Listening for Events

VueJS uses the v-on attribute added in HTML tags to listen to the events.
Usually, the value of the v-on attribute is the name of a method defined in the methods property. The specified method is called when the event is emitted.

Basic syntax:
<button v-on:eventName='methodName'>Try it</button>

<script>
var vm = new Vue({
 //...
 methods:{
 methodName: function(event){
 //code to execute..
 }
 }
})
</script>
- eventName is the name of the event: click, input, mouseenter, etc.

Simple example with the click event:
<div id='demo'>
<h1>{{hi}}</h1>
<button v-on:click='greet'>Try it</button>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {hi: ''},
 methods:{
 greet: function(ev){
 this.hi ='Hello brother';
 }
 }
})
</script>

You can set the event handler from v-on to pass a value, and the event instance to the method:
<button v-on:eventName='methodName("string", $event)'>Try it</button>

<script>
var vm = new Vue({
 //...
 methods:{
 methodName: function(value, event){
 //code to execute..
 }
 }
})
</script>

Test and study the following example:
<div id='demo'>
<h1>{{hi}}</h1>
<a href='/' v-on:click='test("The link not works", $event)'>Open the page</a>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {hi: ''},
 methods:{
 test: function(val, ev){
 this.hi = val;
 if(ev){
 ev.preventDefault();
 alert('Link address: '+ ev.target.href);
 }
 }
 }
})
</script>

Multiple events in v-on

You can add multiple events into an object in a single v-on directive, but without an argument.
- Syntax:
<button v-on={event_1:method_1, event_2:method_2}>Try it</button>

Note: With this method you cannot add modifier, nor pass an argument to the event handler, but it is automatically passed the event instance object.


Example:
<div id='demo'>
<div v-bind:style='styleobj' v-on='{mouseenter:bground, mouseleave:bground}'>Place the mouse here</div>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 styleobj:{background:'#fbfc00', height:'111px', width:'111px'}
 },
 methods:{
 bground: function(ev){
 let bgr ={mouseenter:'#bbbbfe', mouseleave:'#00e000'};
 this.styleobj.background = bgr[ev.type];
 }
 }
})
</script>

Event Modifiers

Vue has event modifiers available on v-on attribute. They are directive postfixes denoted by a dot.
Syntax:
<button v-on:eventName.modifier='methodName'>Try it</button>

Examples with some modifiers available in Vue:
prevent - calls event.preventDefault().
 
<form action='/' id='frm1' v-on:submit.prevent='test'>
The submit event will not reload the page, but will call a Vue method.<br>
<input type='text' name='inp1' value='MarPlo'/> <input type='submit' value='Send'/>
<div>{{text}}</div>
</form>

<script>
var vue_ob = new Vue({
 el: '#frm1',
 data: {
 text:''
 },
 methods:{
 test: function(ev){
 this.text = ev.target.inp1.value;
 }
 }
})
</script>
once - trigger handler at most once.
<div id='demo'>
Nr. clicks: <b>{{nrc}}</b><br>
Only one click is emitted: <button v-on:click.once='nrc +=1'>Click</button>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 nrc:0
 }
})
</script>
self - only trigger handler if event was dispatched from this element, not from a child element.
<div id='demo'>
Click event is registered with '<b>self</b>' modifier on Parent.<br>
If you click on the Child element, it will have no effect.<br>
Nr. clicks: <b>{{nrc}}</b><br>
<div style='background:#b0b0fe; height:144px; text-align:center; width:220px' v-on:click.self='nrc +=1'>
Parent
<div style='background:#00da00; margin:12px; height:88px; width:115px'>Child</div>
</div>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 nrc:0
 }
})
</script>
left | middle | right - only trigger handler for left, middle or right button mouse events.
<div id='demo'>
It registers the clicks on the right button mouse: <b>v-on:click.right.prevent</b>.<br>
The <b>prevent</b> modifier is added to stop default behavior (here to stop opening menu on right click).<br><br>
Nr. clicks: <b>{{nrc}}</b><br>
<div style='background:#afaffe; height:144px; text-align:center; width:222px' v-on:click.right.prevent='nrc +=1'>
Right click here
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 nrc:0
 }
})
</script>

You can chain multiple modifiers on an event with v-on, see in the example above: v-on:click.right.prevent.


Key Modifier

Vue.js allows adding key modifiers to v-on when listening for key events.
- Syntax:
v-on.eventname.keyname
We can make use of multiple keynames. For example: v-on.onkeydown.ctrl.enter

- Example:
<div id='demo'>
Add some text, then press enter.
<form action='/' v-on:keydown.enter.prevent='submitFrm'>
<input type='text' value='MarPlo'/>
<div v-html='str'></div>
</form>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 str:''
 },
 methods:{
 submitFrm:function(ev){
 this.str ='The form submits the text: <b>'+ ev.target.value +'</b>';
 }
 }
})
</script>

v-on.event shorthand

You can replace the v-on: with the character @ in Vue syntax, as a shorthand for v-on:.
- Syntax:
<div @eventName='methodName'></div>

Dynamic events

You can also use Dynamic events, which are added between square brackets:
v-on:[event]='methodName'

//OR
@[event]='methodName'

Example:
<div id='demo'>
<h1>{{hi}}</h1>
<button @[ev_name]='greet'>Try it</button>
</div>

<script>
var vue_ob = new Vue({
 el: '#demo',
 data: {
 hi: '',
 ev_name: 'click'
 },
 methods:{
 greet: function(ev){
 this.hi ='Forgive the past, love the present, trust the future.';
 }
 }
})
</script>

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which type of <input> creates input fields that should contain a numeric value?
type="text" type="number" type="date"
<input type="number" name="points" min="5" max="80" />
Which CSS property allows to change the transparency of an element?
font-style opacity color
#id {
  filter:alpha(opacity=40);    /* for IE */
  opacity:0.4;
}
Click on the function that formats a number to specified number of characters.
toPrecision() exp() toFixed()
var num = 12.34567;
alert( num.toPrecision(3) );       // 12.3
Which function randomizes the order of the elements into an array?
natsort() shuffle() sort()
$lang =[10=>"PHP", 20=>"JavaScript", "site"=>"coursesweb.net");
shuffle($lang);
var_export($lang);     // array (0=>"coursesweb.net", 1=>"PHP", 2=>"JavaScript")
Vue JS - Events

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