Get the offset position of the caret in a textarea in pixels [duplicate]

问题

In my project I'm trying to get the offset position of the caret in a textarea in pixels. Can this be done?

Before asking here, I have gone through many links, especially Tim Down's, but I couldn't find a solution which works in IE8+, Chrome and Firefox. It seems Tim Down is working on this.

Some other links which I have found have many issues like not finding the top offset of the caret position.

I am trying to get the offset position of the caret because I want to show an auto-complete suggestion box inside the textarea by positioning it based on the offset position of the caret.

PS: I can't use a contenteditable div because I have written lots of code related to a textarea.

回答1:

Here's an approach using rangyinputs, rangy and jQuery.

It basically copies the whole text from inside the textarea into a div of the same size. I have set some CSS to ensure that in every browser, the textarea and the div wrap their content in exactly the same way.

When the textarea is clicked, I read out at which character index the caret is positioned, then I insert a caret span at the same index inside the div. By only doing that I ended up having an issue with the caret span jumping back to the previous line if the user clicked at the start of a line. To fix that I check if the previous character is a space (which would allow a wrap to occur), if that is true, I wrap it in a span, and I wrap the next word (the one directly after the caret position) in a span. Now I compare the top values between these two span's, if they differ, there was some wrapping going on, so I assume that the top and the left value of the #nextword span are equivalent to the caret position.

This approach can still be improved upon, I'm sure I haven't thought of everything that could possibly go wrong, and even if I have, then I haven't bothered implementing a fix for all of them as I don't have the time to do so at the moment, a number of things that you would need to look at:

~~it doesn't yet handle hard returns inserted with Enter~~ (fixed)
~~positioning breaks when entering multiple spaces in a row~~ (fixed)
I think hyphens would allow a content wrap to occur as well..

Currently it works exactly the same way across browsers here on Windows 8 with the latest versions of Chrome, Firefox, IE and Safari. My testing has not been very rigorous though.

Here's a jsFiddle.

I hope it will help you, at the very least it might give you some ideas to build on.

Some Features:

I have included a ul for you which is positioned in the right spot, and fixed a Firefox issue where the textarea selection was not re-set back to its original spot after the DOM manipulations.
I have added IE7 - IE9 support and fixed the multiple word selection issue pointed out in the comments.
I have added support for hard returns inserted with Enter and multiple spaces in a row.
I have fixed an issue with the default behaviour for the ctrl+shift+left arrow text selection method.

JavaScript

function getTextAreaXandY() {

    // Don't do anything if key pressed is left arrow
    if (e.which == 37) return;     

    // Save selection start
    var selection = $(this).getSelection();
    var index = selection.start;

    // Copy text to div
    $(this).blur();
    $("div").text($(this).val());

    // Get current character
    $(this).setSelection(index, index + 1);
    currentcharacter = $(this).getSelection().text;

    // Get previous character
    $(this).setSelection(index - 1, index)
    previouscharacter = $(this).getSelection().text;

    var start, endchar;
    var end = 0;
    var range = rangy.createRange();

    // If current or previous character is a space or a line break, find the next word and wrap it in a span
    var linebreak = previouscharacter.match(/(\r\n|\n|\r)/gm) == undefined ? false : true;

    if (previouscharacter == ' ' || currentcharacter == ' ' || linebreak) {
        i = index + 1; // Start at the end of the current space        
        while (endchar != ' ' && end < $(this).val().length) {
            i++;
            $(this).setSelection(i, i + 1)
            var sel = $(this).getSelection();
            endchar = sel.text;
            end = sel.start;
        }

        range.setStart($("div")[0].childNodes[0], index);
        range.setEnd($("div")[0].childNodes[0], end);
        var nextword = range.toHtml();
        range.deleteContents();
        var position = $("<span id='nextword'>" + nextword + "</span>")[0];
        range.insertNode(position);
        var nextwordtop = $("#nextword").position().top;
    }

    // Insert `#caret` at the position of the caret
    range.setStart($("div")[0].childNodes[0], index);
    var caret = $("<span id='caret'></span>")[0];
    range.insertNode(caret);
    var carettop = $("#caret").position().top;

    // If preceding character is a space, wrap it in a span
    if (previouscharacter == ' ') {
        range.setStart($("div")[0].childNodes[0], index - 1);
        range.setEnd($("div")[0].childNodes[0], index);
        var prevchar = $("<span id='prevchar'></span>")[0];
        range.insertNode(prevchar);
        var prevchartop = $("#prevchar").position().top;
    }

    // Set textarea selection back to selection start
    $(this).focus();
    $(this).setSelection(index, selection.end);

    // If the top value of the previous character span is not equal to the top value of the next word,
    // there must have been some wrapping going on, the previous character was a space, so the wrapping
    // would have occured after this space, its safe to assume that the left and top value of `#nextword`
    // indicate the caret position
    if (prevchartop != undefined && prevchartop != nextwordtop) {
        $("label").text('X: ' + $("#nextword").position().left + 'px, Y: ' + $("#nextword").position().top);
        $('ul').css('left', ($("#nextword").position().left) + 'px');
        $('ul').css('top', ($("#nextword").position().top + 13) + 'px');
    }
    // if not, then there was no wrapping, we can take the left and the top value from `#caret`    
    else {
        $("label").text('X: ' + $("#caret").position().left + 'px, Y: ' + $("#caret").position().top);
        $('ul').css('left', ($("#caret").position().left) + 'px');
        $('ul').css('top', ($("#caret").position().top + 14) + 'px');
    }

    $('ul').css('display', 'block');
}

$("textarea").click(getTextAreaXandY);
$("textarea").keyup(getTextAreaXandY);

HTML

<div></div>
<textarea>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.</textarea>
<label></label>
<ul>
    <li>Why don't you type this..</li>
</ul>

CSS

body {
    font-family: Verdana;
    font-size: 12px;
    line-height: 14px;
}
textarea, div {
    font-family: Verdana;
    font-size: 12px;
    line-height: 14px;
    width: 300px;
    display: block;
    overflow: hidden;
    border: 1px solid black;
    padding: 0;
    margin: 0;
    resize: none;
    min-height: 300px;
    position: absolute;
    -moz-box-sizing: border-box;
    white-space: pre-wrap;
}
span {
    display: inline-block;
    height: 14px;
    position: relative;
}
span#caret {
    display: inline;
}
label {
    display: block;
    margin-left: 320px;
}
ul {
    padding: 0px;
    margin: 9px;
    position: absolute;
    z-index: 999;
    border: 1px solid #000;
    background-color: #FFF;
    list-style-type:none;
    display: none;
}
@media screen and (-webkit-min-device-pixel-ratio:0) {
    span {
        white-space: pre-wrap;
    }
}
div {
    /* Firefox wrapping fix */
    -moz-padding-end: 1.5px;
    -moz-padding-start: 1.5px;
    /* IE8/IE9 wrapping fix */
    padding-right: 5px\0/;
    width: 295px\0/;
}
span#caret
{
    display: inline-block\0/;
}

回答2:

You can create a separate (invisible) element and fill it with textarea content from start to the cursor position. Textarea and the "clone" should have matching CSS (font properties, padding/margin/border and width). Then stack these elements on top of each other.

Let me start with a working example, then walk through the code: http://jsfiddle.net/g7rBk/

Updated Fiddle (with IE8 fix)

HTML:

<textarea id="input"></textarea>
<div id="output"><span></span></div>
<div id="xy"></div>

Textarea is self-explanatory. Output is a hidden element to which we'll pass text content and make measures. What's important is that we'll use an inline element. the "xy" div is just an indicator for testing purposes.

CSS:

/* identical styling to match the dimensions and position of textarea and its "clone"
*/
#input, #output {
    position:absolute;
    top:0;
    left:0;
    font:14px/1 monospace;
    padding:5px;
    border:1px solid #999;
    white-space:pre;
    margin:0;
    background:transparent;
    width:300px;
    max-width:300px;
}
/* make sure the textarea isn't obscured by clone */
#input { 
    z-index:2;
    min-height:200px;
}

#output { 
    border-color:transparent; 
}

/* hide the span visually using opacity (not display:none), so it's still measurable; make it break long words inside like textarea does. */
#output span {
    opacity:0;
    word-wrap: break-word;
    overflow-wrap: break-word;
}
/* the cursor position indicator */
#xy { 
    position:absolute; 
    width:4px;
    height:4px;
    background:#f00;
}

JavaScript:

/* get references to DOM nodes we'll use */
var input = document.getElementById('input'),
    output = document.getElementById('output').firstChild,
    position = document.getElementById('position'),

/* And finally, here it goes: */
    update = function(){
         /* Fill the clone with textarea content from start to the position of the caret. You may need to expand here to support older IE [1]. The replace /\n$/ is necessary to get position when cursor is at the beginning of empty new line.
          */
         output.innerHTML = input.value.substr( 0, input.selectionStart ).replace(/\n$/,"\n\001");

        /* the fun part! 
           We use an inline element, so getClientRects[2] will return a collection of rectangles wrapping each line of text.
           We only need the position of the last rectangle.
         */
        var rects = output.getClientRects(),
            lastRect = rects[ rects.length - 1 ],
            top = lastRect.top - input.scrollTop,
            left = lastRect.left+lastRect.width;
        /* position the little div and see if it matches caret position :) */
        xy.style.cssText = "top: "+top+"px;left: "+left+"px";
    }

[1] Caret position in textarea, in characters from the start

[2] https://developer.mozilla.org/en/docs/DOM/element.getClientRects

Edit: This example only works for fixed-width textarea. To make it work with user-resizable textarea you'd need to add an event listener to the resize event and set the #output dimensions to match new #input dimensions.

回答3:

There's a much simpler solution for getting the caret position in pixels, than what's been presented in the other answers.

Note that this question is a duplicate of a 2008 one, and I've answered it here. I'll only maintain the answer at that link, since this question should have been closed as duplicate years ago.

Copy of the answer

I've looked for a textarea caret coordinates plugin for meteor-autocomplete, so I've evaluated all the 8 plugins on GitHub. The winner is, by far, textarea-caret-position from Component.

Features

pixel precision
no dependencies whatsoever
browser compatibility: Chrome, Safari, Firefox (despite two bugs it has), IE9+; may work but not tested in Opera, IE8 or older
supports any font family and size, as well as text-transforms
the text area can have arbitrary padding or borders
not confused by horizontal or vertical scrollbars in the textarea
supports hard returns, tabs (except on IE) and consecutive spaces in the text
correct position on lines longer than the columns in the text area
no "ghost" position in the empty space at the end of a line when wrapping long words

Here's a demo - http://jsfiddle.net/dandv/aFPA7/

How it works

A mirror <div> is created off-screen and styled exactly like the <textarea>. Then, the text of the textarea up to the caret is copied into the div and a <span> is inserted right after it. Then, the text content of the span is set to the remainder of the text in the textarea, in order to faithfully reproduce the wrapping in the faux div.

This is the only method guaranteed to handle all the edge cases pertaining to wrapping long lines. It's also used by GitHub to determine the position of its @ user dropdown.

回答4:

JsFiddle of working example: http://jsfiddle.net/42zHC/2/

Basically, we figure out how many columns fit in the width (since it will be monospace). We have to force scrollbars to always be there otherwise the calculation is off. Then we divide the number of columns that fit with the width, and we get the x offset per character. Then we set the line height on the textarea. Since we know how many characters are in a row, we can divide that with the number of characters and we get the row number. With the line height, we now have the y offset. Then we get the scrollTop of the textarea and subtract that, so that once it starts using the scrollbar, it still shows up in the right position.

Javascript:

$(document).ready(function () {
  var cols = document.getElementById('t').cols;
  var width = document.getElementById('t').clientWidth;
  var height = $('textarea').css('line-height');
  var pos = $('textarea').position();
  $('#t').on('keyup', function () {
    el = document.getElementById("t");
    if (el.selectionStart) { 
        selection = el.selectionStart; 
      } else if (document.selection) { 
        el.focus(); 
        var r = document.selection.createRange(); 
        if (r == null) { 
           selection = 0; 
        } 
        var re = el.createTextRange(), 
        rc = re.duplicate(); 
        re.moveToBookmark(r.getBookmark()); 
        rc.setEndPoint('EndToStart', re); 
        selection = rc.text.length; 
      } else { selection = 0 }
    var row = Math.floor((selection-1) / cols);
    var col = (selection - (row * cols));
    var x = Math.floor((col*(width/cols)));
    var y = (parseInt(height)*row);
    $('span').html("row: " + row + "<br>columns" + col + "<br>width: " + width + "<br>x: " + x +"px<br>y: " + y +"px<br>Scrolltop: "+$(this).scrollTop()).css('top',pos.top+y-$(this).scrollTop()).css('left',pos.left+x+10);
  });
});

HTML:

<textarea id="t"></textarea>
<br>
<span id="tooltip" style="background:yellow"></span>

CSS:

textarea {
  height: 80px;
  line-height: 12px;
  overflow-y:scroll;
}
span {
  position: absolute;
}

回答5:

I couldn't get something similar to work, so my solution was to locate the character position of the caret in the textarea, cut out the current paragraph and display this next to the textarea.

Using the offset, I placed a fake cursor (div, display:inline, 1px wide, border-left: 1px solid black) in this view of the editable text.

This way, you can create a visual feedback area where you can show the result of effects (quite like stackoverflow does when you write an answer).

来源：https://stackoverflow.com/questions/16212871/get-the-offset-position-of-the-caret-in-a-textarea-in-pixels

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