v-on & methods: User Interaction

We've talked a lot about data, props and using those in a template. But what we haven't done yet is add any behavior. So here's our next big goal: add a link to the bottom of the sidebar that, when the user clicks it, will collapse or expand the sidebar.

Adding the collapsed Data

Cool! But... where should we start? Let's think about it: in the sidebar template, we're going to need to know whether or not we are currently collapsed or expanded... because we will probably need to render different classes or styles to make that happen. And since this "is collapsed" information is something that will change while our Vue app is running, it needs to live in data.

Ok! Inside data(), add a new property called, how about, collapsed. Set it false so that the component starts not collapsed.

Back on the browser, it doesn't do anything yet, but we can see the new data.

Let's update the template to use this. Add a style attribute. Basically, if collapsed is true, we'll set a really small width. Of course we can't just start referencing the collapsed variable right now because we need to add the : in front of :style to make it dynamic.

To set the width, we could put width: in quotes then end quote, plus, and some dynamic logic that uses the collapsed variable. But... yuck! Because the style attribute can get complex, instead of creating a long string of styles, Vue allows us to set this to an object with a key for each style, like width: '500px' and margin: '10px'. In our case, I'll use the ternary syntax: if collapsed is true, set the width to 70px, else use auto.

Ok! We have a collapsed data and we're using it in the template. Testing time! In the Vue dev tools, click on Sidebar, change collapsed to true and... oh that looks awful! We'll clean that up soon. But it is working: the width dynamically changes.

Adding the Button

Of course, we're not going to expect users to change the collapsed data via their Vue dev tools. Nope: they'll need a button that will change this state.

Back on the template, let's see... after the <ul>, I'll add an <hr>, a div with some positioning classes and a button with class="btn btn-secondary".

Nothing interesting yet! For the button text, if we're currently collapsed, use >>, else, use << Collapse.

Oh ESLint is mad! Hmm, it thinks that the < sign is me trying to open an HTML tag! I could escape this and use < - that's probably a great solution! But as you'll see, Vue will escape this for us.

Back on the browser, I'll inspect the new button, and right click to "Edit as HTML". Yep! Vue automatically escapes any text that you render. That < becomes < automatically.

Hello v-text

Oh, and, by the way, when you have an element and the only thing inside of it is some dynamic text - like our button - there is one other way to render that text. Copy our dynamic code, add a new attribute called v-text, set it to our dynamic code... and delete the curly braces.

Now Vue is happy. v-text is the third Vue directive that we've seen, after v-bind and v-for. It's not a particularly important one... it's just an alternative way to set the "text" of an element and... it's a tiny bit faster. I mostly just wanted you to see it.

Adding a new Method

Let's get back to the real goal: when the user clicks this button, we need to do something! Specifically, we need to change the collapsed data.

We know that the variables that we have access to in the template are the keys from data and props... though we don't have any props in this component. It turns out that there are a couple other ways to add stuff to your template. One allows you to add functions.

Check it out: it doesn't matter where... but I'll do it after data(), add a methods: option set to an object. Create a function called toggleCollapsed() and, to start, just say console.log('CLICKED!').

The idea is that, "on click" of this button, we will tell Vue to call this method.

OnClick with v-on

How... do we do that? By using our fourth directive - and a very, very important one. It's called v-on. Inside the button element, add v-on: and then the name of the normal DOM event that you want to listen to, so: click. Set this to the name of the method that should be called on click: toggleCollapsed.

Yep, the toggleCollapsed function is now available in the template because we added it to the methods options. PhpStorm even lets you Ctrl or Cmd click to jump to it!

So... let's try it! Back on your browser, click and... then scroll down. These errors up here were from when we were in the middle of working. And... yes! There's our log!

The v-on Shortcut

v-on is one of the most important directives and we're going to use it all the time. Scroll up a little to the :href attribute. We know that this is really v-bind in disguise: v-bind is such a common directive, that Vue gives us this special shortcut.

Vue also has a shortcut for v-on: the @ symbol. Want to run some code "on click", use @click or @mouseover or @ whatever event you want.

Updating the State

Ok: we have a button and when we click it, it calls the toggleCollapsed method. The final step is to change the collapsed state so that the template updates. How? this.collapsed = !this.collapsed.

If you were eagerly waiting for some complex & impressive code to change the data, I'm sorry to disappoint you.

The secret to this working is that any data keys are accessible as a property on this. In the next chapter, we'll look deeper into how that works. But for now, it feels good: we change the value of this.collapsed and... apparently, Vue will re-render.

Sound too good to be true? Let's try it! Click the button. It works! Over on the Vue dev tools, on the Sidebar component, we can watch the collapsed state change.

And... congratulations! We've now talked about the core parts of Vue. Seriously! To get this working, we added a collapsed data, used that in the template, and, on click, called a toggleCollapsed method that we created... which changed the collapsed data. Vue was then smart enough to re-render the template.

We'll see this basic flow over and over again.

But before we do, I want to talk more about how the this variable works in Vue. It's at the heart of Vue's magic. And if we understand it, we'll be just a little bit closer to being unstoppable. That's next.