如何防止可拖动元素移出边界？

Question

通过使用与w3Schools中的示例大致相同的代码，我创建了一个可拖动的元素：

//Make the DIV element draggagle:
dragElement(document.getElementById(("mydiv")));

function dragElement(elmnt) {
  var pos1 = 0, pos2 = 0, pos3 = 0, pos4 = 0;
  if (document.getElementById(elmnt.id + "header")) {
    /* if present, the header is where you move the DIV from:*/
    document.getElementById(elmnt.id + "header").onmousedown = dragMouseDown;
  } else {
    /* otherwise, move the DIV from anywhere inside the DIV:*/
    elmnt.onmousedown = dragMouseDown;
  }

  function dragMouseDown(e) {
    e = e || window.event;
    // get the mouse cursor position at startup:
    pos3 = e.clientX;
    pos4 = e.clientY;
    document.onmouseup = closeDragElement;
    // call a function whenever the cursor moves:
    document.onmousemove = elementDrag;
  }

  function elementDrag(e) {
    e = e || window.event;
    // calculate the new cursor position:
    pos1 = pos3 - e.clientX;
    pos2 = pos4 - e.clientY;
    pos3 = e.clientX;
    pos4 = e.clientY;
    // set the element's new position:
    elmnt.style.top = (elmnt.offsetTop - pos2) + "px";
    elmnt.style.left = (elmnt.offsetLeft - pos1) + "px";
  }

  function closeDragElement() {
    /* stop moving when mouse button is released:*/
    document.onmouseup = null;
    document.onmousemove = null;
  }
}

问题是我需要将元素限制在视口内部以及一小部分填充。我有一个解决方案，你只需检查元素位置是否小于0，如果它被设置为0或如果它大于宽度/高度设置为宽度/高度。这不够好，因为它创造了一种我不想要的反复弹跳效果。

所以我想知道这个问题是否有更明显的解决方案？不使用jQuery也是如此。

Answer 1

您需要跟踪可用的视口（窗口调整大小可能会更改它）并确保元素不会越过边界。如果元素将越过边界，您应该什么也不做，而不是让它交叉然后再设置它。下面是一个工作示例。

请注意，w3schools的代码示例在其样式和功能使用方面已经过时了 - 有更优雅的方式来支持现代HTML中的拖动。

var PADDING = 8;

var rect;
var viewport = {
  bottom: 0,
  left: 0,
  right: 0,
  top: 0
}

//Make the DIV element draggagle:
dragElement(document.getElementById(("mydiv")));

function dragElement(elmnt) {
  var pos1 = 0, pos2 = 0, pos3 = 0, pos4 = 0;
  if (document.getElementById(elmnt.id + "header")) {
    /* if present, the header is where you move the DIV from:*/
    document.getElementById(elmnt.id + "header").onmousedown = dragMouseDown;
  } else {
    /* otherwise, move the DIV from anywhere inside the DIV:*/
    elmnt.onmousedown = dragMouseDown;
  }

  function dragMouseDown(e) {
    e = e || window.event;
    // get the mouse cursor position at startup:
    pos3 = e.clientX;
    pos4 = e.clientY;
    
    // store the current viewport and element dimensions when a drag starts
    rect = elmnt.getBoundingClientRect();
    viewport.bottom = window.innerHeight - PADDING;
    viewport.left = PADDING;
    viewport.right = window.innerWidth - PADDING;
    viewport.top = PADDING;
    
    document.onmouseup = closeDragElement;
    // call a function whenever the cursor moves:
    document.onmousemove = elementDrag;
  }

  function elementDrag(e) {
    e = e || window.event;
    // calculate the new cursor position:
    pos1 = pos3 - e.clientX;
    pos2 = pos4 - e.clientY;
    pos3 = e.clientX;
    pos4 = e.clientY;
    
    // check to make sure the element will be within our viewport boundary
    var newLeft = elmnt.offsetLeft - pos1;
    var newTop = elmnt.offsetTop - pos2;

    if (newLeft < viewport.left
        || newTop < viewport.top
        || newLeft + rect.width > viewport.right
        || newTop + rect.height > viewport.bottom
    ) {
    	// the element will hit the boundary, do nothing...
    } else {
      // set the element's new position:
      elmnt.style.top = (elmnt.offsetTop - pos2) + "px";
      elmnt.style.left = (elmnt.offsetLeft - pos1) + "px";
    }
  }

  function closeDragElement() {
    /* stop moving when mouse button is released:*/
    document.onmouseup = null;
    document.onmousemove = null;
  }
}

#mydiv {
  border: 1px solid black;
  height: 100px;
  left: 8px;
  position: absolute;
  top: 8px;
  width: 100px;
}

<div id="mydiv"></div>