Merge pull request #195 from pacien/scroll-zoom

viewer/LdPicture: implement mousewheel and pinch zoom

1 files changed, 134 insertions, 0 deletions

diff --git a/viewer/src/services/ldzoom.ts b/viewer/src/services/ldzoom.ts
new file mode 100644
index 0000000..c28c2c8
--- /dev/null
+++ b/viewer/src/services/ldzoom.ts
@@ -0,0 +1,134 @@
+/* ldgallery - A static generator which turns a collection of tagged
+--             pictures into a searchable web gallery.
+--
+-- Copyright (C) 2020  Pacien TRAN-GIRARD
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU Affero General Public License for more details.
+--
+-- You should have received a copy of the GNU Affero General Public License
+-- along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+// polyfill still required for IE and Safari, see https://caniuse.com/#feat=resizeobserver
+import ResizeObserver from 'resize-observer-polyfill';
+import "hammerjs";
+
+/**
+ * Mousewheel and pinch zoom handler.
+ */
+export default class LdZoom {
+  readonly containerElement: HTMLDivElement;
+  readonly imageElement: HTMLImageElement;
+  readonly pictureProperties: Gallery.PictureProperties;
+  readonly maxScaleFactor: number;
+  readonly scrollZoomSpeed: number;
+  scaleFactor: number = 0.0;
+
+  constructor(
+    containerElement: HTMLDivElement, imageElement: HTMLImageElement,
+    pictureProperties: Gallery.PictureProperties,
+    maxScaleFactor: number, scrollZoomSpeed: number
+  ) {
+    this.containerElement = containerElement;
+    this.imageElement = imageElement;
+    this.pictureProperties = pictureProperties;
+    this.maxScaleFactor = maxScaleFactor;
+    this.scrollZoomSpeed = scrollZoomSpeed;
+  }
+
+  /**
+   * Register event listeners.
+   */
+  public install() {
+    this.updateImageScale(this.scaleFactor);
+
+    new ResizeObserver(() => {
+      this.updateImageScale(this.scaleFactor);
+    }).observe(this.containerElement);
+
+    this.containerElement.addEventListener('wheel', wheelEvent => {
+      wheelEvent.preventDefault();
+      const scaleFactor = this.scaleFactor - Math.sign(wheelEvent.deltaY) * this.scrollZoomSpeed;
+      this.zoom(wheelEvent.offsetX, wheelEvent.offsetY, scaleFactor);
+    });
+
+    const pinchListener = new Hammer(this.containerElement);
+    pinchListener.get('pinch').set({enable: true});
+    this.installPinchHandler(pinchListener);
+  }
+
+  private installPinchHandler(pinchListener: HammerManager) {
+    let startScaleFactor = 0.0;
+
+    pinchListener.on('pinchstart', (pinchEvent: HammerInput) => {
+      startScaleFactor = this.scaleFactor;
+    });
+
+    pinchListener.on('pinchmove', (pinchEvent: HammerInput) => {
+      const focusX = pinchEvent.center.x + this.containerElement.scrollLeft;
+      const focusY = pinchEvent.center.y + this.containerElement.scrollTop;
+      const scaleFactor = pinchEvent.scale * startScaleFactor;
+      this.zoom(focusX, focusY, scaleFactor);
+    });
+  }
+
+  /**
+   * Returns the picture resolution as it should be displayed.
+   */
+  private getDisplayResolution(): Gallery.Resolution {
+    return {
+      width: this.pictureProperties.resolution.width * this.scaleFactor,
+      height: this.pictureProperties.resolution.height * this.scaleFactor,
+    };
+  }
+
+  /**
+   * Applies scaling to the DOM image element.
+   * To call after internal intermediate computations because DOM properties aren't stable.
+   */
+  private resizeImageElement() {
+    const imageDim = this.getDisplayResolution();
+    this.imageElement.width = imageDim.width;
+    this.imageElement.height = imageDim.height;
+  }
+
+  /**
+   * Centers the image element inside its container if it fits, or stick to the top and left borders otherwise.
+   * It's depressingly hard to do in pure CSS…
+   */
+  private recenterImageElement() {
+    const imageDim = this.getDisplayResolution();
+    const marginLeft = Math.max((this.containerElement.clientWidth - imageDim.width) / 2, 0);
+    const marginTop = Math.max((this.containerElement.clientHeight - imageDim.height) / 2, 0);
+    this.imageElement.style.marginLeft = `${marginLeft}px`;
+    this.imageElement.style.marginTop = `${marginTop}px`;
+  }
+
+  private zoom(focusX: number, focusY: number, scaleFactor: number) {
+    const imageDim = this.getDisplayResolution();
+    const ratioX = focusX / imageDim.width;
+    const ratioY = focusY / imageDim.height;
+    this.updateImageScale(Math.min(scaleFactor, this.maxScaleFactor));
+
+    const newImageDim = this.getDisplayResolution();
+    this.containerElement.scrollLeft -= focusX - ratioX * newImageDim.width;
+    this.containerElement.scrollTop -= focusY - ratioY * newImageDim.height;
+  }
+
+  private updateImageScale(newScaleFactor: number) {
+    const horizontalFillRatio = this.containerElement.clientWidth / this.pictureProperties.resolution.width;
+    const verticalFillRatio = this.containerElement.clientHeight / this.pictureProperties.resolution.height;
+    const minScaleFactor = Math.min(horizontalFillRatio, verticalFillRatio, 1.0);
+    this.scaleFactor = Math.max(newScaleFactor, minScaleFactor);
+    this.resizeImageElement();
+    this.recenterImageElement();
+  }
+}