This sample demonstrates a custom Tool, LinkLabelOnPathDraggingTool, that allows the user to drag the label of a Link, but that keeps the label exactly on the path of the link. The tool is defined at LinkLabelOnPathDraggingTool.js.

The label on the link can be any arbitrarily complex object. It is positioned by the GraphObject.segmentIndex and GraphObject.segmentFraction properties. The segmentIndex is set to NaN such that the whole link path acts as the segment, and the segmentFraction is set by the LinkLabelOnPathDraggingTool. A two-way data binding on segmentFraction automatically remembers any modified value on the link data object in the model.

The tool is derived from a similar tool, LinkLabelDraggingTool.js, that allows the user to drag the label in any direction from the mid-point of the Link path.

GoJS Features in this sample


Tools handle all input events, such as mouse and keyboard interactions, in a Diagram. There are many kinds of predefined Tool classes that implement all of the common operations that users do.

For flexibility and simplicity, all input events are canonicalized as InputEvents and redirected by the diagram to go to the Diagram.currentTool. By default the Diagram.currentTool is an instance of ToolManager held as the Diagram.toolManager. The ToolManager implements support for all mode-less tools. The ToolManager is responsible for finding another tool that is ready to run and then making it the new current tool. This causes the new tool to process all of the input events (mouse, keyboard, and touch) until the tool decides that it is finished, at which time the diagram's current tool reverts back to the Diagram.defaultTool, which is normally the ToolManager, again.

More information can be found in the GoJS Intro.

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GoJS Extensions

GoJS can be extended in a variety of ways. The most common way to change the standard behavior is to set properties on the GraphObject, Diagram, CommandHandler, Tool, or Layout. But when the desired property does not exist, you might need to override methods of CommandHandler, Tool, Layout, Link, or Node. Methods that you can override are documented in the API reference. Various features of GoJS can be overriden, either by replacing a method on an instance (a feature of JavaScript) or by defining a subclass. You should not modify the prototypes of any of the GoJS classes.

In addition to our samples, GoJS provides an extensions gallery, showcasing the creation of custom tools and layouts. Those classes and samples are written in TypeScript, available at ../extensionsJSM/, as ECMAScript/JavaScript modules -- these use the ../release/go-module.js library. We recommend that you copy the files that you need into your project, so that you can adjust how they refer to the GoJS library that you choose and so that you can include them into your own building and packaging procedures.

More information can be found in the GoJS Intro.

Related samples