Flux Application Architecture
facebook.github.io
facebook.github.io
1. All data should be kept in stores. You may have some local component state, but it shouldn't be anything you want to persist if the component is unmounted. We have tried using state several times, and always go back to keeping it in singleton stores. It also works better with the action-dispatcher pattern.
2. Now all your data is in stores, but how do you get it into the specific component that needs it? We started with large top level components which pull all the data needed for their children, and pass it down through props. This leads to a lot of cruft and irrelevant code in the intermediate components. What we settled on, for the most part, is components declaring and fetching the data they need themselves, except for some small, more generic components. Since most of our data is fetched asynchronously and cached, we've created mixins that make it easy to declare which data your component needs, and hook the fetching and listening for updates into the lifecycle methods (componentWillMount, etc).
3. Actions don't have callbacks, as they are by design fire-and-forget. So if you need to be notified when some item has finished being created, for example, you need to listen for the follow up action that the CREATE action fires (yeah, actions firing actions, a bit ugly). Even then, how do you know that CREATE_COMPLETED action correlates to the CREATE that you fired, and not another? Well, actions also come with a payload, so what we ended up doing was passing a context object into the payload and plumbing it all the way down into the CREATE_COMPLETED and CREATE_FAILED actions. Being really strict about actions is a major reason why Flux has scaled well for us.
Perhaps promises would be a good solution to your action spaghetti? They can deliver progress, completed, and error messages targeted to the place that cares about them, rather than passing context through the entire rest of the system.
1) User clicks a button to favorite an object, lets say an Article 2) View (component) listens for the click handler and calls ArticleActions.favorite(articleID) 3) ArticleActions.favorite fires a preliminary ARTICLE_UPDATE action, for any stores that want to update optimistically 4) ArticleActions.favorite calls into the ArticleDAO (the data access abstraction) via ArticleDAO.update, a function that takes an ID, some updates, and two callbacks, success and error. 5) In the success callback, ArticleActions.favorites fires off ARTICLE_UPDATE_COMPLETED, along with the updated article object 6) In the error callback, ArticleAction.favorites fires off ARTICLE_UPDATE_FAILED, along with the error 7) Stores listen for the COMPLETED and FAILED actions to know when to update their data, including stores that show error notices
As you can see,the action layer itself should not really be accepting callbacks/returning promises. It might be easier, however, if the DAO layer returned promises, and this is something we have talked about migrating to.
The dispatcher?
e.g: ArticleActions.favorite fires off an ARTICLE_UPDATE action, the ArticleStore receives this and does the appropriate ArticleDAO asynchronous call and when done emits a change event to update any Views.
I.e: they update themselves (eagerly, hence the name) based on changes from the data-access layer. They're able to choose themselves which transformations to do on the data in order for views/components to query them efficiently.
Is there no scenario in which you would want to show (next to the Favorite button) that favoriting failed but not care about other article-update failures?
The importance of keeping actions fire-and-forget is that data must live in the store; if actions have callbacks, the views would be using state to keep themselves updated with data that should be in the store.
As you say, the models live in stores, so building on my other comment, you would have an ArticleStore that is responsible for providing access to and caching all of the Article objects. As a rule, if you want to mutate data, you do so by calling an action (ArticleActions.update, for example). See my other comment for how the update flow works: https://news.ycombinator.com/item?id=7721381
If you want to fetch data, you go to the store (ArticleStore.getByID, or ArticleStore.query). The ArticleStore will then call into the ArticleDAO (data access abstraction) to fetch data asynchronously, and when it returns the ArticleStore incorporates the data into its cache and "informs", which is basically a pub-sub push (the views/components subscribe to the stores they want to get data from).
I find it mind-blowing: do you only use tools made inside Facebook? Do you develop these frameworks or just use them? If you don't know Backbone which frameworks did you learn of this kind? I'm curious.
Tools such as jQuery and Backbone seem ubiquitous on the web because they are a perfect fit for addressing common problems in the webpage/ajax/dynamic content area. However, there is a sizeable group of developers who work with rich applications, intranet portals, line-of-business apps that required significantly more structure and skeleton than Backbone/Marionette provide. This importance of using an overarching "framework" rather than a "library" increases in proportion to the team size and the code surface area.
In my case, we started with YUI and then migrated to ExtJS. This was before Backbone existed, although it would not have made a difference. In recent years we have evaluated Angular and Ember but did not find compelling reasons to migrate (for a greenfield project the choice may be different, but migrating significant apps carries a significant cost).
Both YIU and ExtJS provided everything we needed under one roof, and there was no use for jQuery or Backbone&co. The downsize of a mega-framework like ExtJS is the overhead - it is ill suited for a simple app. Couple months ago I started using Backbone & co for small isolated mini-apps, but I cannot wait to find a suitable replacement because it feels clumsy and backwards.
To be clear, I am not saying it is impossible to build complex apps primarily driven by Backbone - I know people who have done that (often to their own peril). I am saying that it is entirely possible to be a Facebook-level engineer working on complex applications and have zero experience with Backbone as it is great and solving problems you do not have.
I've really enjoyed working with knockout (so much so I've even written "components" with it (ajax file handler a la gmails but with previews etc) and I've found it to have just enough structure for the stuff I need on the front side.
It surprises me that it's not more popular than it is but maybe there are reasons for that I simply don't know or understand.
The one plus of the event approach I can think of is that if one component causes new data to load on the client (an article is updated), none of the components that rely on that article will show stale data - that is to say, it's extraordinarily difficult for components looking at the same data to ever be out of sync.
MyComponent = React.createClass({
componentDidMount: function(){
this.props.collection.on('request', function(){
this.setState({loading: true});
}.bind(this));
this.props.collection.on('sync', function(){
this.setState({loading: false});
}.bind(this));
this.props.collection.fetch();
},
});
If you don't want the component to own the collection (if you want to share a collection between multiple components), just pass it as a prop from a parent component; otherwise you can instantiate a new collection on getDefaultProps.
[1] https://github.com/swannodette/om [2] https://github.com/swannodette/om/wiki/Cursors
The example is a bit contrived, but hopefully you see the problem? Especially consider if there are a couple more layers between App and whatever leaf component is rendering something,
https://github.com/wingspan/wingspan-cursor/blob/master/js/C...
Alternately, you can pass multiple cursors[1] to a component. In your example, you could pass each ArticleItem a cursor pointing to the right article, and a more general cursor pointing to the author store. This would eliminate the hard coding of "authors", but it would require ArticleList to pass along some sort of "additional-data" cursor.
[1] https://github.com/swannodette/om/wiki/Cursors#using-multipl...
_handleClick: function() {
this.setState({expanded: true});
},
render: function() {
return (
<div className={this.state.expanded ? 'expanded' : 'compact'}>
<button onClick={this._handleClick} />
</div>
);
}I'd love to hear some more about the design of your data store: are these cursors basically paths to data? Are they similar to Om's concept of cursors? https://github.com/swannodette/om/wiki/Cursors
https://github.com/wingspan/wingspan-cursor/blob/master/js/C...
How does this interact with shouldComponentUpdate? Generally it seems that when moving data out of props/state, it's harder to take advantage of the performance hooks that React gives you because you don't have the old and new data to compare when rerendering.
In the dispatcher, it looks like each callback is executed in order if you use waitsFor, but they do not wait for any aysnc behavior to complete.
Do you have stores register listeners with each other during their dispatcher-fired callbacks?
Adding another question; do you try to prevent stores from knowing about each other at all?
One thing that raise my eyebrows about this article though is how says that Flux eschews MVC, and then goes on to say that it has stores that are "somewhat similar to a model", and "the dispatcher exposes a method that allows a view to trigger a dispatch to the stores", which, as far as classic MVC goes, is the exact definition of a controller. What they call controller-view is, imho, just an implementation detail within the view layer: in classic MVC, views were responsible for subscribing to model observables, or, as the article puts it, it "listens for events that are broadcast by the stores that it depends on".
When talking about this architecture with the Mithril community, I find that referring back to the MVC pattern makes it even easier to reason about the flow of data: it's basically M -> V -> C -> M ...
It's unfortunate that the general understanding of the MVC pattern became misunderstood over the course of many frameworks. The whole point of design patterns should be that we could just name them for people to "get" it, rather than having to reinvent new ways of naming things and having everyone relearn the nomenclature.
One thing that threw me off was startComputation() / endComputation(). It seems you have to be explicit about when properties are being updated for views to update. I worry this might be error-prone vs react.js - if you forget an endComputation(), or an exception occurs outside of a try/finally, your views will freeze forever, no?
https://github.com/lhorie/mithril.js/issues/68
Generally, you only need to manually call start/endComputation when integrating to 3rd party code that needs to update Mithril managed bindings.
In the latest release, I tweaked the rendering aggressiveness to allow redraws after exceptions in event handlers, and I updated the docs wrt `try/finally` blocks in integration scenarios.
One other thing people could do that I don't think should go in Mithril core is to call m.redraw inside window.onerror.
Of course, I'm open to other suggestions, if you have any.
I imagine this could have a significant performance impact, but don't know the mithril internals well enough to say.
EDIT: Thinking about it, the virtual DOM must be re-constructed on every redraw since the props aren't tied to a view. This probably wouldn't work well then.
As far as redrawing impact goes, Mithril does rate-limiting, so even attempting to subvert m.prop to force it to spam redraws should still perform ok.
It sounds to me like Flux goes with a model layer that handles all state (internal to the app and not), pushes most binding between pieces of the UI to the view layer, and uses a thin controller layer to mediate the other two layers and coordinate parts of the model layer with overlapping concerns.
I once had the epiphany that an entire web app is pretty much a whole bunch of nested or concatenated MVCs. I think the confusion arrives when people assume that there is one-and-only-one MVC and try to shoehorn all the parts of a new workflow into that rigid structure. I don't think that the proliferation of acronyms purporting to be something different helps with understanding.
To address your second comment: One distinction between the dispatcher and controllers in the MVC framework is that the dispatcher generally doesn't contain any business logic. It's essentially a hub for passing messages through: all sources of changes get funneled through the dispatcher, and the stores listen for those events, but the dispatcher doesn't actually modify or initiate the events. In my talk, I described it as the traffic controller of the system, which might be a more appropriate description.
As things get more complicated, the dispatcher may do more things at the framework level (Bill refers to the dispatcher handling dependencies between stores), but it stays separate from the application logic. In other words, we use the same dispatcher for multiple apps, so it plays a different role from the controller.
Hope that clarifies a bit, I'll see if we can make this distinction clearer in the documentation :)
I could figure out jQuery back in the day pretty easily.
Then we started using Angular and I have tried and tried to understand all its concepts my brain just couldn't keep track of list of: dom transclusion, scopes, controllers, directives, services, dependency injections and so on. More experienced JS developers loved and had no problem picking it up.
But after watching a few videos about React.js and worked through tutorials, I really started to understand better. I really like the concepts and how this library is put together. So far this is looking really good.
View receives event-------\..../----views render new state
Controllers handle event---\../--controllers mediate new state
Models react to new data----\/--models update controllers
I'd be really interested to see how Flux would augment statecharts...
Direct Link: http://youtu.be/nYkdrAPrdcw?t=10m21s
EDIT: So far, the video is much more helpful to me in terms of bringing the concepts of Flux to life. Jing does a terrific job explaining in my opinion.
Is there a name for such a pattern?
http://en.wikipedia.org/wiki/Continuation#In_Web_development
Anyone has a more detailed explanation?
http://www.quora.com/Pete-Hunt/Posts/Facebooks-React-vs-Angu...
http://www.reddit.com/r/javascript/comments/1oo1y8
Though they're both from a member of the React team and might be a little biased, they were very informative.
[1] http://www.confreaks.com/videos/3221-mwjs-be-predictable-not...
Flux is just Facebook's internal usage pattern. Doesn't really have an Angular equivalent other than the somewhat nebulous Angular best practices.
Technically, this is done by simply using an unique dirty bit, which triggers the recomputation of the rendered scene. Which is fast enough in practice, and even supports unchanged hints to reduce the costs of expensive DOM updates.
This is eerly similar to 3d scene rendering. The unchanged hints are the equivalent of viewport clipping, though require more work from the coder to setup right.
For instance: the directives `ng-show` and `ng-hide` simply apply CSS styles. As far as I know it's not possible to completely remove an Angular component from the DOM and then replace it later (without getting too deep into imperative JS). However, that's trivial to do in React.
Angular chokes on large data sets, probably because it's doing everything directly on the DOM. React handles huge numbers of records effortlessly. Getting comparable performance with Angular requires much more imperative voodoo than I'd like out of an ostensibly declarative system.
I also find it harder to reason about Angular's data flow, since there are many ways to pass things around: nested scope, isolate scope, sibling scope, same scope, dependency injection to name a few. Finding where a particular handler or value comes from is sometimes quite the hunt. With React, it's all pretty much self-contained.
Wouldn't ng-if be appropriate for this?
What I'm wondering, given the unidirectional flow and the design of flux as a whole: would Integration Testing be needed at all anymore?
I mean, there's no state outside of the component that could possibly influence the component's consistency. Therefore, all needed testing could be done by simply unit-testing all components in isolation.
That sounds pretty huge.
It'd probably be worth having a barebones starter kit though.