Vue Transitions

When we click the button, our little "Testing transitions" div hides and shows thanks to v-show... but without any transitions. The reason is that Vue hides and shows by adding and removing display: none. Our CSS transition will only work if the opacity changes.

So, instead of display: none, what we really need Vue to do is change the opacity from 1 to 0. Or, more generically, we need Vue to add a new class when it's hidden so that we can set opacity: 0 on that class, just like we're already doing with our hidden class. That would make this works just like before.

Hello <transition>: Adding & Removing Classes

Fortunately, Vue has a feature just for this. The key is to surround the element that we're hiding and showing with a special transition component. Check it out: add <transition before and after the div... and indent to keep ESLint happy.

But I want to be super clear about what this <transition> tag does and does not do. Go back to your browser and do a full refresh. Now, click. Woh! It doesn't even hide anymore! Oh, wait... it did hide... it just took a few seconds. When I click again, it's instantly back. That's... odd.

Inspect element on the div and watch really closely. Thanks to the <transition> component, when I click, it adds a few classes... then after a few seconds, it removes those and hides the element with display: none. When we click again, it removes the display: none and temporarily adds some other classes... which also disappear after a few seconds.

Obviously, these classes on their own do nothing: this did not magically cause the div to fade in and out. But we can use those classes in CSS to add the transitions.

<transition> Reads your CSS Transition Time

So the first takeaway is this: the <transition> component is a fancy mechanism to temporarily add a few classes whenever the item inside is hidden or shown. That's it. Though it is a bit smarter than it first appears.

Head over to the CSS in the component... and temporarily comment-out the transition: opacity 3s.

Now go back to the browser and watch the div again. When I click the button... boom! It instantly hides and does not add those temporary classes! It turns out that the transition component actually looks at the element that's being hidden or shown and detects if it has a CSS transition. If it does, it uses that to know how long to add those temporary classes.

Remove the comment and put the transition back. Now when we click, watch: the classes are added for exactly 3 seconds and then the display: none is set. Vue does this so that our CSS transition - which is not quite working yet - has the time it needs to finish before Vue finally, fully hides the element with display: none.

Adding the Transition CSS

So to get the CSS transition working, all we need to do is use the temporary classes that Vue is adding instead of the hidden class we were using before. Replace .hidden with .v-enter and .v-leave-to. If you're using Vue 3, it's called .v-enter-from instead of .v-enter.

These are both classes that Vue adds in specific situations. Don't worry, we'll talk about exactly how this all works in a minute.

We can also replace the transition-testing class with .v-enter-active and .v-leave-active:

Back up in the template, we can remove the transition-testing class that we're no longer using:

Before we talk about how this all works, let's try it. Move over and... beautiful! It takes 3 seconds to fade out... and 3 seconds to fade in!

All About How/When the Transition Classes are Added

To help explain this, search for "Vue Transitions", click into the Enter/Leave Transitions page on their docs and scroll down a bit. Here they explain when each class is added and removed with a little diagram.

Transition Classes when Hiding / Leaving

When we first click the button, we're hiding the element. So we're on the "leave" side of the diagram. When that happens, Vue cleverly adds several classes at just the right moment so that we can add transitions.

First, Vue adds the v-leave-active class. This remains on the element during the entire transition, so the entire 3 seconds for us. If we go back down to our CSS, the .v-leave-active class establishes that we want a 3 second transition on the opacity property. That doesn't cause the transition - but our browser now knows that we want a transition if the opacity changes.

Next, still at the very start of things, Vue adds the v-leave class, or v-leave-from in Vue 3. That... actually doesn't do anything in our case, which means that the opacity is still the default 1. Then, one frame later, something very important happens: Vue remove the v-leave class and adds v-leave-to at the same moment. Thanks to our CSS, that changes the opacity from 1 to 0 and that causes our browser to transition the opacity for 3 seconds.

After 3 seconds, Vue removes all of the classes and adds display: none. How crazy is that?

Transition Classes when Showing / Entering

A similar thing happens in the other "enter" direction, when we're changing an element from hidden to shown. Once again, before the process starts, Vue adds a v-enter-active class. This establishes that our element should have a 3 second transition for opacity.

Next, still at the very start of the process, Vue adds v-enter, or v-enter-from in Vue 3. This is important because, thanks to our CSS, that sets the opacity to 0. One frame later, Vue removes the display: none from the element. At that moment, the item is technically displayed... but has an opacity of 0. One frame after, Vue removes v-enter and adds v-enter-to. For us, the important part is that it removes v-enter, which changes the opacity from 0 back to the default 1. That triggers the nice 3 second transition. The v-enter-to, in our case, doesn't do anything because we don't need it to. Finally, after the 3 seconds is done, all of these classes are removed.

Phew! Pretty wild, right? We can, "kind of" see this in action. When we click the button, we see some of the classes... but not all of them, because some classes are literally added for 1 frame and then removed: too fast for us to see.

So... Vue transitions are an intelligent way add a set of classes at just the right time when an element is shown or hidden, either via v-show or v-if. By themselves, the classes don't do anything. That's our job with CSS.

Next: let's use this to transition between our two components: the checkout form and the shopping cart. We'll also talk about transition "modes".