The `SourceAnchor` link tool renders a handle above the source anchor of the link (as determined by the anchor function applied on link source). It accepts five additional arguments, which can be passed as an object to the link tool constructor:

redundancyRemoval | boolean | If the user moves the anchor so that it lies on a single line with two (or more) vertices, should the middle one(s) be considered redundant and removed? Default is `true`. Note that this setting is not applied until the user actually moves the anchor; this means that the anchor and vertices can still be arranged in this redundant fashion, using the `link.vertices` function, for example.
restrictArea | boolean | Should the user only be allowed to move the anchor in a restricted area (the area of the bounding box of the source magnet)? Default is `true`.
areaPadding | number | If the `restrictArea` option is set to `true`, the user can only move the anchor in a restricted area (the area of the bounding box of the source magnet). JointJS shows this restriction by drawing a boundary around that area. This option determines whether this boundary area should be visually inflated and if so, by how much. Default is `10`. Note that this is a purely cosmetic setting; regardless of the provided value, the movement stays restricted to the original uninflated bounding box.
snapRadius | number | While the user is moving the anchor, from how far away should the segment snap in order to arrange itself in line with the anchor reference? Default is `10`. (For link source, the anchor reference is the first vertex. If there are no vertices, it is the target anchor.)
snap | function | What snap function should be applied when the user moves the anchor? Default is a simple function that emulates the snapping behavior of Vertices and Segments link tools: If the value of one of the user pointer coordinates is within `snapRadius` of the value of a coordinate of the anchor reference, snap to the reference value. (For link source, the anchor reference is the first vertex. If there are no vertices, it is the target anchor.) The callback function must return the anchor as a `g.Point` and have the signature `function(coords, endView, endMagnet, endType, linkView, toolView)`:
coords | g.Point | A Point object recording the x-y coordinates of the user pointer when the snap function was invoked.
endView | dia.ElementView | The ElementView which contains the restricted area. The Element model can be accessed as `endView.model`; this may be useful for writing conditional logic based on element attributes.
endMagnet | SVGElement | The SVGElement in our page that contains the magnet (element/subelement/port) which serves as the restricted area.
endType | string | Is the end view the source (`'source'`) or target (`'target'`) of the link?
linkView | dia.LinkView | The LinkView to which the tool is attached. The Link model can be accessed as `linkView.model`; this may be useful for writing conditional logic based on link attributes.
toolView | dia.ToolView | This tool. May be useful for writing conditional logic based on tool options (e.g. the default algorithm refers to `toolView.options.snapRadius`).

Example:

    var sourceAnchorTool = new joint.linkTools.SourceAnchor({
        focusOpacity: 0.5,
        redundancyRemoval: false,
        restrictArea: false,
        snapRadius: 20
    });

An example of a useful custom `snap` function is provided below. It snaps the anchor to the center of the closest side of the restricted area.

    var snapAnchor = function(coords, endView, endMagnet) {

        // remove rotation of the restricted area
        var bbox = endView.getNodeUnrotatedBBox(endMagnet);
        var angle = endView.model.angle();
        var origin = endView.model.getBBox().center();
        coords.rotate(origin, angle);

        // identify the side nearest to pointer coords
        var anchor;
        var side = bbox.sideNearestToPoint(coords);
        switch (side) {
            case 'left': anchor = bbox.leftMiddle(); break;
            case 'right': anchor = bbox.rightMiddle(); break;
            case 'top': anchor = bbox.topMiddle(); break;
            case 'bottom': anchor = bbox.bottomMiddle(); break;
        }

        // rotate the anchor according to original rotation of restricted area
        return anchor.rotate(origin, -angle);
    };

    var sourceAnchorTool = new joint.linkTools.SourceAnchor({
        snap: snapAnchor;
    });

If the user moves the anchor away from its original position, the anchor position may be reset by double-clicking the anchor handle.