CSS過渡通過在指定持續(xù)時間內(nèi)平滑插值屬性變化來實現(xiàn)動畫效果。 1. 它監(jiān)聽特定CSS屬性的變化，2. 僅對可動畫屬性如顏色、透明度、尺寸等有效，3. 常用語法為transition: property duration timing-function delay，4. 常見觸發(fā)方式包括懸停、類名變更、焦點狀態(tài)等，5. 注意避免使用all及謹慎選擇可動畫屬性以優(yōu)化性能。

CSS transitions make it possible to animate changes in CSS properties over a specified duration, creating smooth visual effects when elements change state — like when you hover over a button or toggle a menu.

How Transitions Work Under the Hood

At their core, transitions watch for changes in specific CSS properties. When a change happens (for example, from width: 100px to width: 200px ), and if a transition is applied, the browser doesn't apply the new value instantly. Instead, it calculates how to smoothly interpolate between the old and new values over time.

This interpolation only works for animatable properties — things like color, opacity, width, height, transform, etc. You can't transition non-numeric values like display: none to display: block , because there's no in-between state.

Basic Syntax of a Transition

To create a transition, you typically define two things:

• Which property to transition
• How long the transition should take

Here's a basic example:

 .button {
  background-color: blue;
  transition: background-color 0.3s ease;
}

.button:hover {
  background-color: darkblue;
}

In this case, when the user hovers over .button , the background color will change gradually over 0.3 seconds using an "ease" timing function.

You can also use shorthand to define multiple aspects at once:

 transition: property duration timing-function delay;

Some common timing functions include:

• linear : consistent speed
• ease : starts slow, then fast, then ends slow
• ease-in : starts slow
• ease-out : ends slow
• cubic-bezier(x1, y1, x2, y2) : custom curve

Which Properties Can Be Animated?

Not all CSS properties are animatable. For example, you can animate opacity , transform , color , and background-color , but not display , z-index , or float .

A good rule of thumb is that if a property has a numeric value with a range (like pixel values, percentages, colors), it's likely animatable. If it's a switch-like setting (on/off), it usually isn't.

If you're ever unsure, tools like CSS Triggers can help you check which properties are safe to animate and what performance impact they might have.

Triggering a Transition

Transitions don't happen on their own — something has to trigger them. The most common triggers are:

• Hovering ( :hover )
• Adding/removing a class via JavaScript
• Focus states ( :focus )
• Media queries (like changing layout based on screen size)

For example, adding a class dynamically through JavaScript:

 element.classList.add('active');

And in your CSS:

 .box {
  opacity: 0;
  transition: opacity 0.5s;
}

.box.active {
  opacity: 1;
}

This way, when the active class is added, the box fades in smoothly.

One thing to note: transitions only work when the property changes from one value to another. So if you set the same value again (like toggling the same color), nothing will happen visually.

A Few Gotchas to Watch For

• Multiple transitions : You can transition multiple properties by separating them with commas:

   transition: background-color 0.3s ease, transform 0.4s linear;

• Shorthand pitfalls : Using all as the property might seem convenient, but it can lead to unintended animations. It's usually better to be explicit about which properties you want to animate.

• Performance : Animating expensive properties like width or height can cause layout thrashing. For smoother results, prefer animating transform and opacity .

• Browser support : While modern browsers support transitions well, always test across platforms, especially if targeting older devices or mobile environments.

基本上就這些。

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