Instance Initializers in Emberjs - javascript

We are getting through the long slog of updating our ember-cli application to its latest iteration. We fell very much behind. I am at the stage where instance initializers have been introduced and I am getting the feeling this is going to break the way in which I have implemented a certain initializer currently.
export function initialize(container, application) {
var store = container.lookup('store:main');
// We need a basket to be present when
// the application loads. Wait for this
// to happen before continuing.
application.deferReadiness();
store.findOrCreateRecord('order', basketToken).then(function(basket) {
container.register('basket:main', basket, { instantiate: false });
application.inject('controller:basket', 'model', 'basket:main');
// Let the application know we have
// a basket and can continue.
application.advanceReadiness();
});
}
What is now recommended is that I split this up into a "normal" initializer to register the basket object and an instance initializer to grab the store and make the call to our API server. Doing this however I would not have access to the registry within the instance initializer to register the returned object from my promise which I would then inject into my controller. I assume I am thinking about this all wrong, but I have not been able to wrap my head around it. Any suggestions how I should by updating this?

I think it's reasonable to post #tomdale explanation here as an answer to help others with understanding initializers.
#tomdale: "It's not possible to defer app readiness in an instance initializer, since by definition instance initializers are only run after the app has finished booting.
Sidebar on the semantics of application booting: "App readiness" (as in, deferReadiness() and advanceReadiness()) refers to whether all of the code for the application has loaded. Once all of the code has loaded, a new instance is created, which is your application.
To restate, the lifecycle of an Ember application running in the browser is:
Ember loads.
You create an Ember.Application instance global (e.g.
App).
At this point, none of your classes have been loaded yet.
As your JavaScript file is evaluated, you register classes on the
application (e.g. App.MyController = Ember.Controller.extend(…);)
Ember waits for DOM ready to ensure that all of your JavaScript
included via <script> tags has loaded.
Initializers are run.
If you need to lazily load code or wait for additional setup, you can call deferReadiness().
Once everything is loaded, you can call advanceReadiness().
At this point, we say that the Application is
ready; in other words, we have told Ember that all of the classes
(components, routes, controllers, etc.) that make up the app are
loaded.
A new instance of the application is created, and instance
initializers are run.
Routing starts and the UI is rendered to the
screen.
If you want to delay showing the UI because there is some runtime setup you need to do (for example, you want to open a WebSocket before the app starts running), the correct solution is to use the beforeModel/model/afterModel hooks in the ApplicationRoute. All of these hooks allow you to return a promise that will prevent child routes from being evaluated until they resolve.
Using deferReadiness() in an initializer is an unfortunate hack that many people have come to rely on. I call it a hack because, unlike the model promise chain in the router, it breaks things like error and loading substates. By blocking rendering in initializers, IMO you are creating a worse experience for users because they will not see a loading or error substate if the promise is slow or rejects, and most of the code I've seen doesn't have any error handling code at all. This leads to apps that can break with just a blank white screen and no indication to the user that something bad has happened."

Related

How to understand the effectscope in Vue?

The offical RFC
There is a example for effect
function createSharedComposable(composable) {
let subscribers = 0
let state, scope
const dispose = () => {
if (scope && --subscribers <= 0) {
scope.stop()
state = scope = null
}
}
return (...args) => {
subscribers++
if (!state) {
scope = effectScope(true)
state = scope.run(() => composable(...args))
}
onScopeDispose(dispose)
return state
}
}
I know what it will do, it will force all components to calculate only once when we use useMouse API
But I can't understand the concept of effect, and how does it work?
Espeically some APIs for effect like getCurrentScope. I tried to see the return values of getCurrentScope, but i have gained nothing.
Please help me!
effect is a common term used in reactive frameworks (both VueJS and React) to refer to (I believe) side effect. If you are familiar with functional programming, you probably already know that it is called side effect because it is not "pure function", because it mutates shared or global state.
Ignore the academic terminology, effect in these systems merely refers to any application defined method that does something bespoke, like
const foo = () => {
// I do something bespoke
}
The meaning of effect is really that broad. What your method actually does in its body does not matter to the framework. All that the framework knows is foo does some unstructured things. What VueJS does in extra, is to monitor, through its reactivity system, if your effect depends on any reactive data. And if it does, VueJS will re-run your effect every time the data it depends on changes.
effect (or side effect) isn't something bad or special or advanced. In fact, your application is all about making effects/side effects. For example, the commonest effect in a VueJS application is DOM manipulation. It is so common that VueJS extracts it into a different abstraction: template. Behind the scene, templates are compiled to render functions - which look a lot like the foo above - that get re-evaluated every time some dependent reactive data changes. That is how VueJS keeps your UI up to date.
Another extreme of common effects are those really bespoke ones, e.g. you just want to do some old fashion imperative things (like the jQuery style) whenever your data changes. And VueJS let you do it through watchEffect: you give VueJS a lambda, and VueJS will blindly call it every time its dependency changes without asking what it is doing.
VueJS discovers your dependency on reactive data by running your effect. As long as your effect accesses any reactive data (say, yourState.bar) during its execution, VueJS will notice that and record a dependency of your effect on yourState.bar
In its essence, the reactivity system is just the modern version of the good-old observable/subscriber pattern. Reactive states are the observables, and effects are the subscribers/observers. If you look beyond the magic layer and think of VueJS in the form of a subscriber pattern, there is one issue it cannot avoid: whenever you have subscribe, you will have to deal with unsubscribe, otherwise you will have memory or resource leaks simply because you keep holding on to subscribers (they in turn hold on to other things) and nothing can be freed. This unsubscribe part is what the RFC calls "effect dispose".
Typically you will have two challenges when dealing with this unsubscribing/disposing/cleaning up/cancelling business:
deciding when to unsubscribe/dispose
knowing how to unsubscribe/dispose
In a typical reactive framework, both of the above are application's responsibility. As the application dev, you are the only one who knows when a subscription is no longer needed and how to reverse the additional resource allocation (if any) you made at the time of creating the subscription.
But in a typical VueJS app you rarely need to manually deal with any kind of cleanup (stopping the DOM patching, watch, or computed etc). That is because VueJS takes care of it automatically. The reactive effects established within a component's setup method will be automatically disposed (whatever needed for a proper clean up) when the component is unmounted. How does that happen? Let's just say some other magic exists inside VueJS to associate all your effects with the life cycle of the corresponding component. Technically, as the RFC says, that magic is effectScope.
Conceptually, each component creates an effectScope. All your effects defined inside component setup method will be associated with that scope. When the component destroys, VueJS automatically destroys the scope, which will clean up the associated effects.
The RFC proposes to make effectScope a public api so that people can use it without using a VueJS component. This is possible because Vue3 is built with modularization. You can use Vue's reactivity module without using the entire VueJS. But without the underlying effectScope, you then have to manually dispose all your effects.
What would making a coffee look like in code?
snowingfox.getCupsOutOfCupboard();
snowingfox.getCoffeeOffShelf();
snowingfox.getMilkOutOfFridge();
snowingfox.boilingWater();
// ...
Now imagine each morning I wake up and have a coffee. You could say I'm making
a coffee in reaction to waking up. How would I run this code repeatedly in
response to an isMorning variable becoming true?
This is what effect solves in Vue 3. It wraps around a chunk of
code that should be executed in response to reactive data being changed. In practise you most likely won't use effect directly, and instead rely on
things like computed and watchEffect (which use effect in their
implementations).
In short: effect is one part of Vue's reactivity system and is Vue's way of
marking and locating code that should be re-run in response to data updates.
Docs: https://v3.vuejs.org/guide/reactivity.html
Course: https://www.vuemastery.com/courses/vue-3-reactivity/vue3-reactivity/
Here's how the initial code could be implemented to be reactive:
import { ref, watchEffect } from 'vue';
const isMorning = ref(false);
watchEffect(() => {
if (!isMorning.value) return;
snowingfox.getCupsOutOfCupboard();
snowingfox.getCoffeeOffShelf();
snowingfox.getMilkOutOfFridge();
snowingfox.boilingWater();
});

Angular 2 - single service provider across multiple browser windows

I am trying to supply an alert once a task is complete - the user may be in any of multiple pages at the time. The alert should display to all pages.
I am using a service implementing BehaviorSubject
The provider for which is in my app.component.ts page - single instance
In my app.component.html I have the two components, one the alert, the other that fires the alert.
<alert></alert>
<submit-service></submit-service>
The service emits to the alert component which renders the alert.
This works fine, but only ever on the page that submits the service (not to any other page) - submission function is also in the alert component.
submit-service utilises
public emit: BehaviorSubject<model> = new BehaviorSubject(new model());
Once the event is completed it then fires off this.emit.next(_model);
In the alert component I subscribe to the event
ngOnInit(): void {
this.service.emit.subscribe(data=> {
this.fireAlert(data);
}
});
}
so I suppose the main question is, how do I have a single service subscribed across multiple instances, across multiple pages?
EDIT 1
Apologies for the ambiguity, by page I mean separate browser window or tab i.e. window.open
Just in case others are having this same issue - there is in fact an answer.
Using global storage events allows the traversing of information across all browser tabs/windows.
In the service instead of using BehaviourSubject, the service updates or 'emits' the data to a local storage item, event listener utilising a HostListener() decorator can then listen for these udpates - which listens across all windows and tabs.
Example:
#HostListener('window:storage', ['$event'])
onStorageChange(ev: StorageEvent) {
console.log(ev.key);
console.log(ev.newValue);
}
See here for further information: Storage events
So there's a couple things at play here. The first is the service that let's your application know that it's time to display the alert. It sounds like you already have that, but for simplicity sake I would make sure you are declaring that in a forRoot() context. I won't go into a crazy amount of detail regarding this topic, but essentially you need to make sure that your service is running in the root context. If you start lazy loading modules, and then subscribing to your service from within the lazy loaded module, it will create it's own Dependency Injection context and you'll start pounding your head against the table wondering why your service isn't updating. (been there :)
The next thing to look at is where you want to render your alert. You'll likely want to use the ComponentFactoryResolver to render your alert in the highest level component you can think of that makes sense. Basically (if I understand your need correctly), you need this to be within the same component, or higher as all of the pages you want to have the alert rendered to. For example I am working on an application that has a dashboard where we have a ComponentFactoryResolver that renders any and all modals we might need throughout the application. This allows us to call modals from anywhere within the dashboard using, like you, a behavior subject that activates the modals. Here's a great article on using the ComponentFactoryResolver.
Update 1
So after realizing that "page" was actually a new browser window this method won't necessarily work. Using BehaviorSubjects will only update within the application context, so opening a new window creates a new application context, i.e. killing the BehaviorSubject of being a viable candidate to make this work. You'll need to have a service that is not instance specific. Web sockets as you mentioned would be a good alternative.
It is worth noting though that if it's possible to refactor the code to open modals instead of new windows, you could maintain the integrity of your Dependency Injection tree, and then use BehaviorSubjects to achieve this. Otherwise you'll need something outside of the application that is maintaining state.

Sharing fetch handler logic defined across multiple service workers

Following This Discussion, Where there is a comment that speaks of
patching a Fetch
by overriding self.fetch, self.XMLHttpRequest, and self.caches (for
cache.add/addAll)? It would seem these would let you intercept network requests and manipulate responses before the main SW script sees them.
I have been looking out for any documentation that speaks of such stuff, and can not seem to find any.
In a scenario where I simply need multiple service workers to co exist in a single scope,
After having an importScripts in one of them to import the event handlers of the other,
How exactly would I patch a fetch / avoid a fetch race / have both the fetch handlers work?
There's a few things to cover here:
Multiple Service Workers in a Single Scope
There can only be one active service worker for a given scope. If you attempt to register two different service worker scripts that each have the same scope, the second registration will trigger the service worker update flow:
// There's an implied default scope of '/'.
// See https://stackoverflow.com/a/33881341/385997
navigator.serviceWorker.register('/sw1.js');
// If called later on, this will trigger the update flow.
// You'll only end up with one of the two being active.
navigator.serviceWorker.register('/sw2.js');
The exact timing for when sw2.js will activate and take control over any existing clients depends on whether you're using self.skipWaiting() and self.clients.claim() inside of sw2.js. Once sw2.js activates, sw1.js will be marked as redundant.
Another way of asking what I think is the same question is whether you can have multiple service workers controlling the same client page at the same time. The answer is no, you can have at most one service worker controlling any client page, and only that service worker will be able to respond to fetch events originating from the page.
Using importScripts to Share Common Handlers
Instead of attempting to register multiple service workers with the same scope, using importScripts() to pull in logic that's defined in a different JavaScript file sounds like a reasonable approach. There are a few things to keep in mind when using importScripts() in this fashion:
importScripts() needs to be called during the initial startup execution of your service worker code, not inside an event handler. I.e. "lazy-loading" of importScripts() is not supported.
importScripts() executes all the of the code inside of the file(s) synchronously, one by one, in the order in which they're listed. You can have multiple importScripts(), or importScripts() inside of files that are themselves imported, and they'll all execute in a defined order.
Inside an imported script, self will be set to the same ServiceWorkerGlobalScope that would be used if the code were in the top-level service worker. I.e., there's no difference between calling self.addEventListener() inside of an imported script or inside of the top-level service worker.
(This isn't directly related to your question, but it's good to know:) The files referenced via importScripts() will be cached by default, using the same mechanism that's built in to the browser for caching your top-level service worker file. While there are some changes to the service worker specification underway to change this, as of right now, those cached importScripts() files will be used indefinitely as long as their filenames don't change. So a best practice is to either include a version number or a hash in the file names of anything referenced with importScripts().
Multiple fetch Event Handlers
What happens when you have multiple calls to self.addEventListener('fetch')?
From the previous section we know that it's not relevant whether those multiple calls originate inside of an importScripts() resource or the top-level service worker. They both operate on the same global scope.
The behavior is well-defined: when a client page makes a request, it will trigger the fetch handlers of the controlling service worker one by one, in the order in which they were registered, until the first call is made to event.respondWith(). One one fetch event handler calls respondWith(), no other fetch event handlers will be triggered, and it's the sole responsibility of that handler to (eventually) return a Response to the client page.
Since the order in which your self.addEventlistener('fetch') calls matter, make sure that you list the files in your importScripts() in an appropriate order, and either include your call to importScripts() before or after you define any fetch event handlers in your top-level service worker, depending on which you want to take precedence.
While you can use conditional logic to determine whether or not to call event.respondWith(), that logic can't be asynchronous, since the service worker won't wait to see whether event.respondWith() is called. It needs to synchronously move on to the next event handler (assuming there is one).
So inside a fetch handler, you can use conditional logic like
// This can be executed synchronously.
if (event.request.url.endsWith('.html')) {
event.respondWith(...);
}
but you can't use conditional logic like:
// caches.match() is asynchronous, and the service worker will have
// moved on to the next `fetch` handler before it completes.
caches.match('index.html').then(response => {
if (response) {
event.respondWith(...);
}
});
There's a live code sample that you can explore if you want to see the multiple-handlers behavior for yourself.

Dependency Injection vs. Managed Dependencies vs. Global Object

I'm working within a Javascript + BackboneJS (an MVC framework) + RequireJS framework, but this question is somewhat OO generic.
Let me start by explaining that in Backbone, your Views are a mix of traditional Views and Controllers, and your HTML Templates are the traditional MVC Views
Been racking my head about this for a while and I'm not sure what the right/pragmatic approach should be.
I have a User object that contains user preferences (like unit system, language selection, anything else) that a lot of code depends on.
Some of my Views do most of the work without the use of templates (by using 3rd party libs, like Mapping and Graphing libs), and as such they have a dependency on the User object to take care of unit conversion, for example. I'm currently using RequireJS to manage that dependency without breaking encapsulation too much.
Some of my Views do very little work themselves, and only pass on Model data to my templating engine / templates, which do the work and DO have a dependency on the User object, again, for things like units conversion. The only way to pass this dependency into the template is by injecting it into the Model, and passing the model into the template engine.
My question is, how to best handle such a widely needed dependency?
- Create an App-wide reference/global object that is accessible everywhere? (YUK)
- Use RequireJS managed dependencies, even though it's generally only recommended to use managed dependency loading for class/object definitions rather than concrete objects.
- Or, only ever use dependency injection, and manually pass that dependency into everything that needs it?
From a purely technical point of view, I would argue that commutable globals (globals that may change), especially in javascript, are dangerous and wrong. Especially since javascript is full of parts of code that get executed asynchronously. Consider the following code:
window.loggedinuser = Users.get("Paul");
addSomeStuffToLoggedinUser();
window.loggedinuser = Users.get("Sam");
doSomeOtherStuffToLoggedinUser();
Now if addSomeStuffToLoggedinUser() executes asynchronously somewhere (e.g. it does an ajax call, and then another ajax call when the first one finishes), it may very well be adding stuff to the new loggedinuser ("Sam"), by the time it gets to the second ajax call. Clearly not what you want.
Having said that, I'm even less of a supporter of having some user object that we hand around all the time from function to function, ad infinitum.
Personally, having to choose between these two evils, I would choose a global scope for things that "very rarely change" --- unless perhaps I was building a nuclear powerstation or something. So, I tend to make the logged in user available globally in my app, taking the risk that if somehow for some reason some call runs very late, and I have a situation where one user logs out and directly the other one logs in, something strange may happen. (then again, if a meteor crashes into the datacenter that hosts my app, something strange may happen as well... I'm not protecting against that either). Actually a possible solution would be to reload the whole app as soon as someone logs out.
So, I guess it all depends on your app. One thing that makes it better (and makes you feel like you're still getting some OO karma points) is to hide your data in some namespaced singleton:
var myuser = MyApp.domain.LoggedinDomain.getLoggedinUser();
doSomethingCoolWith(myuser);
in stead of
doSomethingCoolWith(window.loggedinuser);
although it's pretty much the same thing in the end...
I think you already answered your own question, you just want someone else to say it for you : ) Use DI, but you aren't really "manually" passing that dependency into everything since you need to reference it to use it anyways.
Considering the TDD approach, how would you test this? DI is best for a new project, but JS gives you flexible options to deal with concrete global dependencies when testing, ie: context construction. Going way back, Yahoo laid out a module pattern where all modules were loosely coupled and not dependent on each other, but that it was ok to have global context. That global context can make your app construction more pragmatic for things that are constantly reused. Its just that you need to apply that judiciously/sparingly and there need be very strong cases for those things being dynamic.

Memory versus Speed in Javascript Web Applications

In my former web applications, when the user had a transition from one "page" (no page reload, just a new div) to another, I just hid the one div for later use and created + showed the new one. When the user returned from address management to events management, I only needed to hide the current div and re-show the already used one. Of course, this needs memory, but is faster.
In my new web application, I use Backbone.js, Require.js and jQuery. All my modules are AMD (jquery 1.7.1, backbone.js 0.5.3-optamd3, ...).
After reading Derick Bailey's interesting blogs (http://lostechies.com/derickbailey/2011/09/15/zombies-run-managing-page-transitions-in-backbone-apps/) I now clean my divs before having a transition to a new "page" and re-create it in case the user returns to it.
Likewise, regarding requirejs amd modules I used to have a speed over memory strategy: The heart of my web app navigation is in my only router object. If the user selects a "page"/feature for the first time, I load the amd module (it's a backbone.js view object) for it and all its dependencies with the require command and store this resulting view object (with its model object) for later use in an array in the router object. When the user comes back, I take the the stored view object and re-render the view.
I guess I will switch from this behaviour also into always reloading the module (from cache), but I am unsure.
In order to go the best way, I want to get a better understanding and want to ask 2 questions:
I have 5 AMD modules. When the user needs a feature, I load and execute a module and get a backbone.js view object as a result, which I store in an array in my router object. Every AMD module has Backbone.js (AMD version) as a dependency. When the user has visited all 5 "pages" and all my 5 view objects are stored in my array, do I have 5 copies of backbone.js in my browser memory since every backbone.js dependency is fetched from cache and executed anew, or has the garbage collector removed it?
How do other web application developer think about this speed over memory strategy?
CONTINUED
Today I found a similar question on stackoverflow (http://stackoverflow.com/questions/7866971/how-does-amd-specifically-requirejs-handle-dependancies-across-multiple-module).
The answer was: "It will only be loaded once, both of the above modules will get the same module value ...".
So it seems that it is not so bad to store the results of already loaded+executed amd modules for later use.
Wolfgang
Cache everything (that might be used more than once) . (write to Canvas/ImageData).
You should only have 1 copy of the framework in memory. If you're concerned there's more, rewrite it to force all AMDs to use a single source Backbone.
Memory is speed.
If you want better speed:
extern your js files for browser caching.
Use localstorage
Do most computation at the user
Minimize the server and streamline request flow

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