zulip/static/js/lightbox_canvas.js

var LightboxCanvas = (function () {
    var events = {
        documentMouseup: [],
        windowResize: [],
    };

    window.onload = function () {
        document.body.addEventListener("mouseup", function (e) {
            events.documentMouseup = events.documentMouseup.filter(function (event) {
                // go through automatic cleanup when running events.
                if (!document.body.contains(event.canvas)) {
                    return false;
                }

                event.callback.call(this, e);
                return true;
            });
        });

        window.addEventListener("resize", function (e) {
            events.windowResize = events.windowResize.filter(function (event) {
                if (!document.body.contains(event.canvas)) {
                    return false;
                }

                event.callback.call(this, e);

                return true;
            }.bind(this));
        });
    };

    var funcs = {
        setZoom: function (meta, zoom) {
            // make sure the zoom is above 1 and below the maxZoom.
            meta.zoom = Math.min(Math.max(zoom, 1), meta.maxZoom);
        },

        // this is a function given a canvas that attaches all of the events
        // required to pan and zoom.
        attachEvents: function (canvas, context, meta) {
            var mousedown = false;

            // wheelEvent.deltaMode is a value that describes what the unit is
            // for the `deltaX`, `deltaY`, and `deltaZ` properties.
            const DELTA_MODE = {
                PIXEL: 0,
                LINE: 1,
                PAGE: 2,
            };

            // give object structure in `mousedown`, because its props are only
            // ever set once `mousedown` + `mousemove` is triggered.
            var lastPosition = {};

            // in browsers such as Safari, the `e.movementX` and `e.movementY`
            // props don't exist, so we need to create them as a difference of
            // where the last `layerX` and `layerY` movements since the last
            // `mousemove` event in this `mousedown` event were registered.
            var polyfillMouseMovement = function (e) {
                e.movementX = (e.layerX - lastPosition.x) || 0;
                e.movementY = (e.layerY - lastPosition.y) || 0;

                lastPosition = {
                    x: e.layerX,
                    y: e.layerY,
                };
            };

            // use the wheel event rather than scroll because this isn't
            // actually an element that can scroll. The wheel event will
            // detect the *gesture* of scrolling over an element, without actually
            // worrying about scrollable content.
            canvas.addEventListener("wheel", function (e) {
                e.preventDefault();

                // this is to reverese scrolling directions for the image.
                var delta = meta.direction * e.deltaY;

                if (e.deltaMode === DELTA_MODE.LINE) {
                    // the vertical height in pixels of an approximate line.
                    delta *= 15;
                }

                if (e.deltaMode === DELTA_MODE.PAGE) {
                    // the vertical height in pixels of an approximate page.
                    delta *= 300;
                }

                // this is calculated as the user defined speed times the normalizer
                // (which just is what it takes to take the raw delta and transform
                // it to a normal speed), multiply it against the current zoom.
                // Example:
                // delta = 8
                // normalizedDelta = delta * (1 / 20) * 1 = 0.4
                // zoom = zoom * (0.4 / 100) + 1
                var zoom = meta.zoom * (
                    (meta.speed * meta.internalSpeedMultiplier * delta / 100) + 1
                );

                funcs.setZoom(meta, zoom);
                funcs.displayImage(canvas, context, meta);

                return false;
            });

            // the only valid mousedown events should originate inside of the
            // canvas.
            canvas.addEventListener("mousedown", function () {
                mousedown = true;
            });

            // on mousemove, actually run the pan events.
            canvas.addEventListener("mousemove", function (e) {
                // to pan, there must be mousedown and mousemove, check if valid.
                if (mousedown === true) {
                    polyfillMouseMovement(e);
                    // find the percent of movement relative to the canvas width
                    // since e.movementX, e.movementY are in px.
                    var percentMovement = {
                        x: (e.movementX / canvas.width),
                        y: (e.movementY / canvas.height),
                    };

                    // add the percentMovement to the meta coordinates but divide
                    // out by the zoom ratio because when zoomed in 10x for example
                    // moving the photo by 1% will appear like 10% on the <canvas>.
                    meta.coords.x += percentMovement.x * 2 / meta.zoom;
                    meta.coords.y += percentMovement.y * 2 / meta.zoom;

                    // redraw the image.
                    funcs.displayImage(canvas, context, meta);
                }
            });

            // make sure that when the mousedown is lifted on <canvas>to prevent
            // panning events.
            canvas.addEventListener("mouseup", function () {
                mousedown = false;
                // reset this to be empty so that the values will `NaN` on first
                // mousemove and default to a change of (0, 0).
                lastPosition = {};
            });


            // do so on the document.body as well, though depending on the infra,
            // these are less reliable as preventDefault may prevent these events
            // from propagating all the way to the <body>.
            events.documentMouseup.push({
                canvas: canvas,
                meta: meta,
                callback: function () {
                    mousedown = false;
                },
            });

            events.windowResize.push({
                canvas: canvas,
                meta: meta,
                callback: function () {
                    funcs.sizeCanvas(canvas, meta);
                    funcs.displayImage(canvas, context, meta);
                },
            });
        },

        imageRatio: function (image) {
            return image.naturalWidth / image.naturalHeight;
        },

        displayImage: function (canvas, context, meta) {
            meta.coords.x = Math.max(1 / (meta.zoom * 2), meta.coords.x);
            meta.coords.x = Math.min(1 - (1 / (meta.zoom * 2)), meta.coords.x);

            meta.coords.y = Math.max(1 / (meta.zoom * 2), meta.coords.y);
            meta.coords.y = Math.min(1 - (1 / (meta.zoom * 2)), meta.coords.y);

            var c = {
                x: meta.coords.x - 1,
                y: meta.coords.y - 1,
            };

            var x = (meta.zoom * c.x * canvas.width) + canvas.width / 2;
            var y = (meta.zoom * c.y * canvas.height) + canvas.height / 2;
            var w = canvas.width * meta.zoom;
            var h = canvas.height * meta.zoom;

            canvas.width = canvas.width;
            context.imageSmoothingEnabled = false;

            context.drawImage(meta.image, x, y, w, h);
        },

        // the `sizeCanvas` method figures out the appropriate bounding box for
        // the canvas given a parent that has constraints.
        // for example, if a photo has a ration of 1.5:1 (w:h), and the parent
        // box is 1:1 respectively, we want to stretch the photo to be as large
        // as we can, which means that we check if having the photo width = 100%
        // means that the height is less than 100% of the parent height. If so,
        // then we size the photo as w = 100%, h = 100% / 1.5.
        sizeCanvas: function (canvas, meta) {
            if (typeof meta.onresize === "function") {
                meta.onresize(canvas);
            }

            var parent = {
                width: canvas.parentNode.clientWidth,
                height: canvas.parentNode.clientHeight,
            };

            if (parent.height * meta.ratio > parent.width) {
                canvas.width = parent.width * 2;
                canvas.style.width = parent.width + "px";


                canvas.height = (parent.width / meta.ratio) * 2;
                canvas.style.height = parent.width / meta.ratio + "px";
            } else {
                canvas.height = parent.height * 2;
                canvas.style.height = parent.height + "px";

                canvas.width = parent.height * meta.ratio * 2;
                canvas.style.width = parent.height * meta.ratio + "px";
            }

            blueslip.warn("Please specify a 'data-width' or 'data-height' argument for canvas.");
        },
    };

    // a class w/ prototype to create a new `LightboxCanvas` instance.
    var __LightboxCanvas = function (el) {
        var self = this;

        this.meta = {
            direction: -1,
            zoom: 1,
            image: null,
            coords: {
                x: 0.5,
                y: 0.5,
            },
            speed: 1,
            // this is to normalize the speed to what I would consider to be
            // "standard" zoom speed.
            internalSpeedMultiplier: 0.05,
            maxZoom: 10,
        };

        if (el instanceof Node) {
            this.canvas = el;
        } else if (typeof el === "string") {
            this.canvas = document.querySelector(el);
        } else {
            blueslip.warn("Error. 'LightboxCanvas' accepts either string selector or node.");
            return;
        }

        this.context = this.canvas.getContext("2d");

        this.meta.image = new Image();
        this.meta.image.src = this.canvas.getAttribute("data-src");
        this.meta.image.onload = function () {
            self.meta.ratio = funcs.imageRatio(this);

            funcs.sizeCanvas(self.canvas, self.meta);
            funcs.displayImage(self.canvas, self.context, self.meta);
        };

        this.canvas.image = this.meta.image;

        funcs.attachEvents(this.canvas, this.context, self.meta);
    };

    __LightboxCanvas.prototype = {
        // set the speed at which scrolling zooms in on a photo.
        speed: function (speed) {
            this.meta.speed = speed;
        },

        // set the max zoom of the `LightboxCanvas` canvas as a mult of the total width.
        maxZoom: function (maxZoom) {
            this.meta.maxZoom = maxZoom;
        },

        reverseScrollDirection: function () {
            this.meta.direction = 1;
        },

        setZoom: function (zoom) {
            funcs.setZoom(this.meta, zoom);
            funcs.displayImage(this.canvas, this.context, this.meta);
        },

        resize: function (callback) {
            this.meta.onresize = callback;
        },
    };

    return __LightboxCanvas;
}());

if (typeof module !== 'undefined') {
    module.exports = LightboxCanvas;
}