问题
Is it possible to tell the code to position by the center point of an element, rather than by the top-left point? If my parent element has
width: 500px;
and my child element has
/*some width, for this example let's say it's 200px*/
position: absolute;
left: 50%;
one would assume that based on the 50% positioning, the child element will be in the middle of the parent, leaving 150px of free space on each side. However, it is not, since it is the top-left point of the child that goes to 50% of the parent's width, therefore the whole child's width of 200px goes to the right from there, leaving 250px of free space on the left and only 50px on the right.
So, my question is, how to achieve center positioning?
I found this solution:
position: absolute;
width: 200px;
left: 50%;
margin-left: -100px;
but I don't like it because you need to edit it manually for each element's width - I would like something that works globally.
(For example, when I work in Adobe After Effects, I can set a position for an object and then set specific anchor point of that object that will be put to that position. If the canvas is 1280px wide, you position an object to 640px and you choose the center of the object to be your anchor point, then the whole object will be centered within the 1280px wide canvas.)
I would imagine something like this in CSS:
position: absolute;
left: 50%;
horizontal-anchor: center;
Similarly, horizontal-anchor: right would position the element by its right side, so the whole content would be to the left from the point of its parent's 50% width.
And, the same would apply for vertical-anchor, you get it.
So, is something like this possible using only CSS (no scripting)?
Thanks!
回答1:
If the element must be absolutely positioned (so, margin: 0 auto; is of no use for centering), and scripting is out of the question, you could achieve this with CSS3 transforms.
.centered-block {
width: 100px;
left: 50%;
-webkit-transform: translate(-50%, 0);
position: absolute;
}
See this fiddle for some examples. The important parts: left: 50%; pushes block halfway across its parent (so its left side is on the 50% mark, as you mentioned). transform: translate(-50%, 0); pulls the block half it's own width back along the x-axis (ie. to the left), which will place it right in the center of the parent.
Note that this is unlikely to be cross-browser compatible, and will still require a fall back of some sort for old browsers.
回答2:
If adding another element is an option, consider the following:
(View the following code live here)
HTML:
<div class="anchor" id="el1">
<div class="object">
</div>
</div>
<div class="anchor" id="el2">
<div class="object">
</div>
</div>
CSS:
/* CSS for all objects */
div.object
{
width: 100%;
height: 100%;
position: absolute;
left: -50%;
top: -50%; /* to anchor at the top center point (as opposed to true center) set this to 0 or remove it all together */
}
div.anchor
{
position: absolute; /* (or relative, never static) */
}
/* CSS for specific objects */
div#el1
{
/* any positioning and dimensioning properties for element 1 should go here */
left: 100px;
top: 300px;
width: 200px;
height: 200px;
}
div#el2
{
/* any positioning and dimensioning properties for element 2 should go here */
left: 400px;
top: 500px;
width: 100px;
height: 100px;
}
div#el1 > div.object
{
/* all other properties for element 1 should go here */
background-color: purple;
}
div#el2 > div.object
{
/* all other properties for element 2 should go here */
background-color: orange;
}
Essentially what we're doing is setting up one object to define the position, width and height of the element, and then placing another one inside of it that gets offset by 50%, and gets it's parent dimensions (ie width: 100%).
回答3:
Stumbled upon this question, and I'd like to add another way to center content within a div.
By using the CSS3 display mode 'flexible box', as in setting the following CSS properties to the parent div
display: flex;
position: relative;
align-items: center;
justify-content:center;
And 'at least' setting the following properties to the child div
position:relative;
The browser will automatically center the contents within the parent div, unaffected by their width or height, this particularly comes in handy when you're developing something like an image grid or just want to remove the hassle of calculating a divs position, then having to recalculate it when you change your mind about the set-up of said div.
In my own work, I tend to set-up a class named
.center-center
With the properties I described for the parent div, then just add it to whatever element of which I need its contents centered.
I've created a JSFiddle to support this explanation, feel free to click on the red squares to see the positioning in action.
https://jsfiddle.net/f1Lfqrfs/
For multi-line support, you can add (or uncomment in the JSF) the following CSS property to the parent DIV
flex-wrap: wrap;
回答4:
Here's one way to center a text element in the middle of the container (using a header as an example
CSS:
.header {
text-align: center;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
position: absolute;
}
It centers by a middle anchor point.
回答5:
left:50% doesn't center the center of gravity of this div 50% to the left, it is the distance between the left border and the left border of the parent element, so basically you need to do some calculations.
left = Width(parent div) - [ Width(child div) + Width(left border) + Width(right border) ]
left = left / 2
So you can do a Javascript function...
function Position(var parentWidth, var childWith, var borderWidth)
{
//Example parentWidth = 400, childWidth = 200, borderWidth = 1
var left = (parentWidth - ( childWidth + borderWidth));
left = left/2;
document.getElementById("myDiv").style.left= left+"px";
}
回答6:
I got it working by using transform: translateX with calc for horizonally positioning by element center instead of top-left. Same trick can be used for vertical using transform: transformY. This will work with width of any type (try resizing)
Snippet: transform: translateX(calc(-50% + 223px));
Browser support: https://caniuse.com/#feat=transforms2d, https://caniuse.com/#feat=calc
Codepen for more examples: https://codepen.io/manikantag/pen/KJpxmN
Note on offsetLeft/offsetTop etc: el.offsetLeft will not given proper value for CSS transformed elements. You would require something like el.getBoundingClientRect().left - el.offsetLeft - el.parentNode.offsetLeft (as mentioned here)
Note about flow effect: as detailed in CSS Things That Don’t Occupy Space, transform doesn't effect the following element without considering the offset caused by transform. Sometimes this may not be the intended behavior. This can be fixed using negative margins if width/height are fixed (doesn't work if width/height are using %)
.left-positioned {
margin-left: 223px;
background-color: lightcoral;
}
.center-positioned {
transform: translateX(calc(-50% + 223px)); /*=====> actual solution */
background-color: cyan;
}
.with-width {
width: 343px;
background-color: lightgreen;
}
/* styles not related to actual solution */
div {
resize: both;
overflow: auto;
}
.container {
background-color: lightgray;
}
.ele {
display: inline-block;
height: 70px;
text-align: center;
}<div class="container">
<div class="ele left-positioned">Reference element (left at 223px)</div> <br/>
<div class="ele center-positioned">^<br/>Center positioned element (center at 223px)</div> <br/>
<div class="ele center-positioned with-width">^<br/>Center positioned element with width (center at 223px)</div>
</div>来源：https://stackoverflow.com/questions/15328416/position-by-center-point-rather-than-top-left-point