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Keyboard Accessibility

A chart behavior is easily extendable with the use of Chart Modifiers. So they can be used to add a keyboard accessibility to the chart. See also What is the ChartModifier API, Common ChartModifiers Features.

Custom Keyboard Interaction Modifier​

For this example we created a simple custom modifier - KeyboardZoomPanModifier, which adds an ability to scroll the chart with arrow buttons and to zoom with "+"/"-" buttons. The same approach can be used to add more custom keyboard bindings.

Here is the definition of the KeyboardZoomPanModifier:

import {
    ChartModifierBase2D,
    EClipMode,
    Point,
    EXyDirection
} from "scichart";

// ...
const DEFAULT_SCROLL_DELTA = 100;
const DEFAULT_ZOOM_DELTA = 120;
class KeyboardZoomPanModifier extends ChartModifierBase2D {
    constructor(options) {
        super(options);
        this.type = "KeyboardZoomPan";
        /**
         * Modifies the speed of  zoom, for example growFactor = 0.001 means each 'click'
         * zooms the chart 0.1%
         */
        this.growFactor = options && options.growFactor || 0.001;

        /**
         * Modifies the speed of scroll, for example scrollFactor = 0.001 means each 'click'
         * scrolls the chart 0.1%
         */
        this.scrollFactor = options && options.scrollFactor || 0.001;
        this.handleKeyDown = this.handleKeyDown.bind(this);
    }
    scroll(xDelta, yDelta) {
        // Scroll the X,YAxis by the number of pixels since the last update
        const token = this.parentSurface.suspendUpdates();
        if ([EXyDirection.XDirection, EXyDirection.XyDirection].includes(this.xyDirection)) {
            this.parentSurface.xAxes.asArray().forEach(axis => {
                const delta = (axis.isHorizontalAxis ? xDelta : -yDelta) * this.scrollFactor;
                axis.scroll(axis.flippedCoordinates ? -delta : delta, EClipMode.None);
            });
        }
        if ([EXyDirection.YDirection, EXyDirection.XyDirection].includes(this.xyDirection)) {
            this.parentSurface.yAxes.asArray().forEach(axis => {
                const delta = (axis.isHorizontalAxis ? -xDelta : yDelta) * this.scrollFactor;
                axis.scroll(axis.flippedCoordinates ? -delta : delta, EClipMode.None);
            });
        }
        token.resume();
    }
    onAttach() {
        // set tabIndex attribute of the chart root element if it was not set externally
        this.parentSurface.domChartRoot.tabIndex = this.parentSurface.domChartRoot.tabIndex || 0;
        // subscribe to keyboard input event
        this.parentSurface.domChartRoot.addEventListener("keydown", this.handleKeyDown);
    }
    onDetach() {
        // unsubscribe from keyboard input event
        this.parentSurface.domChartRoot.removeEventListener("keydown", this.handleKeyDown);
    }
    /**
     * Performs the zoom operation around the mouse point
     * @param mousePoint The X,Y location of the mouse at the time of the zoom
     * @param delta the delta factor of zoom
     */
    performZoom(mousePoint, delta) {
        const fraction = this.growFactor * delta;
        if ([EXyDirection.XDirection, EXyDirection.XyDirection].includes(this.xyDirection)) {
            this.parentSurface.xAxes.asArray().forEach(axis => {
                this.growBy(mousePoint, axis, fraction);
            });
        }
        if ([EXyDirection.YDirection, EXyDirection.XyDirection].includes(this.xyDirection)) {
            this.parentSurface.yAxes.asArray().forEach(axis => {
                this.growBy(mousePoint, axis, fraction);
            });
        }
    }
    handleKeyDown(event) {
        // ignore key combinations
        if (event.ctrlKey || event.altKey || event.metaKey) {
            return;
        }
        switch (event.key) {
            case "ArrowUp":
                this.scroll(0, DEFAULT_SCROLL_DELTA);
                break;
            case "ArrowDown":
                this.scroll(0, -DEFAULT_SCROLL_DELTA);
                break;
            case "ArrowRight":
                this.scroll(DEFAULT_SCROLL_DELTA, 0);
                break;
            case "ArrowLeft":
                this.scroll(-DEFAULT_SCROLL_DELTA, 0);
                break;
            case "+": {
                const zoomPoint = new Point(
                    this.parentSurface.seriesViewRect.width / 2,
                    this.parentSurface.seriesViewRect.height / 2
                );
                this.performZoom(zoomPoint, -DEFAULT_ZOOM_DELTA);
                break;
            }
            case "-": {
                const zoomPoint = new Point(
                    this.parentSurface.seriesViewRect.width / 2,
                    this.parentSurface.seriesViewRect.height / 2
                );
                this.performZoom(zoomPoint, DEFAULT_ZOOM_DELTA);
                break;
            }
            default:
                return;
        }
        // prevent default behavior if the key is used by the modifier
        event.preventDefault();
    }
}

 Now let's look closer at the properties and methods of this class.

First of all the class is extended from ChartModifierBase2D📘, which provides it with some properties and methods required on a chart modifier.

Also we defined several optional properties on the class which may be useful for configuring the modifier: type, growFactor, scrollFactor.

To make it possible for a chart to respond to key press events we need to make sure that it is focusable by keyboard navigation and add appropriate event listeners.

We can do this inside the onAttach/onDetach methods, which are called when the modifier is added to the chart:

onAttach() {
    // set tabIndex attribute of the chart root element if it was not set externally
    this.parentSurface.domChartRoot.tabIndex = this.parentSurface.domChartRoot.tabIndex || 0;
    // subscribe to keyboard input event
    this.parentSurface.domChartRoot.addEventListener("keydown", this.handleKeyDown);
}
onDetach() {
    // unsubscribe from keyboard input event
    this.parentSurface.domChartRoot.removeEventListener("keydown", this.handleKeyDown);
}

 KeyboardZoomPanModifier.handleKeyDown method is the one responsible for handling different keyboard inputs.

Depending on the key pressed it will call KeyboardZoomPanModifier.scroll or KeyboardZoomPanModifier.performZoom to updated the visible ranges.

tip

KeyboardZoomPanModifier.scroll uses the SuspendUpdates API for more info check Batching updates or Temporary Suspending Drawing.

Finally, the usage of the modifier simply looks like:

sciChartSurface.chartModifiers.add(new KeyboardZoomPanModifier({ scrollFactor: 0.1 }));

 Make sure the chart root element is focused by keyboard navigation for modifier to work. E.g. you can make the element focused by default:

// focus on scichart root to allow scichart detect keyboard events

sciChartSurface.domChartRoot.focus();

See Also​