Vue GWT

# Event Handling

This page comes from the official Vue.js documentation and has been adapted for Vue GWT.

# Listening to Events

We can use the v-on directive to listen to DOM events and run some JavaScript when they're triggered.

For example:

<div>
  <button v-on:click="counter += 1">Add 1</button>
  <p>The button above has been clicked {{ counter }} times.</p>
</div>

@Component
public class ButtonPlusOneComponent implements IsVueComponent {
    @Data int counter = 0;
}

Result:

# Method Event Handlers

The logic for many event handlers will be more complex though, so keeping your JavaScript in the value of the v-on attribute simply isn't feasible. That's why v-on can also accept the name of a method you’d like to call.

For example:

<button v-on:click="greet">Greet</button>

@Component
public class GreetComponent implements IsVueComponent {
    @JsMethod
    public void greet() {
        Window.alert("Hello from GWT!");
    }
}

Result:

You can also invoke the method in JavaScript. Try this in your browser console:

greetComponent.greet();

# Methods in Inline Handlers

Instead of binding directly to a method name, we can also use methods in an inline JavaScript statement:

<div>
  <button v-on:click='say("hi")'>Say hi</button>
  <button v-on:click='say("what")'>Say what</button>
</div>

@Component
public class HiWhatComponent implements IsVueComponent {
    @JsMethod
    public void say(String message) {
        Window.alert(message);
    }
}

Result:

# Accessing the Event

Sometimes we also need to access the original DOM event in an inline statement handler. You can pass it into a method using the special $event variable:

<button v-on:click='warn("Form cannot be submitted yet.", (Event) $event)'>
    Submit
</button>

// ...
public void warn(String message, Event event) {
    if (event != null) {
        event.preventDefault();
        message += " -> Event Type: " + event.type;
    }

    Window.alert(message);
}

You will notice that we cast the $event variable to Event in our template. This is to indicate the type of the $event variable to Vue GWT.

This means that if your warn method was actually declared like this: void warn(Event event, String message) it would break at compile time with an explicit error.

You don't have to import the Event class to use it in your template. It's already imported by default for convenience.

Event comes from Elemental2. It is the base for all DOM events. You cast it to more specific events like MouseEvent or KeyboardEvent in your Component class.

You'll see further that $event can also be used when communicating between Components. In those case you might cast it to another type, like Todo for example.

# Event Modifiers

It is a very common need to call event.preventDefault() or event.stopPropagation() inside event handlers. Although we can do this easily inside methods, it would be better if the methods can be purely about data logic rather than having to deal with DOM event details.

To address this problem, Vue provides event modifiers for v-on. Recall that modifiers are directive postfixes denoted by a dot.

  • .stop
  • .prevent
  • .capture
  • .self
  • .once
<!-- the click event's propagation will be stopped -->
<a v-on:click.stop="doThis"></a>

<!-- the submit event will no longer reload the page -->
<form v-on:submit.prevent="onSubmit"></form>

<!-- modifiers can be chained -->
<a v-on:click.stop.prevent="doThat"></a>

<!-- just the modifier -->
<form v-on:submit.prevent></form>

<!-- use capture mode when adding the event listener -->
<!-- i.e. an event targeting an inner element is handled here before being handled by that element -->
<div v-on:click.capture="doThis">...</div>

<!-- only trigger handler if event.target is the element itself -->
<!-- i.e. not from a child element -->
<div v-on:click.self="doThat">...</div>

TIP

Order matters when using modifiers because the relevant code is generated in the same order. Therefore using @click.prevent.self will prevent all clicks while @click.self.prevent will only prevent clicks on the element itself.

New in Vue.js 2.1.4

<!-- the click event will be triggered at most once -->
<a v-on:click.once="doThis"></a>

Unlike the other modifiers, which are exclusive to native DOM events, the .once modifier can also be used on component events. If you haven't read about components yet, don't worry about this for now.

# Key Modifiers

When listening for keyboard events, we often need to check for common key codes. Vue also allows adding key modifiers for v-on when listening for key events:

<!-- only call vm.submit() when the keyCode is 13 -->
<input v-on:keyup.13="submit">

Remembering all the keyCodes is a hassle, so Vue provides aliases for the most commonly used keys:

<!-- same as above -->
<input v-on:keyup.enter="submit">

<!-- also works for shorthand -->
<input @keyup.enter="submit">

Here's the full list of key modifier aliases:

  • .enter
  • .tab
  • .delete (captures both "Delete" and "Backspace" keys)
  • .esc
  • .space
  • .up
  • .down
  • .left
  • .right

You can also define custom key modifier aliases via the global VueConfig:

// enable v-on:keyup.f1
Vue.setConfig(new VueConfig().setKeyCode("f1", 112));

# Automatic Key Modifers

New in Vue.js 2.5.0

You can also directly use any valid key names exposed via KeyboardEvent.key as modifiers by converting them to kebab-case:

<input @keyup.page-down="onPageDown">

In the above example, the handler will only be called if $event.key === 'PageDown'.

TIP

A few keys (.esc and all arrow keys) have inconsistent key values in IE9, their built-in aliases should be preferred if you need to support IE9.

# System Modifier Keys

New in Vue.js 2.1.0

You can use the following modifiers to trigger mouse or keyboard event listeners only when the corresponding modifier key is pressed:

  • .ctrl
  • .alt
  • .shift
  • .meta

Note: On Macintosh keyboards, meta is the command key (⌘). On Windows keyboards, meta is the windows key (⊞). On Sun Microsystems keyboards, meta is marked as a solid diamond (◆). On certain keyboards, specifically MIT and Lisp machine keyboards and successors, such as the Knight keyboard, space-cadet keyboard, meta is labeled “META”. On Symbolics keyboards, meta is labeled “META” or “Meta”.

For example:

<!-- Alt + C -->
<input @keyup.alt.67="clear">

<!-- Ctrl + Click -->
<div @click.ctrl="doSomething">Do something</div>

TIP

Note that modifier keys are different from regular keys and when used with keyup events, they have to be pressed when the event is emitted. In other words, keyup.ctrl will only trigger if you release a key while holding down ctrl. It won't trigger if you release the ctrl key alone.

# Mouse Button Modifiers

New in Vue.js 2.2.0

  • .left
  • .right
  • .middle

These modifiers restrict the handler to events triggered by a specific mouse button.

# Why Listeners in HTML?

You might be concerned that this whole event listening approach violates the good old rules about "separation of concerns". Rest assured - since all Vue handler functions and expressions are strictly bound to the ViewModel that's handling the current view, it won't cause any maintenance difficulty. In fact, there are several benefits in using v-on:

  1. It's easier to locate the handler function implementations within your Java code by simply skimming the HTML template.

  2. Since you don't have to manually attach event listeners in Java, your ViewModel code can be pure logic and DOM-free. This makes it easier to test.

  3. When a ViewModel is destroyed, all event listeners are automatically removed. You don't need to worry about cleaning it up yourself.

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