javascript - 如何使用纯 javascript 在屏幕上平滑拖动图像

Question

我正在尝试使用 JavaScript 在屏幕上拖动图像。我已经编写了一个脚本，可以在带有文本的 div 上正常工作，但是当我在图像上使用它时，它的工作非常间歇。

我已将我的代码放在 jsfiddle 上，以便其他人可以理解我的意思。 http://jsfiddle.net/laurence/YNMEX/

如果你运行它你会发现文本块是可以拖放的，但是当你对图像尝试同样的事情时，它会留下图像。就像图像跟不上鼠标的移动一样。

我在下面的 jsfiddle 上重复了代码：

function startDrag(e) {
  // determine event object
  if (!e) {
    var e = window.event;
  }

  // IE uses srcElement, others use target
  var targ = e.target ? e.target : e.srcElement;

  if (targ.className != 'dragme') {
    return
  };
  // calculate event X, Y coordinates
  offsetX = e.clientX;
  offsetY = e.clientY;

  // assign default values for top and left properties
  if (!targ.style.left) {
    targ.style.left = '0px'
  };
  if (!targ.style.top) {
    targ.style.top = '0px'
  };

  // calculate integer values for top and left 
  // properties
  coordX = parseInt(targ.style.left);
  coordY = parseInt(targ.style.top);
  drag = true;

  // move div element
  document.onmousemove = dragDiv;
}

function dragDiv(e) {
  if (!drag) {
    return
  };
  if (!e) {
    var e = window.event
  };
  var targ = e.target ? e.target : e.srcElement;
  // move div element
  targ.style.left = coordX + e.clientX - offsetX + 'px';
  targ.style.top = coordY + e.clientY - offsetY + 'px';
  return false;
}

function stopDrag() {
  drag = false;
}
window.onload = function() {
  document.onmousedown = startDrag;
  document.onmouseup = stopDrag;
}

.dragme {
  position: relative;
  width: 270px;
  height: 203px;
  cursor: move;
}

#draggable {
  background-color: #ccc;
  border: 1px solid #000;
}

<div id="draggable" class="dragme">"Hello World!"</div>
<img src="https://picsum.photos/270/203" alt="drag-and-drop image script" title="drag-and-drop image script" class="dragme">

Answer 1

只需在函数return false末尾添加startDrag即可防止浏览器处理点击事件。

Answer 2

也targ应该只在startDragand上分配global（不带 var）：

http://jsfiddle.net/gigyme/YNMEX/132/

<script type="text/javascript">
    function startDrag(e) {
            // determine event object
            if (!e) {
                var e = window.event;
            }
            if(e.preventDefault) e.preventDefault();

            // IE uses srcElement, others use target
            targ = e.target ? e.target : e.srcElement;

            if (targ.className != 'dragme') {return};
            // calculate event X, Y coordinates
                offsetX = e.clientX;
                offsetY = e.clientY;

            // assign default values for top and left properties
            if(!targ.style.left) { targ.style.left='0px'};
            if (!targ.style.top) { targ.style.top='0px'};

            // calculate integer values for top and left 
            // properties
            coordX = parseInt(targ.style.left);
            coordY = parseInt(targ.style.top);
            drag = true;

            // move div element
                document.onmousemove=dragDiv;
            return false;

        }
        function dragDiv(e) {
            if (!drag) {return};
            if (!e) { var e= window.event};
            // var targ=e.target?e.target:e.srcElement;
            // move div element
            targ.style.left=coordX+e.clientX-offsetX+'px';
            targ.style.top=coordY+e.clientY-offsetY+'px';
            return false;
        }
        function stopDrag() {
            drag=false;
        }
        window.onload = function() {
            document.onmousedown = startDrag;
            document.onmouseup = stopDrag;
        }
</script>

Answer 3

您可以在函数e.preventDefault();末尾添加startDrag

Answer 4

绑定拖动事件的特定元素。

function log() {
    var debug = true;
    if (!debug)
        return;
    if (arguments) {
        for (var i = 0; i < arguments.length; i++) {
            console.log(arguments[i]);
        }
    }
}    

function Drag(element) {
    this.dragging = false;
    this.mouseDownPositionX = 0;
    this.mouseDownPositionY = 0;
    this.elementOriginalLeft = 0;
    this.elementOriginalTop = 0;
    var ref = this;
    this.startDrag = function (e) {
        e.preventDefault();
        ref.dragging = true;
        ref.mouseDownPositionX = e.clientX;
        ref.mouseDownPositionY = e.clientY;
        ref.elementOriginalLeft = parseInt(element.style.left);
        ref.elementOriginalTop = parseInt(element.style.top);
        // set mousemove event
        window.addEventListener('mousemove', ref.dragElement);
        log('startDrag');
    };
    this.unsetMouseMove = function () {
        // unset mousemove event
        window.removeEventListener('mousemove', ref.dragElement);
    };
    this.stopDrag = function (e) {
        e.preventDefault();
        ref.dragging = false;
        ref.unsetMouseMove();
        log('stopDrag');
    };
    this.dragElement = function (e) {
        log('dragging');
        if (!ref.dragging)
            return;
        e.preventDefault();
        // move element
        element.style.left = ref.elementOriginalLeft + (e.clientX - ref.mouseDownPositionX) + 'px';
        element.style.top = ref.elementOriginalTop + (e.clientY - ref.mouseDownPositionY) + 'px';
        log('dragElement');
    };
    element.onmousedown = this.startDrag;
    element.onmouseup = this.stopDrag;
}

var myDrag = new Drag(yourElement);// bind event
console.log(myDrag.dragging);