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问题:

So I feel like I'm a bit out of my league here. But here's what I want to do.

Basically, I want to have a user select part of text within in a paragraph (which may contain many elemnts (i.e. <span> and <a>) to return the value of the id attribute of that paragraph. Here's what I thinking.

function getParaID() //function will be called using a mouseUp event
{
var selObj = window.getSelection();
var selRange = selObj.getRangeAt(0); //btw can anyone explain what this zero means
var paraElement = selRange.commonAncestorContainer;
var paraID = paraElement.getAttribute;

return paraID;
 }

What do you think? Am I close?

回答1:

The selection range's commonAncestorContainer property may be a reference to a text node, or a <span> or <a> element or <body> element, or whatever else you may have in your page. That being the case, you need to work up the DOM tree to find the containing <p> element, if one exists. You also need to be aware that IE < 9 does not support window.getSelection() or DOM Range, although it is possible to do what you want quite easily in IE < 9. Here's some code that will work in all major browsers, including IE 6:

jsFiddle: http://jsfiddle.net/44Juf/

Code:

function getContainingP(node) {
    while (node) {
        if (node.nodeType == 1 && node.tagName.toLowerCase() == "p") {
            return node;
        }
        node = node.parentNode;
    }
}

function getParaID() {
    var p;
    if (window.getSelection) {
        var selObj = window.getSelection();
        if (selObj.rangeCount > 0) {
            var selRange = selObj.getRangeAt(0);
            p = getContainingP(selRange.commonAncestorContainer);
        }
    } else if (document.selection && document.selection.type != "Control") {
        p = getContainingP(document.selection.createRange().parentElement());
    }
    return p ? p.id : null;
}

Regarding the 0 passed to getRangeAt(), that is indicating which selected range you want. Firefox supports multiple selected ranges: if you make a selection and then hold down Ctrl and make another selection, you will see you now have two discontinous ranges selected, which can be accessed via getRangeAt(0) and getRangeAt(1). Also in Firefox, selecting a column of cells in a table creates a separate range for each selected cell. The number of selected ranges can be obtained using the rangeCount property of the selection. No other major browser supports multiple selected ranges.

回答2:

You're quite close. If all you want is the id of the parent element, then you should replace your paraElement.getAttribute with paraElement.id, like:

var paraID = paraElement.id;

Regarding the parameter to getRangeAt(), it is specifying the index of the selection range to return, and it's only really relevant to controls that allow discontinuous selections. For instance, a select box in which the user can use ctrl + click to select several arbitrary groups of rows simultaneously. In such a case you could use the parameter to step from one selected region to the next. But for highlighting text within a paragraph you should never have a discontinuous selection and thus can always pass 0. In essence it means that you're asking for "the first selected region".

Also note that if your interface allows the user's selection to span multiple paragraphs then your commonAncestorContainer may not be a paragraph, it might also be whatever element it is that contains all of your paragraph tags. So you should be prepared to handle that case.

Edit:

After playing with this a bit, here is my suggestion: http://jsfiddle.net/vCsZH/

Basically, instead of relying on commonAncestorContainer this code applies a mouseDown and a mouseUp listener to each paragraph (in addition to the one already applied to the top-level container). The listeners will, in essence, record the paragraphs that the selection range starts and ends at, making it much simpler to reliably work out which paragraph(s) are selected.

If ever there was a case in favor of using dynamic event binding through a framework like jQuery, this is it.