I'm learning about transitions in CSS3 via sololearn.com, where the only examples given for using transitions are in conjunction with the :hover selector.
Everything else I can find uses either that or :active with no explanation of what other selectors can be used or why selectors other than these two are not shown as examples.
Is there a way to start transitions without interaction from the user? Or would that require JavaScript?
You can use any selector to initiate a transition, but you don't really need one for it. All you need is to:
transition properties on the elementYou can do both of the above without using CSS selectors, which are nothing more than a handy method available for setting DOM element properties.
transitions work with any selector whatsoever. But they are not related to any selector and you don't really need selectors to perform CSS transitions.
All you need for a transition to work is to set the following transition properties:
transition-duration,transition-delay,transition-timing-functiontransition-property. (has to be animatable for transition to work).After this, whenever the property named in transition-property changes its value between two animatable values, the change between the initial and the set value happens according to the set transition properties.
This happens regardless of what exactly triggers the change of value for the transitioned property
It could:
Or, of course, by a combination of the two (i.e. use JavaScript to apply a class, which changes the value of the property being transitioned)
So, in effect, the answer to your question is: transitions work with any selector but they also work without one. They work based on DOM element property values, regardless of any selector.
A common way to demonstrate a transition is using :hover state because all one needs to do to switch between selectors (which is to control / demonstrate / observe the transition) is. well,... hover!
On:
For example is there a way to start transitions without interaction from the user?
Of course there is! A very basic example is to add a class on <body> when all page resources have finished loading.
When <body> element is created it doesn't have the class and when all resources have finished loading, the Window's onload event triggers. We can use this event to add the class. If a transition is properly defined on body and the transitioned property changes because a stronger selector now applies on the element, the transition will happen. Without any user interaction.
See it working:
window.onload = function() {
document.querySelector('body').classList.add('loaded')
}body {
background-color: #fff;
transition: background-color 1s linear;
}
.loaded { background-color: #212121;}You could also set a recursive function that toggles the state of an element and calls itself after a set amount of time, effectively toggling the state back after the set time, in a continuous cycle:
For the above example this would mean:
window.onload = toggleBodyClass;
function toggleBodyClass() {
document.querySelector('body').classList.toggle('loaded');
setTimeout(toggleBodyClass, 1234);
}body {
background-color: #fff;
transition: background-color 1s linear;
}
.loaded { background-color: #212121;}If the question is: "Can a selector start/stop applying without the use of JavaScript and/or user interaction?" It really translates to:
"Are there transitions triggered by something else than what usually triggers a transition?", with possible answers varying from:
"Why do you want to know?" to "Is there web browsing without JavaScript and/or user interaction?"
Transitions are, as Paulie_D says, changes in state. What this "state" refers to, is simply the value of any style property (that can be animated, anyway). Even the spec describes it the same way:
Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
This means that you don't actually need a selector to start transitions. Styles can be changed via CSS, JavaScript, or the inline style attribute. All of these are subject to transitions, because they all modify style property values. Selectors just happen to be the most common way of doing it because selectors and declaration blocks are the two fundamental components that make up style rules, themselves the building blocks of CSS.
Most other ways of using transitions with or without user interaction do in fact involve JavaScript because CSS doesn't support much state change without requiring user interaction (outside of animations, which are a different beast from transitions), but that doesn't mean that JavaScript is inherently required for transitions to work, because transitions are about state change, regardless of how that state change is invoked (JS or not).
Most tutorials use :hover and :active to demonstrate transitions simply because user interaction is the easiest and most intuitive way for readers to see transitions in action, and to learn what it means for an element to change state (albeit a different kind of state, but the principle is the same). It's also by far the most common use case for transitions: animating changes in state in response to user interaction.
But you don't actually need to change property values with a user interaction pseudo-class in order for transitions to work. You can change them with any selector, even if user interaction is handled by a different element (and that element doesn't have to start transitions using :hover or :active either)...
label {
transition: color 1s linear;
}
:checked + label {
color: #f00;
}<p><input type=checkbox id=check><label for=check>Label</label>... or by the page itself...
h1 {
transition: color 1s linear;
}
:target {
color: #f00;
}<p><a href=#sec1>Section 1</a> <a href=#sec2>Section 2</a>
<h1 id=sec1>Section 1</h1>
<h1 id=sec2>Section 2</h1>Once you add JavaScript into the mix, you can set property values directly (i.e. not use a selector)...
setTimeout(function() {
document.querySelector('p').style.color = '#f00';
}, 1000);p {
transition: color 1s linear;
}<p>Wait for it...... change an element's class, ID or other attribute...
setTimeout(function() {
document.querySelector('p.foo').className = 'bar';
}, 1000);
setTimeout(function() {
document.getElementById('foo').id = 'bar';
}, 2000);
setTimeout(function() {
document.querySelector('p[data-attr]').dataset.attr = 'bar';
}, 3000);p {
transition: color 1s linear;
}
p.bar { color: #f00; }
p#bar { color: #090; }
p[data-attr=bar] { color: #00f; }<p class=foo>Wait for it...
<p id=foo>Wait for it...
<p data-attr=foo>Wait for it...... or even move elements around in the DOM tree (although this does have limitations — notice that the Foo item fails to start its transition because it's getting detached before being reattached, while the Bar item is able to start its transition once it notices it's now first because it never leaves the DOM tree)...
setTimeout(function() {
var ul = document.querySelector('ul');
ul.appendChild(ul.firstChild);
}, 1000);li {
transition: color 1s linear;
}
li:first-child { color: #f00; }<ul><li>Foo<li>Bar</ul>... and be able to start transitions all the same. Notice that all the JavaScript examples start transitions automatically, no user interaction required.
So, in conclusion, transitions are about state change, and said state change pertains to changes in style property values. These are independent of selectors or JavaScript, although selectors are a fundamental part of CSS and you do need JavaScript for most things.
transitions refer to changes in value of animatable properties designated intransition-property.