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      <div class="px-4 pb-16 w-full overflow-hidden prose">
        
<h1>Extending GoJS</h1>
<p>
  <b>GoJS</b> can be extended in a variety of ways. The most common way to change the standard behavior is to set properties on the <a>GraphObject</a>,
  <a>Diagram</a>, <a>CommandHandler</a>, <a>Tool</a>, or <a>Layout</a>. 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. This page describes how to override methods,
  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
  <b>GoJS</b> classes.
</p>

<p class="box bg-danger">
  Do not modify the prototypes of the <b>GoJS</b> classes.<br />
  Only use the properties and methods documented in the <a href="../api/index.html">API</a>.
</p>

<p class="box bg-info">
  In addition to our samples, <b>GoJS</b> provides an <strong><a href="../extensions/">extensions gallery</a></strong
  >, showcasing the creation of custom tools and layouts. Those classes and samples are written in TypeScript, available at <code>../extensionsJSM/</code>, as
  ECMAScript/JavaScript modules -- these use the <code>../release/go-module.js</code> 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.
</p>

<p class="box bg-danger">
  Note that the API for extension classes may change with any version, even point releases. If you intend to use an extension in production, you should copy the
  code to your own source directory.
</p>

<p class="box bg-info">
  When using modern JavaScript, use arrow functions (<code>e => {}</code>) for event handlers and conversion functions. Use <code>function</code> only when
  overriding a method and you want access to <code>this</code> object whose method you have overridden.
</p>

<h2 id="CommandHandler">Command Handler</h2>
<p>
  Overriding the <a>CommandHandler</a> allows you to alter default functionality and create your own key bindings. See the
  <a href="commands.html">intro page on Commands</a> for more. However, the techniques shown below for overriding methods on Tools and Layouts also applies to
  the CommandHandler.
</p>

<h2 id="ToolsAndLayouts">Tools and Layouts</h2>
<p>
  <b>GoJS</b> operates on nodes and links using many tools and layouts, all of which are subclasses of the <a>Tool</a> and <a>Layout</a> classes. See the
  <a href="tools.html">intro page on Tools</a> for more about Tools, and the <a href="layouts.html">intro page on Layouts</a> for more about Layouts.
</p>
<p>
  Tools can be modified, or they can be replaced in or added to the <a>Diagram.toolManager</a>. All tools must inherit from the <a>Tool</a> class or from a
  class that inherits from Tool.
</p>

<p class="box bg-info">
  Some of our samples, such as the <a href="../samples/pipes.html">Pipes sample</a>, contain examples of custom tools. More custom tool examples are available
  in the <a href="../extensions/">extensions gallery</a>. TypeScript versions of those classes are available in <code>../extensionsJSM/</code>.
</p>

<p>
  Layouts can be modified, or they can be used by setting <a>Diagram.layout</a> or <a>Group.layout</a>. All Layouts must inherit from the <a>Layout</a> class or
  a class that inherits from Layout.
</p>

<p class="box bg-info">
  Some of our samples, such as the <a href="../samples/parseTree.html">Parse Tree sample</a>, contain examples of custom layouts. More custom layout examples
  are available in the <a href="../extensions/">extensions gallery</a>. TypeScript versions of those classes are available in <code>../extensionsJSM/</code>.
</p>

<h2 id="OverridingMethodWithoutDefiningSubclass">Overriding a Method Without Defining a Subclass</h2>
<p>
  Due to a "feature" of JavaScript, often we can avoid subclassing a Tool in its entirety and merely override a single method. This is common when we want to
  make a small change to the behavior of a method of a Tool. Here we show how to override the <code>ClickSelectingTool.standardMouseSelect</code> method by
  modifying the tool instance of a particular Diagram.
</p>

<p>
  One can override Layout methods in this manner also, but that is rarely done -- layouts are almost always subclassed. It cannot be done for layouts that are
  the value of <a>Group.layout</a> because those layouts may be copied and cannot be shared.
</p>

<p>
  Since we are not creating a new (sub)class, we set the method directly on the Diagram's <a>ClickSelectingTool</a>, which is referenced through its
  <a>ToolManager</a>. Typical scaffolding for overriding a method in such a manner is as follows:
</p>

<pre class="lang-js"><code>
  const tool = diagram.toolManager.clickSelectingTool;
  tool.standardMouseSelect = function() { // must not be an arrow function =>
    // Maybe do something else before
    // ...

    // In cases where you want normal behavior, call the base functionality.
    // Note the reference to the prototype and the call to 'call' passing it 'this'.
    go.ClickSelectingTool.prototype.standardMouseSelect.call(this);

    // Maybe do something else after
    // ...
  }
</code></pre>

<p>
  As a concrete example, we will override <a>Tool.standardMouseSelect</a> to select only Nodes and Links that have a width and height of less than 50 diagram
  units. This means that we must find the to-be-selected object using <code>diagram.findPartAt</code>, check its bounds, and quit if the bounds are too large.
  Otherwise, we call the base functionality to select as it normally would.
</p>

<pre class="lang-js" id="tool"><code>
  diagram.nodeTemplate =
    new go.Node("Auto")
      .add(
        new go.Shape("Rectangle", { fill: "white" })
          .bind("fill", "color"),
        new go.TextBlock({ margin: 5 })
          .bind("text")
      );

  const tool = diagram.toolManager.clickSelectingTool;
  tool.standardMouseSelect = function() { // must not be an arrow function =>
    const diagram = this.diagram;
    const e = diagram.lastInput;
    // to select containing Group if Part.canSelect() is false
    const curobj = diagram.findPartAt(e.documentPoint, false);
    if (curobj !== null) {
      const bounds = curobj.actualBounds;
      // End the selection process if the bounds are greater than 50 width or height
      if (bounds.width > 50 || bounds.height > 50) {
        // If this was a left click with no modifier, we want to act the same as if
        // we are clicking on the Diagram background, so we clear the selection
        if (e.left && !e.control && !e.shift) {
          diagram.clearSelection();
        }
        // Then return, so that we do not call the base functionality
        return;
      }
    }
    // otherwise, call the base functionality
    // (would be a <code>super</code> method when in a subclass)
    go.ClickSelectingTool.prototype.standardMouseSelect.call(this);
  }

  diagram.model = new go.Model([
    { text: "Alpha", color: "lightblue" },
    { text: "Epsilon", color: "thistle" },
    { text: "Psi", color: "lightcoral" },
    { text: "Gamma", color: "lightgreen" }
  ]);
</code></pre>
<p>
  Running this code, we see that the "Epsilon" and "Gamma" nodes are not selectable, because they are both wider than 50. Note that this custom tool does not
  change the behavior of other tools that might select, such as the <a>DraggingTool</a> or the <a>DragSelectingTool</a>.
</p>
<script>
  goCode('tool', 300, 130);
</script>

<h2 id="OverridingMethodsBySubclassingLayout">Overriding Methods by Subclassing a Layout</h2>
<p>Layouts and Tools can be subclassed to create custom classes that inherit the properties and methods of the predefined class.</p>

<p>In modern JavaScript (ECMAScript) or in TypeScript, you can use newer syntax for defining classes:</p>
<pre class="lang-js"><code>
  export class CascadeLayout extends go.Layout {
    // new data properties (fields) get declared and initialized here

    constructor() {
      super();
      // other initializations can be done here
    }

    // optionally, define property getters and setters here

    // override or define methods
    public doLayout(coll) {
      // Layout logic goes here.
    }
  }
</code></pre>

<p>
  You can define property getters and setters:
</p>
<pre class="lang-js"><code>
  this._offset = new go.Size(12, 12);

  get offset() { return this._offset; }
  set offset(val) {
    if (!(val instanceof go.Size)) {
      throw new Error("new value for CascadeLayout.offset must be a Size, not: " + val);
    }
    if (!this._offset.equals(val)) {
      this._offset = val;
      this.invalidateLayout();
    }
  }
</code></pre>

<p>
  If the layout might be used as the value of <a>Group.layout</a>, you will need to make sure the instance that you set in the Group template can be copied
  correctly.
</p>
<pre class="lang-js"><code>
  cloneProtected(copy) {
    super.cloneProtected(copy);
    copy._offset = this._offset;
  }
</code></pre>

<p>
  Lastly we'll define a <a>Layout.doLayout</a>, being sure to look at the documentation and accommodate all possible input, as <code>doLayout</code> has one
  argument that can either be a <a>Diagram</a>, or a <a>Group</a>, or an <a>Iterable</a> collection.
</p>
<p>
  All together, we can see the cascade layout in action:
</p>
<pre class="lang-js" id="example"><code>
/**
* This layout arranges nodes in a cascade specified by the offset property
*/
class CascadeLayout extends go.Layout {
  constructor() {
    super();
    this._offset = new go.Size(12, 12);
  }

  cloneProtected(copy) {
    super.cloneProtected(copy);
    copy._offset = this._offset;
  }

  get offset() { return this._offset; }
  set offset(val) {
    if (!(val instanceof go.Size)) {
      throw new Error("new value for CascadeLayout.offset must be a Size, not: " + val);
    }
    if (!this._offset.equals(val)) {
      this._offset = val;
      this.invalidateLayout();
    }
  }

  /**
  * This method positions all Nodes and ignores all Links.
  * @param {Diagram|Group|Iterable} coll the collection of Parts to layout.
  */
  doLayout(coll) {
    // get the Nodes and Links to be laid out
    const parts = this.collectParts(coll);

    // Start the layout at the arrangement origin, a property inherited from Layout
    let x = this.arrangementOrigin.x;
    let y = this.arrangementOrigin.y;
    const offset = this.offset;

    for (let node of parts) {
      if (!(node instanceof go.Node)) continue;  // ignore Links
      node.move(new go.Point(x, y));
      x += offset.width;
      y += offset.height;
    }
  }
}  // end of CascadeLayout

  // Regular diagram setup:

  diagram.layout = new CascadeLayout();

  diagram.nodeTemplate =
    new go.Node("Auto")
      .add(
        new go.Shape("Rectangle", { fill: "white" })
          .bind("fill", "color"),
        new go.TextBlock({ margin: 5 })
          .bind("text")
      );

  diagram.model = new go.Model([
    { text: "Alpha", color: "lightblue" },
    { text: "Beta", color: "thistle" },
    { text: "Delta", color: "lightcoral" },
    { text: "Gamma", color: "lightgreen" }
  ]);
</code></pre>
<script>
  goCode('example', 300, 200);
</script>

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