Analysis of Vue drag and drop component development steps
这次给大家带来Vue拖拽组件开发步骤解析，Vue拖拽组件开发的注意事项有哪些，下面就是实战案例，一起来看一下。
为什么选择Vue?
主要原因：对于前端开发来说，兼容性是我们必须要考虑的问题之一。我们的项目不需要兼容低版本浏览器。项目本身也是一个数据驱动型的。加之，Vue本身具有以下主要特性：
•使用虚拟DOM；
•轻量级框架；
•高效的数据绑定；
•灵活的组件系统；
•完整的开发生态链。
这就是我们为什么选择Vue框架的一些原因。
为什么要封装成一个Vue组件？
主要目的是可提高代码的复用性和可维护性。
•复用性：组件化后，一些样式和逻辑均通过配置参数的方式去差异化体现，所以参数的可配置性提高了组件的复用率和灵活性。
•可维护性：组件化后，组件内部的逻辑只对组件负责，外部的逻辑只通过配置参数适配，所以提高了代码的逻辑清晰度，可以快速定位代码出现问题的地方。
组件化搭建页面图示：
上图可看出，在Vue中，所谓组件化搭建页面，简单来说，页面实际上是由一个个功能独立的组件搭建而成。这些组件之间可以组合、嵌套，最终形成了我们的页面。
组件构成
下面是一个完成的组件构成：
// 组件内模板 // 组件内逻辑代码 <script type="text/javascript"></script> // 组件内封装的样式 <style lang="scss" scoped></style>
开发Vue移动拖拽组件为例
拖拽原理
手指在移动的过程中，实时改变元素的位置即top和left值，使元素随着手指的移动而移动。
拖拽实现
•开始拖动时：获取到接触点相对于整个视图区的坐标clientX，clientY；获取元素距离视图上侧和左侧的距离 initTop，initLeft；计算接触点距离元素上侧和左侧的距离 elTop = clientY - initTop, elLeft = clientX - initLeft；
•拖动过程中：通过 currTop = clientY - elTop, currLeft = clientX - elLeft实时获取元素距离视图上侧和左侧的距离值，并赋值给元素，使元素跟着手指的移动而动起来；
•拖动结束，定位元素。
Vue中的实现
使用Vue，最大的不同之处是我们几乎不去操作DOM，要充分利用Vue的数据驱动来实现拖拽功能。本例中，我们只需在垂直方向上拖动元素，所以只需考虑垂直方向的移动即可。
上图中，通过data中的dragList渲染拖拽区域列表，代码如下：
template:
<p class="drag-title">拖拽可调整顺序</p>
<ul class="drag-list">
<li class="drag-item">{{item.txt}}</li>
</ul>
script:
export default {
data() {
return {
dragList:null
}
},
created() {
this.dragList = [
{
isDrag: false,
txt: '列表1',
isShow: false
}
...
]
},
}假设我们将元素从位置1拖至位置3，本质上是数组的顺序发生了改变。这就有必要提一下Vue的最大特性：数据驱动。
所谓的数据驱动就是当数据发生变化时，通过修改数据状态，使用户界面发生相应的改变，开发者不需要手动的去修改DOM。
Vue的数据驱动是通过MVVM这种框架来实现的，MVVM框架主要包含3个部分:Model、View、Viewmodel。
– Model：数据部分；
– View：视图部分；
– Viewmodel：连接视图与数据的中间件。
顺着这个思路走下去，我们知道：
– oldIndex：元素在数组中的初始索引index；
– elHeight：单个元素块的高；
– currTop = clientY - elTop：元素在拖动过程中距离可视区上侧距离；
– currTop - initTop > 0：得知元素是向上拖拽；
– currTop - initTop < 0：得知元素是向下拖拽。
我们以向下拖拽来说：
– 首先，我们要在拖拽结束事件touchend中判断元素从拖动开始到拖动结束时拖动的距离。若小于某个设定的值，则什么也不做；
– 然后，在touchmove事件中判断，若(currTop - initTop) % elHeight>= elHeight/2成立，即当元素拖至另一个元素块等于或超过1/2的位置时，即可将元素插入到最新的位置为newIndex = (currTop - initTop) / elHeight + oldIndex。
– 最后，若手指离开元素，那么我们在touchend事件中，通过this.dragList.splice(oldIndex, 1)，this.dragList.splice(newIndex, 0, item)重新调整数组顺序。页面会根据最新的dragList渲染列表。
写到这里，我们俨然已经用Vue实现了移动端的拖拽功能。但是拖拽体验并不好，接下来，我们对它进行优化。
优化点：我们希望，在元素即将可能落到的位置，提前留出一个可以放得下元素的区域，让用户更好的感知拖拽的灵活性。
方案：（方案已被验证是可行的）将li的结构做一下修改，代码如下：
<ul>
<li class="drag-item">
<p class="leave-block"></p>
// 向上拖拽时留空
<p class="">{{item.txt}}</p>
<p class="leave-block"></p>
// 向下拖拽时留空</li>
</ul>•拖拽开始：将元素的定位方式由static设置为absolute，z-index设置为一个较大的值，防止元素二次拖拽无效；
•拖拽过程中：将元素即将落入新位置的那个li下p的item.isShow设置为true，其他li下p的item.isShow均设置为false;
•拖拽结束：将所有li下p的item.isShow 均设置为false，将元素定位方式由absolute设置为static。
贴一段伪代码：
touchStart(e){
// 获取元素距离视口顶部的初始距离
initTop = e.currentTarget.offsetTop;
// 开始拖动时，获取鼠标距离视口顶部的距离
initClientY = e.touches[0].clientY;
// 计算出接触点距离元素顶部的距离
elTop = e.touches[0].clientY - initTop;
},
touchMove(index, item, e){
// 将拖拽结束时，给元素设置的static定位方式移除，防止元素二次拖拽无效
e.target.classList.remove('static');
// 给拖拽的元素设置绝对定位方式
e.target.classList.add('ab');
// 获取元素在拖拽过程中距离视口顶部距离
currTop = e.touches[0].clientY - elTop;
// 元素在拖拽过程中距离视口顶部距离赋给元素
e.target.style.top = currTop ;
// 获取元素初始位置
oldIndex = index;
// 获取拖拽元素
currItem = item;
// 若元素已经拖至区域外
if(e.touches[0].clientY > (this.dragList.length) * elHeight){
// 将元素距离上侧的距离设置为拖动区视图的高
currTop = (this.dragList.length) * elHeight;
return;
}
// 向下拖拽
if(currTop > initTop ){
// 若拖拽到大于等于元素的一半时，即可将元素插入到最新的位置
if((currTop - initTop) % elHeight>= elHeight / 2){
// 计算出元素拖到的最新位置
newIndex = Math.round((currTop - initTop) / elHeight) + index;
// 确保新元素的索引不能大于等于列表的长度
if(newIndex < this.dragList.length){
// 将所有列表留空处隐藏
for(var i = 0;i< this.dragList.length;i++){
this.dragList[i].isShow = false;
}
// 将元素即将拖到的新位置的留空展示
this.dragList[newIndex].isShow = true;
}
else {
return;
}
}
}
// 向上拖拽，原理同上
if(currTop < initTop){ ... } }, touchEnd(e){ // 若拖动距离大于某个设定的值，则按照上述，执行相关代码 if(Math.abs(e.changedTouches[0].clientY - initClientY ) > customVal){
this.dragList.splice(oldIndex, 1);
this.dragList.splice(newIndex, 0, currItem);
for(var i = 0;i< this.dragList.length;i++){
this.dragList[i].isShow = false;
this.dragList[i].isShowUp = false;
}
}
e.target.classList.remove('ab');
e.target.classList.add('static');
}优化后，如下图所示：
以上便是用Vue实现移动端拖拽组件的过程。我们知道，有些项目是需要在PC端用Vue实现此功能。这里简单提一下PC与移动端的区别如下：
•PC端可以使用的事件组有两种：第一种：H5新特性draggable，dragstart，drag，dragend；第二种：mousedown，mousemove，mouseup;
•PC端获取鼠标坐标是通过e.clientX，clientY，区别于移动端的e.touches[0].clientX，e.touches[0].clientY。
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