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Vue Directives

The last article was a little heavy on discussion. In today's article, we’ll dive into more code examples by addressing some important native Vue directives.

A Vue directive is essentially a special type of command that can be added to HTML content and often appears as a prefixed HTML attribute. We’ll first revisit the v-on directive since we’ve already used it in one of the earlier articles.

Event Handling with v-on

The v-on directive can be used to create event listeners within the DOM to enable us to do something when a particular event happens.

In article #2, the v-on directive was used to call an instance changeGreeting() method when a button was clicked:

<button v-on:click="changeGreeting">
  Change Greeting
</button>

Instead of triggering a method in the instance, we’re also able to run JavaScript inline in the template:

<button v-on:click="greeting = 'Hi there!'">
  Change Greeting
</button>

Though inline JavaScript works just as well, calling methods bound to the instance is often preferable when intended functionality change gets more complicated.

It's important to keep in mind that the v-on directive can be used with virtually any native DOM event:

<h1 v-on:click="method">Click me!</h1>
<h1 v-on:dblclick="method">Double Click me!</h1>
<form v-on:submit="method">...</form>
<input v-on:keydown="method"
  placeholder="Press down on keys" />
<input v-on:keyup="method"
  placeholder="Release keys" />
<!-- ... -->

Here’s a code sample that shows some different event listeners with different expected outcomes:

HTML

{lang=html,line-numbers=off} <<src/v-on-example/index.html

JS

{lang=javascript,line-numbers=off} <<src/v-on-example/main.js

<iframe src='./src/v-on-example/index.html' height="250" scrolling="no" > </iframe>

Live version - https://30dofv-von.surge.sh

From the code sample above, you may have noticed the v-on directive be used on a keyup.enter event. keyup.enter is one of the unique key modifiers Vue provides to allow us to react to events from commonly used keys like the Enter key.

Finally, event handlers usually have an event object that contains details about the event. In the v-on directive, we’re able to access this original event object by explicitly passing in the $event variable:

<h1 v-on:click="method($event)">Click me!</h1>

The v-on directive is essentially what we use to detect and handle events in Vue applications.

Attribute binding with v-bind

The simplest form of data binding in Vue is the Mustache Syntax (i.e. double curly braces) which is used to bind data values on to text content of HTML elements.

In the code samples prepared in the first and second articles, we used the Mustache Syntax to bind greeting, user, and city properties defined in our instance on to the template:

<html>
  <body>
    <div id="app">
      <h2>{{ greeting }}</h2>
      <p>by {{ user }} who lives in {{city}}</p>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

We’re unable to use the Mustache syntax to bind data to standard HTML attributes like href, id, src, etc. To bind HTML attributes, Vue provides the native v-bind directive.

Here’s an example of using the v-bind directive to bind a data property (named cityImage) to the src attribute of an img element:

HTML

{lang=html,line-numbers=off} <<src/v-bind-example/index.html

JS

{lang=javascript,line-numbers=off} <<src/v-bind-example/main.js

With the cityImage appropriately bound to a hosted image of Toronto, we’ll be presented with a view of Toronto’s skyline:

<iframe src='./src/v-bind-example/index.html' height="325" scrolling="no" > </iframe>

Live version - https://30dofv-vbind.surge.sh

Conditional rendering with v-if (or v-show)

Oftentimes, we may find ourselves interested in conditionally rendering content based on the value of an expression. In Vue, we can achieve this with the help of the v-if and v-show directives.

To see an example of conditional rendering, let’s first add a button to our previous example that would allow us to change the city title and image from Toronto to Lagos:

HTML

<html>
  <body>
    <div id="app">
      <h2>{{ greeting }}</h2>
      <p>by {{ user }} who lives in {{ city }}</p>
      <img v-bind:src="cityImage" />
      <button v-on:click="changeCity">
        Change City
      </button>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

JS

{lang=javascript,line-numbers=off} <<src/v-if-example/main.js

The changeCity() method first checks if the city data value is 'Toronto', if so - it changes the city to 'Lagos' and the cityImage to a hosted image of the Lekki-Ikoyi Link Bridge in Lagos state. If the city information has been already been changed, the changeCity() information simply reverts the information back to Toronto.

If we wanted to render some content in certain conditions (e.g. when city === 'Toronto'), the v-if or v-show directive would prove useful. Since I (Hassan) live in Toronto, we can change the text content in the template to better reflect my presence. For example, we’ll use two separate <p> tags each containing a v-if directive to display the appropriate description text content:

{lang=html,line-numbers=off} <<src/v-if-example/index.html

The text content of the first <p> tag that says by Hassan who lives in Toronto will only render if the city property is equal to 'Toronto'. If city is equal to 'Lagos', the second <p> tag will instead only be displayed and say by Hassan who wishes to visit Lagos:

<iframe src='./src/v-if-example/index.html' height="325" scrolling="no" > </iframe>

Live version - https://30dofv-vif.surge.sh

I> Vue also provides the v-else directive to describe an else block and the v-else-if directive to describe an else-if-block.

Instead of the v-if directive, we could also use the v-show directive to conditionally render content:

{lang=html,line-numbers=off} <<src/v-show-example/index.html

<iframe src='./src/v-show-example/index.html' height="325" scrolling="no" > </iframe>

Live version - https://30dofv-vshow.surge.sh

Though they achieve a similar outcome, the v-if and v-show directives differ from one another. The v-if directive does not render the element only until the stated condition is true. The v-show directive, on the other hand, always renders the element but controls the CSS display property depending on whether the stated condition is true.

The v-if directive is usually preferred as long as you don’t need the element to always be present in the DOM and you don’t expect the toggling between displaying/hiding the element to happen very often. If we expect the element to toggle often at runtime - the v-show directive would be more appropriate.

Shorthand syntax with v-on and v-bind

Vue provides unique shorthand syntax only for the commonly used v-bind and v-on directives. The v-bind directive can be shortened with the : symbol:

<!-- the full syntax -->
<img v-bind:src="dataProperty" />

<!-- the shorthand syntax -->
<img :src="dataProperty" />

And the v-on directive can be shortened with the @ symbol:

<!-- the full syntax -->
<button v-on:click="methodName"></button>

<!-- the shorthand syntax -->
<button @click="methodName"></button>

Though the shorthand syntax is entirely optional and achieves the exact same outcome, we’ll stick with using the shorthand syntax for the rest of the course.

Awesome! We'll stop here for today. In the next article, we’ll spend some time discussing how the v-for directive can be used to help render lists of elements.