删除触摸开始目标后，触摸移动事件不会触发

Question

我正在尝试使用下一个模式实现类似拖动的功能：

订阅标记指针向下事件。
当 Down 事件触发时订阅 Window Pointer Move 和 Up 事件并删除标记。
移动时执行一些操作。
当 Up 事件触发时取消订阅 Move and Up。

这适用于鼠标事件，但不适用于触摸事件。在删除 Touch Start 目标元素后，它们不会触发。我尝试使用 Pointer Events Polyfill 但它也不起作用。

我正在使用 Chrome 开发工具来模拟触摸事件。查看示例：

initTestBlock('mouse', {
  start: 'mousedown',
  move: 'mousemove',
  end: 'mouseup'
});
initTestBlock('touch', {
  start: 'touchstart',
  move: 'touchmove',
  end: 'touchend'
});
initTestBlock('touch-no-remove', {
  start: 'touchstart',
  move: 'touchmove',
  end: 'touchend'
}, true);

function initTestBlock(id, events, noRemove) {
  var block = document.getElementById(id);
  var parent = block.querySelector('.parent');
  var target = block.querySelector('.target');
  target.addEventListener(events.start, function(e) {
    console.log(e.type);
    if (!noRemove) {
      setTimeout(function() {
        // Remove target
        target.parentElement.removeChild(target);
      }, 1000);
    }

    function onMove(e) {
      console.log(e.type);
      var pt = getCoords(e);
      parent.style.left = pt.x + 'px';
      parent.style.top = pt.y + 'px';
    }

    function onEnd(e) {
      console.log(e.type);
      window.removeEventListener(events.move, onMove);
      window.removeEventListener(events.end, onEnd);
    }

    window.addEventListener(events.move, onMove);
    window.addEventListener(events.end, onEnd);

  });
}

// Returns pointer coordinates
function getCoords(e) {
  if (e instanceof TouchEvent) {
    return {
      x: e.touches[0].pageX,
      y: e.touches[0].pageY
    };
  }
  return {
    x: e.pageX,
    y: e.pageY
  };
}

window.addEventListener('selectstart', function() {
  return false;
}, true);

.parent {
  background: darkred;
  color: white;
  width: 10em;
  height: 10em;
  position: absolute;
}
.target {
  background: orange;
  width: 4em;
  height: 4em;
}
#mouse .parent {
  left: 0em;
}
#touch .parent {
  left: 11em;
}
#touch-no-remove .parent {
  left: 22em;
}

<div id="mouse">
  <div class="parent">Mouse events
    <div class="target">Drag here</div>
  </div>
</div>
<div id="touch">
  <div class="parent">Touch events
    <div class="target">Drag here</div>
  </div>
</div>
<div id="touch-no-remove">
  <div class="parent">Touch (no remove)
    <div class="target">Drag here</div>
  </div>
</div>

Answer 1

诀窍是在触摸移动完成之前隐藏元素，而不是将其移除。这是一些示例（在 Chrome 开发工具和 select 某些设备中启用触摸模式或使用真实设备）： https://jsfiddle.net/alexanderby/na3rumjg/

var marker = document.querySelector('circle');
var onStart = function(startEvt) {
  startEvt.preventDefault(); // Prevent scroll
  marker.style.visibility = 'hidden'; // Hide target element
  var rect = document.querySelector('rect');
  var initial = {
    x: +rect.getAttribute('x'),
    y: +rect.getAttribute('y')
  };
  var onMove = function(moveEvt) {
    rect.setAttribute('x', initial.x + moveEvt.touches[0].clientX - startEvt.touches[0].clientX);
    rect.setAttribute('y', initial.y + moveEvt.touches[0].clientY - startEvt.touches[0].clientY);
  };
  var onEnd = function(endEvt) {
    window.removeEventListener('touchmove', onMove);
    window.removeEventListener('touchend', onEnd);
    marker.removeEventListener('touchstart', onStart);
    marker.parentElement.removeChild(marker); // Remove target element
  };
  window.addEventListener('touchmove', onMove);
  window.addEventListener('touchend', onEnd);
};
marker.addEventListener('touchstart', onStart);

<svg>
  <circle r="20" cx="50" cy="20" cursor="move"/>
  <rect x="10" y="50" width="80" height="80" />
</svg>

Answer 2

确实，according to the docs，

If the target element is removed from the document, events will still be targeted at it, and hence won't necessarily bubble up to the window or document anymore. If there is any risk of an element being removed while it is being touched, the best practice is to attach the touch listeners directly to the target.

事实证明，解决方案是将 touchmove 和 touchend 监听器附加到 event.target 本身，例如：

element.addEventListener("touchstart", (event) => {
    const onTouchMove = () => {
        // handle touchmove here
    }
    const onTouchEnd = () => {
        event.target.removeEventListener("touchmove", onTouchMove);
        event.target.removeEventListener("touchend", onTouchEnd);
        // handle touchend here
    }
    event.target.addEventListener("touchmove", onTouchMove);
    event.target.addEventListener("touchend", onTouchEnd);
    // handle touchstart here
});

即使从 DOM 中删除 event.target 元素，事件仍将继续正常触发并提供正确的坐标。

删除触摸开始目标后，触摸移动事件不会触发

Touch Move event don't fire after Touch Start target is removed

javascript

google-chrome

events

touch

pointer-events