纯javascript可拖动元素

Question

我知道网上有这样做的例子，但每个例子都不一样，我自己的实现也是如此。我试图弄清楚我的实现出了什么问题，因为它没有按预期工作。

代码sn-p：

var mousePressX = -1;
var mousePressY = -1;

document.getElementById("contextMenu").addEventListener("mousedown", function(event) {
    mousePressX = event.clientX;
    mousePressY = event.clientY;
}, false);

document.getElementById("contextMenu").addEventListener("mouseup", function(event) {
    mousePressX = -1;
    mousePressY = -1;
}, false);

document.getElementById("contextMenu").addEventListener("mousemove", function(event) {
    if (mousePressX != -1 && mousePressY != -1) {
        repositionElement(event.target, event.clientX, event.clientY);
    }
}, false);

function repositionElement(element, currentMouseX, currentMouseY) {
    element.style.left = element.offsetLeft + (currentMouseX - mousePressX) + 'px';
    element.style.top = element.offsetTop + (currentMouseY - mousePressY) + 'px';
}

jsfiddle： http://jsfiddle.net/4rLctko8/

无法真正找出问题所在，但我注意到如果我更改以下几行：

element.style.left = element.offsetLeft + (currentMouseX - mousePressX) + 'px';
element.style.top = element.offsetTop + (currentMouseY - mousePressY) + 'px';

到：

element.style.left = currentMouseX - mousePressX + 'px';
element.style.top = currentMouseY - mousePressY + 'px';

只有当元素被拖动到正 x 轴（向右）和正 y 轴（到底部）时，元素才会被正确拖动，并且当触发 mouseup 时，它将定位在附近的某个地方最左上角区域（0,0左右）

Answer 1

很有趣，到目前为止只用 jQuery 做过。稍微改写一下，并确保每个 mousemove 之后都删除了它的事件侦听器 - 这将是内存泄漏，否则您可能会开始注意到：

http://jsfiddle.net/8wtq17L8/

var contextmenu = document.getElementById('contextMenu');
var initX, initY, mousePressX, mousePressY;

contextmenu.addEventListener('mousedown', function(event) {

    initX = this.offsetLeft;
    initY = this.offsetTop;
    mousePressX = event.clientX;
    mousePressY = event.clientY;

    this.addEventListener('mousemove', repositionElement, false);

    window.addEventListener('mouseup', function() {
      contextmenu.removeEventListener('mousemove', repositionElement, false);
    }, false);

}, false);

function repositionElement(event) {
    this.style.left = initX + event.clientX - mousePressX + 'px';
    this.style.top = initY + event.clientY - mousePressY + 'px';
}

似乎运作良好。 :-)

++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++

编辑 - 我想我也会添加一个支持触摸的版本（尽管较新的设备似乎可以模拟鼠标事件）。不像 Jquery，您可以选择多个事件侦听器，因此它主要是重复，仅使用触摸事件：

http://codepen.io/Shikkediel/pen/VLZKor?editors=011

var object = document.getElementById('element'),
initX, initY, firstX, firstY;

object.addEventListener('mousedown', function(e) {

    e.preventDefault();
    initX = this.offsetLeft;
    initY = this.offsetTop;
    firstX = e.pageX;
    firstY = e.pageY;

    this.addEventListener('mousemove', dragIt, false);

    window.addEventListener('mouseup', function() {
        object.removeEventListener('mousemove', dragIt, false);
    }, false);

}, false);

object.addEventListener('touchstart', function(e) {

    e.preventDefault();
    initX = this.offsetLeft;
    initY = this.offsetTop;
    var touch = e.touches;
    firstX = touch[0].pageX;
    firstY = touch[0].pageY;

    this.addEventListener('touchmove', swipeIt, false);

    window.addEventListener('touchend', function(e) {
        e.preventDefault();
        object.removeEventListener('touchmove', swipeIt, false);
    }, false);

}, false);

function dragIt(e) {
    this.style.left = initX+e.pageX-firstX + 'px';
    this.style.top = initY+e.pageY-firstY + 'px';
}

function swipeIt(e) {
    var contact = e.touches;
    this.style.left = initX+contact[0].pageX-firstX + 'px';
    this.style.top = initY+contact[0].pageY-firstY + 'px';
}