是否可以限制div可以拥有的最大孩子数量？

Question

So I want to limit the number of children a div can have.

The idea is that you have a selected commands div where you can drag and drop whichever commands (essentially buttons) are available to you at any given time, from another div called available commands , but only up to a maximum of four at a time.

Here's the fiddle: http://jsfiddle.net/v9Fg8/2/

How can I limit the number of maximum possible children this div can have, so that when it reaches 4, one of the children gets replaced on the drop event.

Answer 1

You could check if the container has too many elements and save the parent element when dragging like this

function drag(ev) {
    itemParent = ev.target.parentElement;
    //console.log(itemParent);
    ev.dataTransfer.setData("Text", ev.target.id);
};

function drop(ev) {
    ev.preventDefault();
    if (ev.target.childNodes.length == 4) {
        var data = ev.dataTransfer.getData("text");
        console.log(itemParent);
        itemParent.appendChild(document.getElementById(data));
    } else {
        var data = ev.dataTransfer.getData("Text");
        ev.target.appendChild(document.getElementById(data));
    }
};

DEMO

with jQuery

function drop(ev) {
    ev.preventDefault();
    if ($('.commands').not(itemParent).children().length == 4) {
        var data = ev.dataTransfer.getData("text");
        console.log(itemParent);
        itemParent.appendChild(document.getElementById(data));
    } else {
        var data = ev.dataTransfer.getData("Text");
        $('.commands').not(itemParent).append($('#' + data));
        //ev.target.appendChild(document.getElementById(data));
    }
};

This code is better, so you can drop the elements on elements so you don't append to the boxes.

DEMO

Answer 2

It can be super simple with getElementsBtClassName :

var dropped = document.getElementById('selected').getElementsByClassName('button');

and inside drop handler you just check dropped.length :

function drop(ev) {
    ev.preventDefault();
    var data = ev.dataTransfer.getData("Text");
    ev.target.appendChild(document.getElementById(data));

    // Check number of children
    if (dropped.length > 4) {
        ev.target.removeChild(dropped[0]);
    }
};

Demo: http://jsfiddle.net/v9Fg8/6/