Note: This is a follow-up question of https://stackoverflow.com/questions/32536037/flux-store-collection-by-criteria-vs-single-item but it is independent to understand and answer.
Imagine we have an application for managing (CRUD) Tasks. One operation is a Task editing.
First the edit view loads the Task using an action creator that asynchronously fetches it from the server and dispatches TASK_LOAD_SUCCESS event together with the Task payload. Next a Task Store stores the Task and emits a change event so that the edit view can read it and fill the form.
When the user submits the form the changes should be saved and the edit view should be closed.
On the submit the edit view tells action creator to asynchronously save the Task. On AJAX success the TASK_SAVE_SUCCESS is dispatched (to the Task store).
Q1: What should the Task Store do? Should it update its internal flag that a task has been saved then emit the change event and then the view should read that flag from the store and close itself if it is true?
Q2: Should the Store find the Task in the collection of the previously loaded Tasks and update it there? Other Tasks in the collection will remain stale (see Q2 in https://stackoverflow.com/questions/32536037/flux-store-collection-by-criteria-vs-single-item).
Q3: What if we edit the Task again? The Store still has the flag that the Task has been successfully saved and it closes itself immediately. But it was from the previous save. How to deal with it?
Simmilar problem arises if we want to delete a Task. We use an optimistic locking and therefore we must first read the Task from the server then show the confirmation dialog and finally delete the Task on the server (providing ETag from the first read).
Q4: How to use the store to signal that the Task has been loaded for the deletion? During this AJAX call there might another asynchronous read operation become complete and it would clash with this one. Should there be a separate Store for a Task deletion?
Q5: This is same as Q1. After the deletion how to tell the view that it is done so it can close the confirmation dialog?
Q1-Q3: you may store an edit_timestamp in TaskStore and open_timestamp for confirmation dialog. On emitChange you may compare if edit_timestamp > open_timestamp.
Q4: you may cache request Promise for each taskId on fetch request. So instead doing the same request twice (read/delete fetch for the same taskId), you may subscribe on the existed Promise. That allow you to keep only the single instance of task, and I hope avoid Q5 problems:
//To imagine how to arrange promise-based async interaction you may look here http://mjw56.github.io/handling-asynchronous-data-flow-in-flux/index.html
var promises = {};
//Returns Promise
var asyncGetCall = function(taskId) {...}
var getTaskForDelete, getTaskForRead;
getTaskForDelete = getTaskForRead = function(taskId) {
if (!promises[taskId]) {
promises[taskId] = asyncGetCall(taskId);
}
return promises[taskId];
}
getTaskForDelete(10).then(function() {...}); //do asyncGetCall
getTaskForRead(10).then(function() {...}); // do nothing, wait for the first req results
I would like to start a service that once in awhile on all platforms has checked is there a notification to appear or not. Is there any nuget to connect all platforms or some examples?
You can use the Device.StartTimer(TimeSpan minutes) method to start a background task that will repeat after the given time span. Here is a code example:
var minutes = TimeSpan.FromMinutes (3);
Device.StartTimer (minutes, () => {
// call your method to check for notifications here
// Returning true means you want to repeat this timer
return true;
});
This is included with Xamarin Forms, so you don't need any platform specific logic.
http://iosapi.xamarin.com/index.aspx?link=M%3AXamarin.Forms.Device.StartTimer(System.TimeSpan%2CSystem.Func%7BSystem.Boolean%7D)
I think that the best that you can do is following:
Unfortunately, the way that these two platforms have evolved to handle executing background code is completely different. As such, there is no way that we can abstract the backgrounding feature into the Xamarin.Forms library. Instead, we going to continue to rely on the native APIs to execute our shared background task.
Further information for this topic can be found here:
https://robgibbens.com/backgrounding-with-xamarin-forms/
I m developping a Winjs/HTML windows Store application .
I have to do some tests every period of time so let's me explain my need.
when i navigate to my specific page , I have to test (without a specific time in advance=loop)
So when my condition is verified it Will render a Flyout(Popup) and then exit from the Promise. (Set time out need a specific time but i need to verify periodically )
I read the msdn but i can't fullfill this goal .
If someone has an idea how to do it , i will be thankful.
Every help will be appreciated.
setInterval can be used.
var timerId = setInternal(function ()
{
// do you work.
}, 2000); // timer event every 2s
// invoke this when timer needs to be stopped or you move out of the page; that is unload() method
clearInternal(timerId);
Instead of polling at specific intervals, you should check if you can't adapt your code to use events or databinding instead.
In WinJS you can use databinding to bind input values to a view model and then check in its setter functions if your condition has been fulfilled.
Generally speaking, setInterval et al should be avoided for anything that's not really time-related domain logic (clocks, countdowns, timeouts or such). Of course there are situations when there's no other way (like polling remote services), so this may not apply to your situation at hand.
I'd like to understand some details of the relations between command handlers, aggregates, the repository and the event store in CQRS-based systems.
What I've understood so far:
Command handlers receive commands from the bus. They are responsible for loading the appropriate aggregate from the repository and call the domain logic on the aggregate. Once finished, they remove the command from the bus.
An aggregate provides behavior and an internal state. State is never public. The only way to change state is by using the behavior. The methods that model this behavior create events from the command's properties, and apply these events to the aggregate, which in turn call an event handlers that sets the internal state accordingly.
The repository simply allows loading aggregates on a given ID, and adding new aggregates. Basically, the repository connects the domain to the event store.
The event store, last but not least, is responsible for storing events to a database (or whatever storage is used), and reloading these events as a so-called event stream.
So far, so good.
Now there are some issues that I did not yet get:
If a command handler is to call behavior on a yet existing aggregate, everything is quite easy. The command handler gets a reference to the repository, calls its loadById method and the aggregate is returned. But what does the command handler do when there is no aggregate yet, but one should be created? From my understanding the aggregate should later-on be rebuilt using the events. This means that creation of the aggregate is done in reply to a fooCreated event. But to be able to store any event (including the fooCreated one), I need an aggregate. So this looks to me like a chicken-and-egg problem: I can not create the aggregate without the event, but the only component that should create events is the aggregate. So basically it comes down to: How do I create new aggregates, who does what?
When an aggregate triggers an event, an internal event handler responses to it (typically by being called via an apply method) and changes the aggregate's state. How is this event handed over to the repository? Who originates the "please send the new events to the repository / event store" action? The aggregate itself? The repository by watching the aggregate? Someone else who is subscribed to the internal events? ...?
Last but not least I have a problem understanding the concept of an event stream correctly: In my imagination, it's simply something like an ordered list of events. What's of importance is that it's "ordered". Is this right?
The following is based on my own experience and my experiments with various frameworks like Lokad.CQRS, NCQRS, etc. I'm sure there are multiple ways to handle this. I'll post what makes most sense to me.
1. Aggregate Creation:
Every time a command handler needs an aggregate, it uses a repository. The repository retrieves the respective list of events from the event store and calls an overloaded constructor, injecting the events
var stream = eventStore.LoadStream(id)
var User = new User(stream)
If the aggregate didn't exist before, the stream will be empty and the newly created object will be in it's original state. You might want to make sure that in this state only a few commands are allowed to bring the aggregate to life, e.g. User.Create().
2. Storage of new Events
Command handling happens inside a Unit of Work. During command execution every resulting event will be added to a list inside the aggregate (User.Changes). Once execution is finished, the changes will be appended to the event store. In the example below this happens in the following line:
store.AppendToStream(cmd.UserId, stream.Version, user.Changes)
3. Order of Events
Just imagine what would happen, if two subsequent CustomerMoved events are replayed in the wrong order.
An Example
I'll try to illustrate the with a piece of pseudo-code (I deliberately left repository concerns inside the command handler to show what would happen behind the scenes):
Application Service:
UserCommandHandler
Handle(CreateUser cmd)
stream = store.LoadStream(cmd.UserId)
user = new User(stream.Events)
user.Create(cmd.UserName, ...)
store.AppendToStream(cmd.UserId, stream.Version, user.Changes)
Handle(BlockUser cmd)
stream = store.LoadStream(cmd.UserId)
user = new User(stream.Events)
user.Block(string reason)
store.AppendToStream(cmd.UserId, stream.Version, user.Changes)
Aggregate:
User
created = false
blocked = false
Changes = new List<Event>
ctor(eventStream)
isNewEvent = false
foreach (event in eventStream)
this.Apply(event, isNewEvent)
Create(userName, ...)
if (this.created) throw "User already exists"
isNewEvent = true
this.Apply(new UserCreated(...), isNewEvent)
Block(reason)
if (!this.created) throw "No such user"
if (this.blocked) throw "User is already blocked"
isNewEvent = true
this.Apply(new UserBlocked(...), isNewEvent)
Apply(userCreatedEvent, isNewEvent)
this.created = true
if (isNewEvent) this.Changes.Add(userCreatedEvent)
Apply(userBlockedEvent, isNewEvent)
this.blocked = true
if (isNewEvent) this.Changes.Add(userBlockedEvent)
Update:
As a side note: Yves' answer reminded me of an interesting article by Udi Dahan from a couple of years ago:
Don’t Create Aggregate Roots
A small variation on Dennis excellent answer:
When dealing with "creational" use cases (i.e. that should spin off new aggregates), try to find another aggregate or factory you can move that responsibility to. This does not conflict with having a ctor that takes events to hydrate (or any other mechanism to rehydrate for that matter). Sometimes the factory is just a static method (good for "context"/"intent" capturing), sometimes it's an instance method of another aggregate (good place for "data" inheritance), sometimes it's an explicit factory object (good place for "complex" creation logic).
I like to provide an explicit GetChanges() method on my aggregate that returns the internal list as an array. If my aggregate is to stay in memory beyond one execution, I also add an AcceptChanges() method to indicate the internal list should be cleared (typically called after things were flushed to the event store). You can use either a pull (GetChanges/Changes) or push (think .net event or IObservable) based model here. Much depends on the transactional semantics, tech, needs, etc ...
Your eventstream is a linked list. Each revision (event/changeset) pointing to the previous one (a.k.a. the parent). Your eventstream is a sequence of events/changes that happened to a specific aggregate. The order is only to be guaranteed within the aggregate boundary.
I almost agree with yves-reynhout and dennis-traub but I want to show you how I do this. I want to strip my aggregates of the responsibility to apply the events on themselves or to re-hydrate themselves; otherwise there is a lot of code duplication: every aggregate constructor will look the same:
UserAggregate:
ctor(eventStream)
foreach (event in eventStream)
this.Apply(event)
OrderAggregate:
ctor(eventStream)
foreach (event in eventStream)
this.Apply(event)
ProfileAggregate:
ctor(eventStream)
foreach (event in eventStream)
this.Apply(event)
Those responsibilities could be left to the command dispatcher. The command is handled directly by the aggregate.
Command dispatcher class
dispatchCommand(command) method:
newEvents = ConcurentProofFunctionCaller.executeFunctionUntilSucceeds(tryToDispatchCommand)
EventDispatcher.dispatchEvents(newEvents)
tryToDispatchCommand(command) method:
aggregateClass = CommandSubscriber.getAggregateClassForCommand(command)
aggregate = AggregateRepository.loadAggregate(aggregateClass, command.getAggregateId())
newEvents = CommandApplier.applyCommandOnAggregate(aggregate, command)
AggregateRepository.saveAggregate(command.getAggregateId(), aggregate, newEvents)
ConcurentProofFunctionCaller class
executeFunctionUntilSucceeds(pureFunction) method:
do this n times
try
call result=pureFunction()
return result
catch(ConcurentWriteException)
continue
throw TooManyRetries
AggregateRepository class
loadAggregate(aggregateClass, aggregateId) method:
aggregate = new aggregateClass
priorEvents = EventStore.loadEvents()
this.applyEventsOnAggregate(aggregate, priorEvents)
saveAggregate(aggregateId, aggregate, newEvents)
this.applyEventsOnAggregate(aggregate, newEvents)
EventStore.saveEventsForAggregate(aggregateId, newEvents, priorEvents.version)
SomeAggregate class
handleCommand1(command1) method:
return new SomeEvent or throw someException BUT don't change state!
applySomeEvent(SomeEvent) method:
changeStateSomehow() and not throw any exception and don't return anything!
Keep in mind that this is pseudo code projected from a PHP application; the real code should have things injected and other responsibilities refactored out in other classes. The ideea is to keep aggregates as clean as possible and avoid code duplication.
Some important aspects about aggregates:
command handlers should not change state; they yield events or
throw exceptions
event applies should not throw any exception and should not return anything; they only change internal state
An open-source PHP implementation of this could be found here.
I have a simple viewPart offering some text fields to enter parameters for a selenium test. After filling out these fields the user may start the test which approx. needs 30-45 minutes to run. I want my GUI to be alive during this test giving users the chance to do other things. I need a progress monitor.
I tried to put the selenium test in a job containing Display.getDefault().asyncExec to run it. But my GUI freezes after some seconds giving the busyindicator. The selenium does not update any other view but the progress monitor.
Is there another way to ensure that the job wont block my GUI?
Best,
Mirco
Everything executed in (a)syncExec is using the display thread and therefore blocking your UI until it returns. I suggest you use Eclipse Jobs. This will use the progress indicator that the workbench already offers out of the box.
I would suggest to split your code into code that updates the UI and the code that executes other business. Execute all of it in a separate thread, and when you need to retrieve or set some action to the UI then use the "Display.getDefault().asyncExec".
Thread thread = new Thread("Testing") {
// some shared members
public void run() {
someBusiness();
// or use syncExec if you need your thread
// to wait for the action to finish
Display.getDefault().asyncExec(new Runnable() {
#Override
public void run() {
// UI stuff here
// data retrieval
// values setting
// actions trigging
// but no business
}
});
someBusiness();
};
thread.start();