Lightning Web Components - The subtleties of Tracking and Wiring

Following on from yesterday's investigations into the behaviour of '@track', and its effect on untracked properties, I figured I should expand my remit to '@wire'.

My main reason for that was the statement in the documentation:

In the wire adapter’s configuration object, prefix a property with $ to reference a property of the component instance. The $ prefix tells the wire service to treat it as a property of the class and evaluate it as this.propertyName. The property is reactive. If the property’s value changes, new data is provisioned and the component rerenders.

This comment relates to code along the lines of:

    @api recordId;

    @wire(getRecord, { recordId: '$recordId', fields })
    contact;

The bit that really piqued my interest was 'The property is reactive'.

In all the examples, it looked like the property being referenced was always tracked or set as an api field (the latter implying the former)

That's the case in the above example - the property passed as a parameter 'recordId' as referenced by '$recordId' is defined as an api field, and is therefore tracked.

There's also that recurring point 'the component rerenders', which we saw in my previous post. Recalling that, it essentially meant that a change to a tracked property caused all untracked properties in the same component (and I am assured, but yet to prove, child components) to be re-rendered

So, what is the actual behaviour in this case? Are the implications the same?

You can code that illustrates the below examples, as well as the points from yesterday's post in this bitbucket repository if you want to explore the behaviours further. I suggest you take a look.

Tracked property used as a parameter to a wired property

The natural first example is much like the one exampled above. We have an Apex method that takes a parameter. We wire that parameter to a javascript property, and reference a single parameter.

Javascript component

    @track searchString;

    @wire(searchContacts, { searchString: '$searchString' })
    contacts;

When the searchString property is updated:

• The searchString's change causes the wired function to be re-evaluated.
• Therefore the value of 'contacts' changes.
• The component treats contacts as if it was tracked and causes the component to be re-rendered.

OK. That's pretty decent, and what the documentation says will happen. No surprises there!

But then I got to thinking: So, what happens if the property isn't tracked?

My first guess was that maybe the Apex method wouldn't get re-executed, but I wasn't entirely convinced - after all, the documentation only really makes a big deal about rendering. So I changed the code and tried again...

Javascript component

    searchString;

    @wire(searchContacts, { searchString: '$searchString' })
    contacts;

This time, when the searchString property is updated:

• The searchString's change causes the wired function to be re-evaluated.
• Therefore the value of 'contacts' changes.
• The component treats contacts as if it was tracked and causes the component to be re-rendered.

Erm. OK. It's the same. Hmmm....

Looking at what's actually going on, that does make sense. It's the '$searchString' reference that tells the framework that searchString is reactive, as respect to the wiring, so it's that causing the Apex to re-execute. And once the method is re-evaluated, the value of 'contacts' changes, and that causes the component to re-render.

That got me to thinking - so what does the '@track' do against the searchString in the original example. Experience tells me that these things generally will have an effect.

So I added something else to my example...

    renderedCallback() {
        console.log( 'renderedCallback was called' );
    }

This hooks into the component's lifecycle, as described here, and will tell us when the component gets re-rendered.

It turns out that my initial understanding of the first example was slightly wrong, though not in a way that would generally have much of an impact.

That is, if the parameter is tracked, you end up with the following:

• The searchString's change causes:
  • The component to be re-rendered (since it is tracked).
  • The wired function to be re-evaluated (since it is referenced as a '$' parameter).
• The execution of the wired function causes the value of 'contacts' to change.
• The component treats contacts as if it was tracked and causes the component to be re-rendered.

The result is that the component is re-rendered twice!

And sure enough, if you take the tracking off the searchString parameter, the component is only re-rendered once.

So, this does reinforce another point that the documentation makes here:

Don’t overuse @track. Track a property only if you need the component to rerender when the property’s value changes.

Personally, I think I'd call out this particular behaviour and remind people - you don't need to 'track' a property in order for the wired method to re-execute - but maybe it's just my lack of attention that missed that little point.

And I'd also state that you should only track a property that is being used as a reactive wired parameter if you need the property to be re-rendered before the Apex you are calling returns with its result.

Lightning Web Components - the art of Tracking

On Friday I wrote an excited blog post on the introduction of Lightning Web Components.

One of the things I mentioned was that I didn't quite get how '@track' was supposed to work

Since then, I've found the documentation (add /docs/component-library/documentation/lwc/lwc.get_started_introduction to the end of your Salesforce URL), read it, got confused by my memory, wrote a test-rig and looked at it all in depth.

On the surface, it's simple, but there are some complications, which were the cause of my confusion...

In the documentation it says the following:

Both @track and @api mark a property as reactive. If the property’s value changes, the component re-renders.

OK, nice and simple. So what does that mean?

A single untracked property

Testing with a simple component that contains a single property that is not tracked I found the following behaviour:

Javascript component

    export default class TrackExperiments extends LightningElement {
        primitiveUntracked;

        handlePrimitiveUntrackChanged( event ) {
            this.primitiveUntracked = event.target.value;
        }
    }

Template

    <p>The value is: {primitiveUntracked}</p>
    <lightning-input label="Input without an on-change handler"
                        type="Text"
                        value={primitiveUntracked}></lightning-input>
    <lightning-input label="Input with an on-change handler"
                        type="Text"
                        value={primitiveUntracked}
                        onchange={handlePrimitiveUntrackChanged}></lightning-input>

• Changing the value defined in either of the inputs will not result in the rendered value changing.
• Changing the value defined in the input without the onchange handler will not change the Javascript property.
• Changing the value defined in the input that does have an onchange handler will update the Javascript property.

So, fairly straightforward, and maybe in-line with what you would expect:

• Untracked properties are not re-rendered in the UI when they change
• Untracked properties that are bound to inputs are not automatically updated when the input changes unless there is an onchange handler defined that updates the Javascript property

A single untracked property

Testing with a simple component that contains a single property that is tracked, I found the following behaviour:

Javascript component

    export default class TrackExperiments extends LightningElement {
        @track primitiveTracked;

        handlePrimitiveTrackChanged( event ) {
            this.primitiveTracked = event.target.value;
        }
    }

Template

    <p>The value is: {primitiveTracked}</p>
    <lightning-input label="Input without an on-change handler"
                        type="Text"
                        value={primitiveTracked}></lightning-input>
    <lightning-input label="Input with an on-change handler"
                        type="Text" value={primitiveTracked}
                        onchange={handlePrimitiveTrackChanged}></lightning-input>

• Whenever the value of the Javascript property changes, it is reflected on the outputted page.
• Changing the value defined in the input without the onchange handler will not change the Javascript property, and so the rendered property does not change.
• Changing the value defined in the input that does have an onchange handler will update the Javascript property, and both the rendered property and the value in the other input is updated.

OK, in my opinion, slightly odd - I was expecting the input binding to be two-way - that's not the case:

Lightning Web Components contain 1-way binding.

Still, it may have surprised me, but it's easy to remember, and no big deal once you know that's the behaviour.

• Tracked properties are re-rendered in the UI when they change.
• Tracked properties that are bound to inputs are not automatically updated when the input changes unless there is an onchange handler defined that updates the Javascript property.
• Inputs that are bound to tracked properties are automatically updated when the underlying property is updated.

A tracked property and an untracked property on the same page

Javascript component

    export default class TrackExperiments extends LightningElement {
        @track primitiveTracked;
        primitiveUntracked;

        handlePrimitiveTrackChanged( event ) {
            this.primitiveTracked = event.target.value;
        }

        handlePrimitiveUntrackChanged( event ) {
            this.primitiveUntracked = event.target.value;
        }
}

Template

    <p>The value is: {primitiveTracked}</p>
    <lightning-input label="Input without an on-change handler"
                        type="Text"
                        value={primitiveTracked}></lightning-input>
    <lightning-input label="Input with an on-change handler"
                        type="Text" value={primitiveTracked}
                        onchange={handlePrimitiveTrackChanged}></lightning-input>

    <p>The value is: {primitiveUntracked}</p>
    <lightning-input label="Input without an on-change handler"
                        type="Text"
                        value={primitiveUntracked}></lightning-input>
    <lightning-input label="Input with an on-change handler"
                        type="Text"
                        value={primitiveUntracked}
                        onchange={handlePrimitiveUntrackChanged}></lightning-input>

Now things start to get a little surprising.

• Changing the inputs related to the tracked property works as described above, as if it was the only property on page.
• Then, changing the inputs related to the untracked property as described above.
• However, changing the tracked property inputs after changing the untracked input, causes both the tracked and untracked properties to be re-rendered.

The last point really took me by surprise - I was expecting that only the tracked property would be updated. This led me to go back to the documentation again and re-read it.

That is (bold added for emphasis):

Both @track and @api mark a property as reactive. If the property’s value changes, the component rerenders.

Yep, I think I'm reading that right, and it looks like it's reflected in the behaviour I see (without delving too deep into it). When you change the value of a tracked property, the whole component re-renders. This means that any untracked properties will also be re-rendered.

I've put together a more complete illustration of that, which you can find in this git repo.

Lightning Web Components - the dawn of (another) new era

Note that this post was put together before the documentation for Lightning Web Components went live at developer.salesforce. That site should be your first port for any explanation of a concept in LWC!

Salesforce have a new technology. Lightning Components look like they’re on the way out, and are being replaced with a new technology ‘Lightning Web Components’.

The reasons behind that, and the main principles behind its design are covered very nicely in this article on developer.salesforce.com.

From that we can then get to a series of examples here.

(Note: some of the code snippets used below, to illustrate points, are taken from the recipes linked above)

Now I’m a big supporter of evolution, and I love to see new tools being given to developers on the Salesforce platform, so, with a couple of hours to play with it - what’s the immediate impression?

This is an article on early impressions, based on reviewing and playing with the examples - I fully expect there to be misunderstandings, bad terminology, and mistakes in here - If you're OK with that, I'm OK with that. I admit, I got excited and wanted to post something as quickly as possible before my cynical side took over. So here it is - mistakes and all.

WOW. Salesforce UI development has grown up.

Salesforce aren’t lying when they’ve said that they’re trying to bring the development toolset up to the modern standards.

We get imports, what look like annotations and decorators, and there’s even mention of Promises. Maybe there’s some legs in this…

It’s easy to dismiss this as ‘Oh no, yet another change’, but the thing is - the rest of industry develops and improves its toolset - why shouldn’t Salesforce?

The only way to keep the product on point IS to develop the frameworks, replace the technology, upgrade, move on. If you don’t do that then the whole Salesforce Ecosystem starts to stagnate.

Or to put it another way - in every other part of the developer community, learning from what was built yesterday and evolving is seen as a necessity. It’s good to see Salesforce trying to keep up.

So what are the big things that I’ve spotted immediately?

import is supported, and that makes things clearer

Import is a massive addition to Javascript that natively allows us to define the relationships between javascript files within javascript, rather than at the HTML level.

Essentially, this replaces the use of most ‘script’ tags in traditional Javascript development.

For Lightning Web Components, we use this to bring in capabilities from the framework, as well as static resources.

E.g. Importing modules from the Lightning Web Components framework:

 import { LightningElement, track } from 'lwc';

Importing from Static Resources:

 import { loadScript } from 'lightning/platformResourceLoader’;
 import chartjs from '@salesforce/resourceUrl/chart';

What this has allowed Salesforce to do is to split up the framework into smaller components. If you don’t need to access Apex from your web component, then you don’t need to import the part of the framework that enables that capability.

This *should* make individual components much more lightweight and targeted - only including the capabilities that are required, when they are required.

Getting data on screen is simpler

Any javascript property is visible to the HTML template.

E.g.

    export default class WebAppComponentByMe extends LightningElement {
     contacts;

We can then render this property in the HTML with {contacts} (none of those attributes to define and none of those pesky v dot things to forget).

Much neater, much more concise.

We track properties

Looking at the examples, my assumption was that if we want to perform actions when a property is changed, we mark the property trackable using the @track decorator.

For example:

    export default class WebAppComponentByMe extends LightningElement {
     @track contacts;

I was thinking that, at this point, anything that references this property (on page, or in Javascript) will be notified whenever that property changes.

However, at this point I can't really tell what the difference is between tracked and non-tracked properties - a mystery for another day

Since writing this, things have become a little clearer, and I've started posting how and why here.

Wiring up to Apex is much simpler

One of the big criticisms of Lightning Components that I always had was the amount of code you need to write in order to call an Apex method. OK, so you have force:recordData for a lot of situations, but there are many times when only an Apex method will do.

In Web Components, this is much simpler.

In order to connect to Apex, we import the ‘wire’ module, and then import functions into our javascript

 import { LightningElement, wire } from 'lwc';
 import getContactList from '@salesforce/apex/ContactController.getContactList';

The first line imports the wire capabilities from the framework, the second then imports the Apex method as a javascript method, therefore making it available to the component.

We can then connect a javascript property up to the method using the wire decorator:

 @wire(getContactList) contacts;

Or wire up a javascript method:

 @wire(getContactList)
 wiredContacts({ error, data }) {
  if (data) {
   this.contacts = data;
  } else if (error) {
   this.error = error;
  }
 }

When the component is initialised, the getContactList method will be executed.

If the method has parameters, that’s also very simple (E.g. wiring to a property):

 @wire(getContactList, { searchKey: '$searchKey' })
 contacts;

Changing the value of a property causes Apex to re-execute

Having wired up a property as a parameter to an Apex bound Javascript function, any changes to that property will cause the function to be re-executed

For example, if we:

 searchKey = '';

 @wire(findContacts, { searchKey: '$searchKey' })
 contacts;

Whenever the searchKey property changes, the Apex method imported as ‘findContacts’ will be executed and the contacts property is updated.

Thankfully, we can control when that property changes, as it looks like changing the property in the UI does not automatically fire a change the property on the Javascript object. In order to do that, we need to change the property directly.

E.g. Let’s say we extend the previous example and there’s an input that is bound to the property, and there’s an onchange event defined:

And the handler does the following:

 handleKeyChange(event) {
  this.searchKey = event.target.value;
 }

This will cause the findContacts method to fire whenever the value in the input is changed.

Note that it is the assignment to this.searchKey that causes the event to fire - it looks like the binding from the HTML is 1-way. I admit that I need to investigate this further.

Events do not require configuration to be implemented

Events work in a completely different way - but then that’s not a problem - Application and Component events were different enough to cause headaches previously. The model is actually much simpler.

The example in the above referenced repository to look at is ‘PubSub’.

It’s much too involved to into detail here, but the result is that you need to:

• Implement a Component that acts as the messenger (implementing registerListener, unregisterListener and fireEvent)
• Any component that wants to fire an event, or listen for an event will import that component to do so, firing events or registering listeners.

This would seem to imply that (at least a certain amount of) state within components is shared - looking like those defined with 'const'

Whatever the precise nature of the implementation, a pure Javascript solution is surely one that anyone involved in OO development will welcome.

I suspect that, in a later release, this will become a standard component.

Summary

Some people will be thinking "Man, glad I didn’t migrate from Classic / Visualforce to Lightning Experience / Components - maybe I’ll just wait a little longer for it all to settle down”.

You’re wrong - it won’t settle, it’ll continue to evolve and the technologies will be continually replaced by new ones. Eventually, the jump from what you have to where you need to get to will be so huge that you’ll find it incredibly hard. There’s a reason why Salesforce pushes out 3 releases a year, whether you want it or not, these technology jumps are just the same. The more you put it off, the more painful it’ll be.

The change from Lightning Components to Lightning Web Components is vast - a lot more than a single 3 letter word would have you suspect. The only real similarities between the two frameworks that I’ve seen up to now are:

• Curlies are used to bind things
• The Base Lightning Components are the same
• You need to know Javascript

Other than that, they’re a world apart.

Also, I couldn’t find any real documentation - only examples - although those examples are a pretty comprehensive starting point.

Now, obviously it's early days - we're in pre-release right now, but what I've seen gives me great hope for the framework, it's a significant step forward and I can't wait to see what happens next. I wonder if a Unit Testing framework might follow (I can but hope)

You could wait, but hey, really, what are you waiting for? Come on, jump in. The change is exciting...

LinkedIn, and the GDPR age

I should start this post by saying I’m neither a lawyer, nor a GDPR expert.  Possibly both of those facts will become massively apparent in the text that follows.

Also, I’m not a LinkedIn Premium user - so it’s possible I’m missing something obvious by not having access to it.

But anyway, I’ve been thinking about how LinkedIn fits into a GDPR world, and it doesn’t something doesn’t seem quite right to me at the moment.

LinkedIn are in the data business, and they’re very good at protecting that asset.  They tend to be (quite rightly) pro-active in stopping people from extracting data from their systems and pushing it into their own systems.

As such, businesses (recruiters particularly) are encouraged to contact directly within LinkedIn, and they are offered tools to discover people and commence that communication.

Unfortunately, this lack of syncing between LinkedIn and in-house systems can cause a big problem with GDPR.

That is:

What happens if someone says to a recruitment organisation - “Please forget me, and do not contact me again”

In this situation, the organisation is obliged to ‘remove' them from their systems.

At some point in the future another recruiter from the same organisation then finds the person on LinkedIn, without reference to their own systems and messages them using LinkedIn.

What happens next?

By the letter of the law, the organisation may not have done anything wrong.

• The person is no longer in the organisation’s system, they were found on LinkedIn.
• The person was not sent an e-mail, or phoned, they were messaged within LinkedIn.
• The person has consented to have their data held by LinkedIn for the expressed purpose of being contacted by potential recruiters via the platform.

With all this in mind, it may be interpreted that it’s fair game to contact anyone on LinkedIn, regardless of their expressed desire not to be contacted by a particular company.

However, whilst this may be within the definition of the law, it’s pretty clear it’s not in the spirit of the law.

Note - Again I’m not a GDPR expert, nor a lawyer, so can't say for certain that it IS within the definition of the law - nor am I asserting that it is - just that I can imagine that it might be interpreted that way by some people.

And this is where things get complicated for LinkedIn.  I can see a few outcomes of this, but two of them could be extremely worrying for the future on LinkedIn.

Scenario - LinkedIn Premium is seen as an extension of a subscribing organisation’s IT systems.

It could be argued that, whilst LinkedIn is in independent entity, when they provide services to another organisation, their systems then become part of the remit of that subscribing organisation.

I.E. within LinkedIn, any action by a user and the storage of data of that action falls solely within the responsibility of the employer of the user that performs that action.  LinkedIn are not responsible for the use of the data in any way.

On first glance, this looks ideal to LinkedIn - no responsibility!

However, that’s not true - if there’s ever a test case that proves this point, then suddenly LinkedIn becomes a big risk to any organisation that uses it.

Over the course of the last 2 years or so, every data holding organisation in the EU has looked carefully at their data retention and use policies and systems and done what they can to protect themselves - in may cases I’m sure they have changed suppliers and systems since the existing systems have not proven up to scratch in the light of GDPR legislation.

Up to now, I’m not sure that many people have scrutinised LinkedIn in the same way.

At the moment it might be argued that LinkedIn is not supplying the tools to subscribers to allow them to comply with the GDPR legislation.  For example, I’m not aware of any functionality that allows an organisation to state "I wish to completely forget this person, and ensure that I cannot connect, view data on or contact them without their expressed consent”.  If that’s a minimum requirement of any internal system, why would it not be a minimum requirement for LinkedIn?

It could be that once that test case comes, a lot of organisations will take a look at LinkedIn and decide it doesn’t stand up, and it’s no longer worth the risk.

Scenario - LinkedIn, as the data controller, is responsible for the contact made by any users within the system.

This is potentially even worse for LinkedIn.  Since LinkedIn hold the data about people, provide the tools for discovering those people, provide the tools for contacting people, and for relaying those messages, it may be argued that it is up to LinkedIn to provide the mechanism to allow Users to state that they do not wish to be visible to or contacted by a given organisation.

That is, whilst it is another user who is sending the message, it may be that a future test case could state that LinkedIn are responsible for keeping track of who has ‘forgotten’ who.

By not providing that mechanism, and allowing users on the system to make contact when the contact is not welcome and against the target’s wishes, it’s possible that LinkedIn could be argued as being responsible for the unwelcome contact and therefore misuse of data.

Summary

Today, it seems that LinkedIn is in a bit of limbo.

There may be a recognised way to use LinkedIn in the GDPR era - find someone, check in my system that I’m allowed to contact them, go back to LinkedIn and contact them - but in order for that to work it requires the due diligence of recruiters to ensure that the law isn’t broken.

Realistically, something will have to change, or that test case is coming; at some point, someone is going to get an email that is going to break the limbo.

When that happens, I wonder which way it will go..?

Things I still believe in

Over 10 years ago I wrote a blog post on things that I believe in - as a developer, and when I re-read it recently I was amazed at how little has changed.

I'm not sure if that's a good thing, or a bad thing - but it's certainly a thing.

Anyway - here's that list - slightly updated for 2018... it you've seen my talk on Unit Testing recently, you might recognise a few entries.

(opinions are my own, yada yada yada)

• It's easier to re-build a system from its tests than to re-build the tests from their system.


• You can measure code complexity, adherence to standards and test coverage; you can't measure quality of design.


• Formal and flexible are not mutually exclusive.


• The tests should pass, first time, every time (unless you're changing them or the code).


• Test code is production code and it deserves the same level of care.


• Prototypes should always be thrown away.


• Documentation is good, self documenting code is better, code that doesn't need documentation is best.


• If you're getting bogged down in the process then the process is wrong.


• Agility without structure is just hacking.


• Pair programming allows good practices to spread.


• Pair programming allows bad practices to spread.


• Team leaders should be inside the team, not outside it.


• Project Managers are there to facilitate the practice of developing software, not to control it.


• Your customers are not idiots; they always know their business far better than you ever will.


• A long list of referrals for a piece of software does not increase the chances of it being right for you, and shouldn't be considered when evaluating it.


• You can't solve a problem until you know what the problem is. You can't answer a question until the question's been asked.


• Software development is not complex by accident, it's complex by essence.


• Always is never right, and never is always wrong.


• Interesting is not the same as useful.


• Clever is not the same as right.


• The simplest thing that will work is not always the same as the easiest thing that will work.


• It's easier to make readable code correct than it is to make clever code readable.


• If you can't read your tests, then you can't read your documentation.


• There's no better specification document than the customer's voice.


• You can't make your brain bigger, so make your code simpler.


• Sometimes multiple exit points are OK. The same is not true of multiple entry points.


• Collective responsibility means that everyone involved is individually responsible for everything.


• Sometimes it's complex because it needs to be; but you should never be afraid to double check.


• If every time you step forward you get shot down you're fighting for the wrong army.


• If you're always learning you're never bored.


• There are no such things as "Best Practices". Every practice can be improved upon.


• Nothing is exempt from testing. Not even database upgrades or declarative tools.


• It's not enough to collect data, you need to analyse, understand and act upon that data once you have it.


• A long code freeze means a broken process.


• A test hasn't passed until it has failed.


• A test that can't fail isn't a test.


• If you give someone a job, you can't guarantee they'll do it well; If you give someone two jobs you can guarantee they'll do both badly


• Every meeting should start with a statement on its purpose and context, even if everyone in the meeting already knows.