使用pdfjs实现pdf预览，但是pdfjs本身不支持手势缩放。现两种方式实现PDF手势缩放。

1、调用pdfjs原生方法实现

• 在/pdfjs/web/viewer.html 页面添加如下代码。
• 此方法缩放时有卡顿，不够丝滑。

<script type="text/javascript">
 
    var touchState = null;   //记录触屏手势触点信息的对象
    
    //绑定触屏事件
    function addPinchListener() {
      let element = document.getElementById("viewerContainer");
      element.addEventListener("touchstart", onTouchStart, false);
      element.addEventListener("touchmove", onTouchMove, false);
      element.addEventListener("touchend", onTouchEnd, false);
    }
 
    //记录触屏触点坐标 记录起始和结束点
    function onTouchStart(evt) {
      touchState = {
        //多点触屏的第一点
        startX: evt.touches[0].pageX,
        startY: evt.touches[0].pageY,
        endX: evt.touches[0].pageX,
        endY: evt.touches[0].pageY,
 
        //多点触屏的第二点  单点触屏时记录坐标为 -1 
        startX2: evt.touches[1] ? evt.touches[1].pageX : -1,
        startY2: evt.touches[1] ? evt.touches[1].pageY : -1,
        endX2: evt.touches[1] ? evt.touches[1].pageX : -1,
        endY2: evt.touches[1] ? evt.touches[1].pageY : -1
      };
    }
 
    //记录触屏触点坐标 触屏移动时更新结束点坐标
    function onTouchMove(evt) {
      if (touchState === null) {
        return;
      }
      touchState.endX = evt.touches[0].pageX;
      touchState.endY = evt.touches[0].pageY;
      touchState.endX2 = evt.touches[1] ? evt.touches[1].pageX : -1;
      touchState.endY2 = evt.touches[1] ? evt.touches[1].pageY : -1;
    }
 
 
    //触屏结束时 判断是否放大缩小
    function onTouchEnd(evt) {
      if (touchState === null) {
        return;
      }
      //计算两点间距离
      var getDistance = function (startX, startY, endX, endY) {
        return Math.hypot(endX - startX, endY - startY);
      };
 
      if (touchState.startX2 != -1 && touchState.endX2 != -1 && touchState.startY2 != -1 && touchState.endY2 != -1) {
        let distanceStart = getDistance(touchState.startX, touchState.startY, touchState.startX2, touchState.startY2);
        let distanceEnd = getDistance(touchState.endX, touchState.endY, touchState.endX2, touchState.endY2);
        //起始时两点距离和结束时两单距离进行比较，判断是方法还是缩小
        if (distanceStart < distanceEnd) {
          PDFViewerApplication.pdfViewer.increaseScale();
        } else if (distanceStart > distanceEnd) {
          PDFViewerApplication.pdfViewer.decreaseScale();
        }
      }
    }
    
    addPinchListener();
  </script>

2、手势缩放的同时pdf同步缩放

• 在/pdfjs/web/viewer.html 页面添加如下代码。
• 此方法安卓基本正常使用，且比较缩放丝滑，但是ios缩放会页面崩溃。

 <script>
    let agent = navigator.userAgent.toLowerCase();
    if(!agent.includes("iphone")) {
        let pinchZoomEnabled = false;
        function enablePinchZoom(pdfViewer) {
            let startX = 0,
                startY = 0;
            let initialPinchDistance = 0;
            let pinchScale = 1;
            const viewer = document.getElementById("viewer");
            const container = document.getElementById("viewerContainer");
            const reset = () => {
                startX = startY = initialPinchDistance = 0;
                pinchScale = 1;
            };
            // Prevent native iOS page zoom
            //document.addEventListener("touchmove", (e) => { if (e.scale !== 1) { e.preventDefault(); } }, { passive: false });
            document.addEventListener("touchstart", (e) => {
                if (e.touches.length > 1) {
                    startX = (e.touches[0].pageX + e.touches[1].pageX) / 2;
                    startY = (e.touches[0].pageY + e.touches[1].pageY) / 2;
                    initialPinchDistance = Math.hypot(
                        e.touches[1].pageX - e.touches[0].pageX,
                        e.touches[1].pageY - e.touches[0].pageY
                    );
                } else {
                    initialPinchDistance = 0;
                }
            });
            document.addEventListener(
                "touchmove",
                (e) => {
                    if (initialPinchDistance <= 0 || e.touches.length < 2) {
                        return;
                    }
                    if (e.scale !== 1) {
                        e.preventDefault();
                    }
                    const pinchDistance = Math.hypot(
                        e.touches[1].pageX - e.touches[0].pageX,
                        e.touches[1].pageY - e.touches[0].pageY
                    );
                    const originX = startX + container.scrollLeft;
                    const originY = startY + container.scrollTop;
                    pinchScale = pinchDistance / initialPinchDistance;
                    viewer.style.transform = `scale(${pinchScale})`;
                    viewer.style.transformOrigin = `${originX}px ${originY}px`;
                }, {
                    passive: false
                }
            );
            document.addEventListener("touchend", (e) => {
                if (initialPinchDistance <= 0) {
                    return;
                }
                viewer.style.transform = `none`;
                viewer.style.transformOrigin = `unset`;
                PDFViewerApplication.pdfViewer.currentScale *= pinchScale;
                const rect = container.getBoundingClientRect();
                const dx = startX - rect.left;
                const dy = startY - rect.top;
                container.scrollLeft += dx * (pinchScale - 1);
                container.scrollTop += dy * (pinchScale - 1);
                reset();
            });
        }
        document.addEventListener("DOMContentLoaded", () => {
            if (!pinchZoomEnabled) {
                pinchZoomEnabled = true;
                enablePinchZoom();
            }
        });
    }

  </script>

每次进来pdf默认第一页和初始缩放比例

• 在view.js中搜到setInitialView方法 ，添加如下代码

    // 进来默认第一页
    this.pdfViewer.currentPageNumber = 1
    // 进来pdf显示默认缩放比例
    this.pdfViewer.currentScaleValue = _ui_utils.DEFAULT_SCALE_VALUE;