前言

Swipe，常用来做轮播图，需要翻页的场景，最经典的开源库 swipe-js-iso ，不过更推荐使用 React 组件 react-swipe，它封装了 swipe-js-ios 组件，而 swipe-js-ios 组件则封装了 Swipe

HelloWord

ReactSwipe

如果单独使用的化，创建一个 swipe，dom 必须是三层结构，最里面一层是放 slide 的。

<div id="slider" class="swipe">
  <div class="swipe-wrap">
    <div></div>
    <div></div>
    <div></div>
  </div>
</div>

CSS 子元素 float: left; container 的宽度为自定义，swipe-wrap 的宽度为子页面数 * container 的 width，每一个 slide 的宽度为 container 的 width

.swipe {
  overflow: hidden;
  visibility: hidden;
  position: relative;
}
.swipe-wrap {
  overflow: hidden;
  position: relative;
}
.swipe-wrap > div {
  float: left;
  width: 100%;
  position: relative;
}

load 以后，创建 Swipe 即可

const mySwipe = Swipe(document.getElementById('slider'));

源码解析

swipe-js-ios 使用立即函数导出了一个 Swipe 模块，使用 typeof module !== 'undefined' && module.exports 兼容 Node 和 浏览器环境，如果是 Node 环境，将会有 module.export 那么则使用 module.export 导出，否则使用 root.Swipe 全局变量导出

(function(root, factory) {
  if (typeof module !== 'undefined' && module.exports) {
    module.exports = factory();
  } else {
    root.Swipe = factory();
  }
})(this, function() {
  'use strict';
  return function Swipe(container, options) {
        ....
  };
});

检查浏览器的环境，算是一种规范吧，风别检查触摸事件和 transition 的支持
⚠️在浏览器，手机模式下，触摸事件是存在的，而普通浏览器下是不存在的，所以该组件不能在普通浏览器中使用。

var browser = {
      addEventListener: !!window.addEventListener,
      touch:
        'ontouchstart' in window ||
        (window.DocumentTouch && document instanceof window.DocumentTouch),
      transitions: (function(temp) {
        var props = [
          'transitionProperty',
          'WebkitTransition',
          'MozTransition',
          'OTransition',
          'msTransition'
        ];
        for (var i in props)
          if (temp.style[props[i]] !== undefined) return true;
        return false;
      })(document.createElement('swipe'))
    };

创建时会调用 setup，继而添加事件，touch 触摸事件、transitionend 移动事件，resize 重新布局事件

    // trigger setup
    setup();

    // start auto slideshow if applicable
    if (delay) begin();

    // add event listeners
    if (browser.addEventListener) {
      // set touchstart event on element
      if (browser.touch) {
        element.addEventListener('touchstart', events, false);
        element.addEventListener('touchforcechange', function() {}, false);
      }

      if (browser.transitions) {
        element.addEventListener('webkitTransitionEnd', events, false);
        element.addEventListener('msTransitionEnd', events, false);
        element.addEventListener('oTransitionEnd', events, false);
        element.addEventListener('otransitionend', events, false);
        element.addEventListener('transitionend', events, false);
      }

      // set resize event on window
      window.addEventListener('resize', events, false);
    } else {
      window.onresize = function() {
        setup();
      }; // to play nice with old IE
    }

setup 函数的实现，slides 就是容器里面的页面，continuous 是否自动轮播，slidePos 记录了每一个页面的位置，width 是每一个页面的宽度，此处需要剪掉widthOfSiblingSlidePreview 的大小，可以预览前后页。element 的宽度是 **页数 * width **

function setup() {
      // cache slides
      slides = element.children;
      length = slides.length;

      // set continuous to false if only one slide
      continuous = slides.length < 2 ? false : options.continuous;

      // create an array to store current positions of each slide
      slidePos = new Array(slides.length);

      // determine width of each slide
      width =
        Math.round(
          container.getBoundingClientRect().width || container.offsetWidth
        ) -
        widthOfSiblingSlidePreview * 2;

      element.style.width = slides.length * width + 'px';
    }

初始化时，需要更新页面的位置

      var pos = slides.length;
      while (pos--) {
        var slide = slides[pos];

        slide.style.width = width + 'px';
        slide.setAttribute('data-index', pos);

        if (browser.transitions) {
          slide.style.left = pos * -width + widthOfSiblingSlidePreview + 'px';
          move(pos, index > pos ? -width : index < pos ? width : 0, 0);
        }
      }

如果支持轮播的化，需要把左边和右边也填充，然后把 container.style.visibility 设置为 visible，如果不支持 transition 的话，只需要设置 element.style.left 即可

  // reposition elements before and after index
      if (continuous && browser.transitions) {
        move(circle(index - 1), -width, 0);
        move(circle(index + 1), width, 0);
      }

      if (!browser.transitions)
        element.style.left = index * -width + widthOfSiblingSlidePreview + 'px';

      container.style.visibility = 'visible';

move 的实现，translate，更新 slidePos 的位置

    function move(index, dist, speed) {
      translate(index, dist, speed);
      slidePos[index] = dist;
    }

translate 三个参数，index：需要移动的页，dist：移动的位置，speed：移动速度，移动只需要给 页面设置 style 的 transform 就OK了，之后就会以动画移动过去了

function translate(index, dist, speed) {
      var slide = slides[index];
      var style = slide && slide.style;

      if (!style) return;

      style.webkitTransitionDuration = style.MozTransitionDuration = style.msTransitionDuration = style.OTransitionDuration = style.transitionDuration =
        speed + 'ms';

      style.webkitTransform = 'translate(' + dist + 'px,0)' + 'translateZ(0)';
      style.msTransform = style.MozTransform = style.OTransform =
        'translateX(' + dist + 'px)';
    }

prev 对外提供接口，手动翻页使用

   function prev() {
      if (continuous) slide(index - 1);
      else if (index) slide(index - 1);
   }

slide 移动函数，指定移动的页 index 和速度

function slide(to, slideSpeed) {
      // do nothing if already on requested slide
      if (index == to) return;

      if (browser.transitions) {
        var direction = Math.abs(index - to) / (index - to); // 1: backward, -1: forward

        // get the actual position of the slide
        if (continuous) {
          var natural_direction = direction;
          direction = -slidePos[circle(to)] / width;

          // if going forward but to < index, use to = slides.length + to
          // if going backward but to > index, use to = -slides.length + to
          if (direction !== natural_direction)
            to = -direction * slides.length + to;
        }

        var diff = Math.abs(index - to) - 1;

        // move all the slides between index and to in the right direction
        while (diff--)
          move(
            circle((to > index ? to : index) - diff - 1),
            width * direction,
            0
          );

        to = circle(to);

        move(index, width * direction, slideSpeed || speed);
        move(to, 0, slideSpeed || speed);

        if (continuous) move(circle(to - direction), -(width * direction), 0); // we need to get the next in place
      } else {
        to = circle(to); 
        animate(index * -width, to * -width, slideSpeed || speed);
        //no fallback for a circular continuous if the browser does not accept transitions
      }

      index = to;
      offloadFn(options.callback && options.callback(index, slides[index]));
    }

如果浏览器不支持 transition，那么则使用 setInterval 实现渐进移动， animation 是对整个页面进行移动，而 move 是移动每一个子页面

function animate(from, to, speed) {
      // if not an animation, just reposition
      if (!speed) {
        element.style.left = to + 'px';
        return;
      }

      var start = +new Date();

      var timer = setInterval(function() {
        var timeElap = +new Date() - start;

        if (timeElap > speed) {
          element.style.left = to + 'px';

          if (delay) begin();

          options.transitionEnd &&
            options.transitionEnd.call(event, index, slides[index]);

          clearInterval(timer);
          return;
        }

        element.style.left =
          (to - from) * (Math.floor((timeElap / speed) * 100) / 100) +
          from +
          'px';
      }, 4);
    }

接下来研究一下触摸事件的处理，首先是 start，start 事件会记录起始触摸位置以及时间，并且添加 touchmove 和 touchend 事件，如果没有 start 事件，触摸事件是不存在的， end 的时候会被移除。

start: function(event) {
        var touches = event.touches[0];

        // measure start values
        start = {
          // get initial touch coords
          x: touches.pageX,
          y: touches.pageY,

          // store time to determine touch duration
          time: +new Date()
        };

        // used for testing first move event
        isScrolling = undefined;

        // reset delta and end measurements
        delta = {};

        // attach touchmove and touchend listeners
        element.addEventListener('touchmove', this, false);
        element.addEventListener('touchend', this, false);
      },

move 事件，delta 将手指移动距离记下，最后视同 translate 移动

 move: function(event) {
        // ensure swiping with one touch and not pinching
        if (event.touches.length > 1 || (event.scale && event.scale !== 1))
          return;

        if (options.disableScroll) return;

        var touches = event.touches[0];

        // measure change in x and y
        delta = {
          x: touches.pageX - start.x,
          y: touches.pageY - start.y
        };

        // determine if scrolling test has run - one time test
        if (typeof isScrolling == 'undefined') {
          isScrolling = !!(
            isScrolling || Math.abs(delta.x) < Math.abs(delta.y)
          );
        }

        // if user is not trying to scroll vertically
        if (!isScrolling) {
          // prevent native scrolling
          event.preventDefault();

          // stop slideshow
          stop();

          // increase resistance if first or last slide
          if (continuous) {
            // we don't add resistance at the end

            translate(
              circle(index - 1),
              delta.x + slidePos[circle(index - 1)],
              0
            );
            translate(index, delta.x + slidePos[index], 0);
            translate(
              circle(index + 1),
              delta.x + slidePos[circle(index + 1)],
              0
            );
          } else {
            delta.x =
              delta.x /
              ((!index && delta.x > 0) || // if first slide and sliding left
              (index == slides.length - 1 && // or if last slide and sliding right
                delta.x < 0) // and if sliding at all
                ? Math.abs(delta.x) / width + 1 // determine resistance level
                : 1); // no resistance if false

            // translate 1:1
            translate(index - 1, delta.x + slidePos[index - 1], 0);
            translate(index, delta.x + slidePos[index], 0);
            translate(index + 1, delta.x + slidePos[index + 1], 0);
          }
          options.swiping && options.swiping(-delta.x / width);
        }
      },

end 事件，主要判断本次触摸滑动是否有效，并持续接下来的操作，最后将会 remove 掉监听事件。

end: function(event) {
        // measure duration
        var duration = +new Date() - start.time;

        // determine if slide attempt triggers next/prev slide
        var isValidSlide =
          (Number(duration) < 250 && // if slide duration is less than 250ms
            Math.abs(delta.x) > 20) || // and if slide amt is greater than 20px
          Math.abs(delta.x) > width / 2; // or if slide amt is greater than half the width

        // determine if slide attempt is past start and end
        var isPastBounds =
          (!index && delta.x > 0) || // if first slide and slide amt is greater than 0
          (index == slides.length - 1 && delta.x < 0); // or if last slide and slide amt is less than 0

        if (continuous) isPastBounds = false;

        // determine direction of swipe (true:right, false:left)
        var direction = delta.x < 0;

        // if not scrolling vertically
        if (!isScrolling) {
          if (isValidSlide && !isPastBounds) {
            if (direction) {
              if (continuous) {
                // we need to get the next in this direction in place

                move(circle(index - 1), -width, 0);
                move(circle(index + 2), width, 0);
              } else {
                move(index - 1, -width, 0);
              }

              move(index, slidePos[index] - width, speed);
              move(
                circle(index + 1),
                slidePos[circle(index + 1)] - width,
                speed
              );
              index = circle(index + 1);
            } else {
              if (continuous) {
                // we need to get the next in this direction in place

                move(circle(index + 1), width, 0);
                move(circle(index - 2), -width, 0);
              } else {
                move(index + 1, width, 0);
              }

              move(index, slidePos[index] + width, speed);
              move(
                circle(index - 1),
                slidePos[circle(index - 1)] + width,
                speed
              );
              index = circle(index - 1);
            }

            options.callback && options.callback(index, slides[index]);
          } else {
            if (continuous) {
              move(circle(index - 1), -width, speed);
              move(index, 0, speed);
              move(circle(index + 1), width, speed);
            } else {
              move(index - 1, -width, speed);
              move(index, 0, speed);
              move(index + 1, width, speed);
            }
          }
        }

        // kill touchmove and touchend event listeners until touchstart called again
        element.removeEventListener('touchmove', events, false);
        element.removeEventListener('touchend', events, false);
        element.removeEventListener('touchforcechange', function() {}, false);
      },

总结

总的来说，swipe-js-ios 充分利用了 transition，来实现移动动画，搞清楚触摸事件就比较容易能写出来可滑动的 swipe