in Dart's current execution model.
Asynchronous execution is contagious. A synchronous function must return before any other asynchronous events can execute, so there is no way to synchronously wait for asynchronous execution.
Execution in Dart is single-threaded and event based. There is no way for the resync function to block without it also blocking all other execution in the same isolate, so the pending async operations will never happen.
To block the synchronous execution, and continue executing something else, you need to preserve the entire call stack up to that point, and reinstate it later when the synchronous operations have completed. If you have that functionality, then there are probably better ways to do things than Future and Stream :)
Also, waiting for "all async execution" isn't well-defined in an event based system. There might be a broadcast Stream emitting events coming in from the network, a periodic timer, or a receive port getting data from another isolate, or some other source of events that you can't wait for because they come from outside the isolate, or event the process. When the current isolate shuts down, it might send a final shut-down message to another isolate, so effectively the "async execution" isn't over until the isolate dies.
Using the async/await syntax, you won't get synchronous operation, but it will be easier to code the similar asynchronous operation:
function() async {
var result = await asyncFunction();
return result;
}
It won't wait for async operations that aren't reflected in the Future returned by asyncFunction, but that's the job of asyncFunction to not complete until its operations are complete.
Dart is inherently async. Trying to avoid asynchronity won't work out.
There are sync versions of some API calls for example in dart:io and in some situations it might seem simpler to use them instead but because there aren't sync versions for all methods/functions you can't avoid async entirely.
With the recent introduction of the async/await feature programming async become much simpler and the code looks almost like sync code (but it isn't).
If a call went async it stays async. As far as I know there is nothing you can do about it.
import 'package:synchronized_lite/synchronized_lite.dart';
import 'dart:async';
// Using Lock as a mixin to further mimic Java-style synchronized blocks
class SomeActivity with Lock {
bool _started = false;
Future<bool> start() async {
// It's correct to return a Future returned by synchronized()
return synchronized(() async {
if(_started)
return false;
// perform the start operation
await Future.delayed(Duration(seconds: 1));
print("Started");
_started = true;
return true;
});
}
Future<void> stop() async {
// It's also correct to await a synchronized() call before returning
// It's incorrect to neither await a synchronized() call nor return its Future.
await synchronized(() async {
if(!_started)
return;
// perform the stop operation`enter code here`
await Future.delayed(Duration(seconds: 1));
print("Stopped");
_started = false;
});
}
}
// Prints:
// Started
// Stopped
main() async {
var a = SomeActivity();
print("Hello");
a.start();
a.start();
a.stop();
await a.stop();
}
/*Since the Await statement can be used in only asynchronous methods. Then we do two methods.I thinking first we call the async method and then we constantly query the null result for the non-async method. Then we get a synchronized model. In this way, we will wait for the answer in the non-async method. Such a method comes to my mind. But as far as I can see, there is no escape from the async working model in flutter dart language. Need to get used to it.It may be unprofessional, but I wanted to share the solution that came to my mind. hope it helps.
Stock resultStockQueryByBarcodeAsync;
bool waitStockQueryByBarcodeAsyncCompleted = false;
Stock WaitStockQueryByBarcodeAsync(String barcode, int timeOut) {
CallStockQueryByBarcodeAsync(barcode);
var startTime = new DateTime.now();
while (!waitStockQueryByBarcodeAsyncCompleted) {
Duration difference = DateTime.now().difference(startTime);
if (difference.inMilliseconds > timeOut) {
throw TimeoutException("Timeout Exceeded");
}
//we must scope time. Because it can be enter endless loop.
}
return resultStockQueryByBarcodeAsync;
}
void CallStockQueryByBarcodeAsync(String barcode) async {
waitStockQueryByBarcodeAsyncCompleted = false;
resultStockQueryByBarcodeAsync = null;
var stock = await StockQueryByBarcodeAsync(barcode);/*your target async method*/
waitStockQueryByBarcodeAsyncCompleted = true;
resultStockQueryByBarcodeAsync = stock;
}
In my case, I had to initialize the database connection from constructor. I am pretty new in Flutter and I don't know what are the best practices right now. But, here is what I did.
class Storage {
late Database database;
Storage() {
getConnection().then((value) => database = value);
}
Future<Database> getConnection() async {
return await openDatabase('ims.db');
}
}
All I have done, is used the callback method to assign the value when the value is available.
Here's a solution based on staggering the start of the async function with start times at least 1 second apart, when calls come in almost simultaneously.
Steps:
Use the lastKnownTime to calculate the delta, where the initial value is 0
Once the delta is not some huge number, you know it's a duplicate call.
class StartConversationState extends State<StartConversationStatefulWidget> {
#override
Widget build(BuildContext context) {
_delayPush(); // this is the call that gets triggered multiple times
}
int lastKnownTime = 0;
int delayMillis = 3000;
_delayPush() async {
delayMillis += 1500;
await new Future.delayed(Duration(milliseconds: delayMillis));
int millisSinceEpoch = new DateTime.now().millisecondsSinceEpoch;
int delta = millisSinceEpoch - lastKnownTime;
// if delta is less than 10 seconds, means it was a subsequent interval
if (delta < 10000) {
print('_delayPush() , SKIPPING DUPLICATE CALL');
return;
}
// here is the logic you don't want to duplicate
// eg, insert DB record and navigate to next screen
}