Custom Modifiers - the GestureModifierBase API

The GestureModifierBase provides an abstract base class for all Gesture-Based Modifiers within SciChart. Using this API, you can create gestures that you can attach to a chart to perform any gesture you like. Any time you want to do something to alter the behavior of any built-in gesture modifiers, you should be thinking about creating a custom gesture-based modifier to do it.

Note

Example of the Custom Gesture-Based Modifier can be found in the SciChart Android Examples Suite as well as on GitHub:

Custom Gesture-Based Modifier

An example below shows how to use GestureModifierBase API to create a chart custom gesture-based modifier.

As an example, we will implement a single finger vertical pinch-zoom gesture-based modifier to zoom our chart on X-Axis. Also, we want to enable it by double-tap (similar to how it works in Google Maps App).

Note

It's highly recommended to inherit from GestureModifierBase since it gives you some of the base implementations for free.

Let's get into the code.

• Java
• Java with Builders API
• Kotlin

class CustomZoomGestureModifier extends GestureModifierBase {
    private boolean isScrolling = false;
    private boolean isZoomEnabled = false;

    private float touchSlop;
    private final PointF start = new PointF();
    private float lastY;

    @Override
    public void attachTo(IServiceContainer services) {
        super.attachTo(services);

        final Context context = getContext();
        if (context == null) return;

        this.touchSlop = ViewConfiguration.get(context).getScaledTouchSlop() * 2;
    }

    @Override
    public boolean onDoubleTap(MotionEvent e) {
        start.set(e.getX(), e.getY());
        lastY = e.getY();
        isZoomEnabled = true;

        return true;
    }

    @Override
    public void onTouch(ModifierTouchEventArgs args) {
        super.onTouch(args);

        final MotionEvent motionEvent = args.e;
        if (isZoomEnabled && motionEvent.getAction() == MotionEvent.ACTION_MOVE) {
            onScrollInYDirection(motionEvent.getY());
        }
    }

    private void onScrollInYDirection(float y) {
        final float distance = Math.abs(y - start.y);
        if (distance < touchSlop || Math.abs(y - lastY) < 1f) return;

        this.isScrolling = true;

        final float prevDistance = Math.abs(lastY - start.y);
        final double diff = prevDistance > 0 ? distance / prevDistance - 1 : 0;

        growBy(start, getXAxis(), diff);

        this.lastY = y;
    }

    // zoom axis relative to the start point using fraction
    private void growBy(PointF point, IAxis axis, double fraction) {
        final int size = axis.getAxisViewportDimension();
        final float coord = size - point.y;

        double minFraction = (coord / size) * fraction;
        double maxFraction = (1 - coord / size) * fraction;

        axis.zoomBy(minFraction, maxFraction);
    }

    @Override
    protected void onUp(MotionEvent e) {
        // need to disable zoom after finishing scrolling
        if (isScrolling) {
            isScrolling = isZoomEnabled = false;
            start.set(Float.NaN, Float.NaN);
            lastY = Float.NaN;
        }
    }
}

Add it onto your chart like any other modifier to see how it works.

Common Gesture Modifier Properties

As mentioned above, all GestureModifiers within SciChart derive from the abstract GestureModifierBase class, which conforms to OnGestureListener, OnDoubleTapListener interfaces and uses a GestureDetector internally.

Please see the list of common properties below:

Note

To learn more about common ChartModifier features - please read the Chart Modifier APIs article. Also visit related IChartModifier API documentation.