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132
133
134
135
136<h1>Events</h1>
137<p>
138There are three basic kinds of events that <b>GoJS</b> deals with:
139<a>DiagramEvent</a>s, <a>InputEvent</a>s, and <a>ChangedEvent</a>s.
140This page discusses the first two; see <a href="changedEvents.html">Changed Events</a> for the last kind of event.
141</p>
142
143<h2 id="DiagramEvents">Diagram Events</h2>
144<p>
145<a>DiagramEvent</a>s represent general user-initiated changes to a diagram.
146You can register one or more diagram event handlers by calling <a>Diagram.addDiagramListener</a>.
147You can also register a diagram event handler in <a>Diagram</a> initialization when calling <a>GraphObject,make</a>.
148Each kind of diagram event is distinguished by its name.
149</p>
150<p>
151Currently defined diagram event names include:
152</p>
153<ul>
  <li id="InitialAnimationStarting" onclick="window.location.hash = '#AnimationStarting'">
      "<b>InitialAnimationStarting</b>", the initial default animation is about to start;
      do not modify the diagram or its model in the event listener.
      This can be useful for modifying the <a>AnimationManager.defaultAnimation</a> to make a custom initial animation.
      See <a>AnimationManager.initialAnimationStyle</a> for details.
    </li>
<li id="AnimationStarting" onclick="window.location.hash = '#AnimationStarting'">
  "<b>AnimationStarting</b>", a default animation (<a>AnimationManager.defaultAnimation</a>) is about to start;
  do not modify the diagram or its model in the event listener.
</li>
<li id="AnimationFinished" onclick="window.location.hash = '#AnimationFinished'">
  "<b>AnimationFinished</b>", a default animation (<a>AnimationManager.defaultAnimation</a>) just completed;
  do not modify the diagram or its model in the event listener.
</li>
<li id="BackgroundSingleClicked" onclick="window.location.hash = '#BackgroundSingleClicked'">
  "<b>BackgroundSingleClicked</b>", when a mouse left-button single-click happened in the background of the Diagram, not on a Part;
  if you make any changes, start and commit your own transaction.
</li>
<li id="BackgroundDoubleClicked" onclick="window.location.hash = '#BackgroundDoubleClicked'">
  "<b>BackgroundDoubleClicked</b>", when a mouse left-button double-click happened in the background of the Diagram, not on a Part;
  if you make any changes, start and commit your own transaction.
</li>
<li id="BackgroundContextClicked" onclick="window.location.hash = '#BackgroundContextClicked'">
  "<b>BackgroundContextClicked</b>", when a mouse right-button single-click happened in the background of the Diagram, not on a Part;
  if you make any changes, start and commit your own transaction.
</li>
<li id="ChangingSelection" onclick="window.location.hash = '#ChangingSelection'">
  "<b>ChangingSelection</b>", an operation is about to change the <a>Diagram.selection</a> collection,
  which is also the value of the <a>DiagramEvent.subject</a>;
  do not make any changes to the selection or the diagram or the model in the event listener;
  note that just setting <a>Part.isSelected</a> will not raise this event, but tools and commands will.
</li>
<li id="ChangedSelection" onclick="window.location.hash = '#ChangedSelection'">
  "<b>ChangedSelection</b>", an operation has just changed the <a>Diagram.selection</a> collection,
  which is also the value of the <a>DiagramEvent.subject</a>;
  do not make any changes to the selection or the diagram or the model in the event listener;
  note that just setting <a>Part.isSelected</a> will not raise this event, but tools and commands will.
</li>
<li id="ClipboardChanged" onclick="window.location.hash = '#ClipboardChanged'">
  "<b>ClipboardChanged</b>", Parts have been copied to the clipboard by <a>CommandHandler.copySelection</a>;
  the <a>DiagramEvent.subject</a> is the collection of Parts;
  if you make any changes, start and commit your own transaction.
</li>
<li id="ClipboardPasted" onclick="window.location.hash = '#ClipboardPasted'">
  "<b>ClipboardPasted</b>", Parts have been copied from the clipboard into the Diagram by <a>CommandHandler.pasteSelection</a>;
  the <a>DiagramEvent.subject</a> is the <a>Diagram.selection</a>,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="DocumentBoundsChanged" onclick="window.location.hash = '#DocumentBoundsChanged'">
  "<b>DocumentBoundsChanged</b>", the area of the diagram's Parts, <a>Diagram.documentBounds</a>, has changed;
  the <a>DiagramEvent.parameter</a> is the old Rect.
</li>
<li id="ExternalObjectsDropped" onclick="window.location.hash = '#ExternalObjectsDropped'">
  "<b>ExternalObjectsDropped</b>", Parts have been copied into the Diagram by drag-and-drop from outside of the Diagram;
  the <a>DiagramEvent.subject</a> is the set of Parts that were dropped (which is also the <a>Diagram.selection</a>),
  the <a>DiagramEvent.parameter</a> is the source Diagram,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="GainedFocus" onclick="window.location.hash = '#GainedFocus'">
  "<b>GainedFocus</b>", the diagram has gained keyboard focus, such as after a call to <a>Diagram.focus</a>.
</li>
<li id="InitialLayoutCompleted" onclick="window.location.hash = '#InitialLayoutCompleted'">
  "<b>InitialLayoutCompleted</b>", the whole diagram layout has updated for the first time since a major change to the Diagram,
  such as replacing the Model;
  if you make any changes, you do not need to perform a transaction.
</li>
<li id="LayoutCompleted" onclick="window.location.hash = '#LayoutCompleted'">
  "<b>LayoutCompleted</b>", the whole diagram layout has just been updated;
  if you make any changes, you do not need to perform a transaction.
</li>
<li id="LinkDrawn" onclick="window.location.hash = '#LinkDrawn'">
  "<b>LinkDrawn</b>", the user has just created a new Link using <a>LinkingTool</a>;
  the <a>DiagramEvent.subject</a> is the new Link,
  and this is called within a transaction, so that you do not have to start and commit your own transaction..
</li>
<li id="LinkRelinked" onclick="window.location.hash = '#LinkRelinked'">
  "<b>LinkRelinked</b>", the user has just reconnected an existing Link using <a>RelinkingTool</a> or <a>DraggingTool</a>;
  the <a>DiagramEvent.subject</a> is the modified Link,
  the <a>DiagramEvent.parameter</a> is the GraphObject port that the link was disconnected from,
  and this is called within a transaction, so that you do not have to start and commit your own transaction..
</li>
<li id="LinkReshaped" onclick="window.location.hash = '#LinkReshaped'">
  "<b>LinkReshaped</b>", the user has just rerouted an existing Link using <a>LinkReshapingTool</a>;
  the <a>DiagramEvent.subject</a> is the modified Link,
  the <a>DiagramEvent.parameter</a> is the List of Points of the link's original route,
  and this is called within a transaction, so that you do not have to start and commit your own transaction..
</li>
<li id="LostFocus" onclick="window.location.hash = '#LostFocus'">
  "<b>LostFocus</b>", the diagram has lost keyboard focus ("blur").
</li>
<li id="Modified" onclick="window.location.hash = '#Modified'">
  "<b>Modified</b>", the <a>Diagram.isModified</a> property has been set to a new value --
  useful for marking a window as having been modified since the last save;
  do not modify the Diagram or its Model in the event listener.
</li>
<li id="ObjectSingleClicked" onclick="window.location.hash = '#ObjectSingleClicked'">
  "<b>ObjectSingleClicked</b>", a click that occurred on a GraphObject;
  the <a>DiagramEvent.subject</a> is the GraphObject;
  if you make any changes, start and commit your own transaction.
</li>
<li id="ObjectDoubleClicked" onclick="window.location.hash = '#ObjectDoubleClicked'">
  "<b>ObjectDoubleClicked</b>", a double-click that occurred on a GraphObject;
  the <a>DiagramEvent.subject</a> is the GraphObject;
  if you make any changes, start and commit your own transaction.
</li>
<li id="ObjectContextClicked" onclick="window.location.hash = '#ObjectContextClicked'">
  "<b>ObjectContextClicked</b>", a context-click that occurred on a GraphObject;
  the <a>DiagramEvent.subject</a> is the GraphObject;
  if you make any changes, start and commit your own transaction.
</li>
<li id="PartCreated" onclick="window.location.hash = '#PartCreated'">
  "<b>PartCreated</b>", the user inserted a new Part by <a>ClickCreatingTool</a>;
  the <a>DiagramEvent.subject</a> is the new Part,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="PartResized" onclick="window.location.hash = '#PartResized'">
  "<b>PartResized</b>", the user has changed the size of a GraphObject by <a>ResizingTool</a>;
  the <a>DiagramEvent.subject</a> is the GraphObject,
  the <a>DiagramEvent.parameter</a> is the original Size,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="PartRotated" onclick="window.location.hash = '#PartRotated'">
  "<b>PartRotated</b>", the user has changed the angle of a GraphObject by <a>RotatingTool</a>;
  the <a>DiagramEvent.subject</a> is the GraphObject,
  the <a>DiagramEvent.parameter</a> is the original angle in degrees,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionMoved" onclick="window.location.hash = '#SelectionMoved'">
  "<b>SelectionMoved</b>", the user has moved selected Parts by <a>DraggingTool</a>;
  the <a>DiagramEvent.subject</a> is a Set of the moved Parts,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionCopied" onclick="window.location.hash = '#SelectionCopied'">
  "<b>SelectionCopied</b>", the user has copied selected Parts by <a>DraggingTool</a>;
  the <a>DiagramEvent.subject</a> is Set of the newly copied Parts,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionDeleting" onclick="window.location.hash = '#SelectionDeleting'">
  "<b>SelectionDeleting</b>", the user is about to delete selected Parts by <a>CommandHandler.deleteSelection</a>;
  the <a>DiagramEvent.subject</a> is the <a>Diagram.selection</a> collection of Parts to be deleted,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionDeleted" onclick="window.location.hash = '#SelectionDeleted'">
  "<b>SelectionDeleted</b>", the user has deleted selected Parts by <a>CommandHandler.deleteSelection</a>;
  the <a>DiagramEvent.subject</a> is the collection of Parts that were deleted,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionGrouped" onclick="window.location.hash = '#SelectionGrouped'">
  "<b>SelectionGrouped</b>", the user has made a new Group out of the selected Parts by <a>CommandHandler.groupSelection</a>;
  the <a>DiagramEvent.subject</a> is the new Group,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SelectionUngrouped" onclick="window.location.hash = '#SelectionUngrouped'">
  "<b>SelectionUngrouped</b>", the user has removed a selected Group but kept its members by <a>CommandHandler.ungroupSelection</a>;
  the <a>DiagramEvent.subject</a> is the collection of Groups that were ungrouped,
  the <a>DiagramEvent.parameter</a> is the collection of former member Parts that were ungrouped,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SubGraphCollapsed" onclick="window.location.hash = '#SubGraphCollapsed'">
  "<b>SubGraphCollapsed</b>", the user has collapsed selected Groups by <a>CommandHandler.collapseSubGraph</a>;
  the <a>DiagramEvent.subject</a> is the collection of Groups that were collapsed,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="SubGraphExpanded" onclick="window.location.hash = '#SubGraphExpanded'">
  "<b>SubGraphExpanded</b>", the user has expanded selected Groups by <a>CommandHandler.expandSubGraph</a>;
  the <a>DiagramEvent.subject</a> is the collection of Groups that were expanded,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="TextEdited" onclick="window.location.hash = '#TextEdited'">
  "<b>TextEdited</b>", the user has changed the string value of a TextBlock by <a>TextEditingTool</a>;
  the <a>DiagramEvent.subject</a> is the edited TextBlock,
  the <a>DiagramEvent.parameter</a> is the original string,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="TreeCollapsed" onclick="window.location.hash = '#TreeCollapsed'">
  "<b>TreeCollapsed</b>", the user has collapsed selected Nodes with subtrees by <a>CommandHandler.collapseTree</a>;
  the <a>DiagramEvent.subject</a> is the collection of Nodes that were collapsed,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="TreeExpanded" onclick="window.location.hash = '#TreeExpanded'">
  "<b>TreeExpanded</b>", the user has expanded selected Nodes with subtrees by <a>CommandHandler.expandTree</a>;
  the <a>DiagramEvent.subject</a> is the collection of Nodes that were expanded,
  and this is called within a transaction, so that you do not have to start and commit your own transaction.
</li>
<li id="ViewportBoundsChanged" onclick="window.location.hash = '#ViewportBoundsChanged'">
  "<b>ViewportBoundsChanged</b>", the visible area of the Diagram, <a>Diagram.viewportBounds</a>, has changed;
  the <a>DiagramEvent.subject</a> is an object whose "scale" property is the old <a>Diagram.scale</a> value,
  whose "position" property is the old <a>Diagram.position</a> value,
  and whose "bounds" property is the old <a>Diagram.viewportBounds</a> value;
  the <a>DiagramEvent.parameter</a> is also the old viewportBounds Rect.
  Do not modify the Diagram position or scale (i.e. the viewport bounds) in the listener.
</li>
346</ul>
347
348<p>
349DiagramEvents do not necessarily correspond to mouse events or keyboard events or touch events.
350Nor do they necessarily correspond to changes to the diagram's model --
351for tracking such changes, use <a>Model.addChangedListener</a> or <a>Diagram.addModelChangedListener</a>.
352DiagramEvents only occur because the user did something, perhaps indirectly.
353</p>
354<p>
355<p>
356In addition to the DiagramEvent listeners there are also circumstances where detecting such changes is common
357enough to warrant having properties that are event handlers.
358Because these events do not necessarily correspond to any particular input or diagram event,
359these event handlers have custom arguments that are specific to the situation.
360</p>
361<p>
362One very common such event property is <a>GraphObject.click</a>, which if non-null is a function that is called
363whenever the user clicks on that object.
364This is most commonly used to specify behavior for "Button"s, but it and the other "click" event properties,
365"doubleClick" and "contextClick", can be useful on any GraphObject.
366</p>
367<p>
368Another common event property is <a>Part.selectionChanged</a>,
369which (if non-null) is called whenever <a>Part.isSelected</a> changes.
370In this case the event hander function is passed a single argument, the Part.
371There is no need for additional arguments because the function can check the current value of <a>Part.isSelected</a> to decide what to do.
372</p>
373</p>
374<p>
375Model <a>ChangedEvent</a>s are more complete and reliable than depending on <a>DiagramEvent</a>s.
376For example, the "LinkDrawn" DiagramEvent is not raised when code adds a link to a diagram.
377That DiagramEvent is only raised when the user draws a new link using the <a>LinkingTool</a>.
378Furthermore the link has not yet been routed, so <a>Link.points</a> will not have been computed.
379In fact, creating a new link may invalidate a <a>Layout</a>, so all of the nodes may be moved in the near future.
380</p>
381
382<p class="box bg-danger">
383Sometimes you want to update a database as the user makes changes to a diagram.
384Usually you will want to implement a <a>Model</a> <a>ChangedEvent</a> listener,
385by calling <a>Model.addChangedListener</a> or <a>Diagram.addModelChangedListener</a>,
386that notices the changes to the model and decides what to record in the database.
387See the discussion of <a href="changedEvents.html">Changed Events</a> and the <a href="../samples/UpdateDemo.html">Update Demo</a>.
388</p>
389
390<p>
391This example demonstrates handling several diagram events: <b>"ObjectSingleClicked"</b>,
392<b>"BackgroundDoubleClicked"</b>, and <b>"ClipboardPasted"</b>.
393</p>
394<pre class="lang-js" id="diagramEvents"><code>
function showMessage(s) {
  document.getElementById("diagramEventsMsg").textContent = s;
}
398
diagram.addDiagramListener("ObjectSingleClicked",
    function(e) {
      var part = e.subject.part;
      if (!(part instanceof go.Link)) showMessage("Clicked on " + part.data.key);
    });
404
diagram.addDiagramListener("BackgroundDoubleClicked",
    function(e) { showMessage("Double-clicked at " + e.diagram.lastInput.documentPoint); });
407
diagram.addDiagramListener("ClipboardPasted",
    function(e) { showMessage("Pasted " + e.diagram.selection.count + " parts"); });
410
var nodeDataArray = [
  { key: "Alpha" },
  { key: "Beta", group: "Omega" },
  { key: "Gamma", group: "Omega" },
  { key: "Omega", isGroup: true },
  { key: "Delta" }
];
var linkDataArray = [
  { from: "Alpha", to: "Beta" },  // from outside the Group to inside it
  { from: "Beta", to: "Gamma" },  // this link is a member of the Group
  { from: "Omega", to: "Delta" }  // from the Group to a Node
];
diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
424</code></pre>
425<script>goCode("diagramEvents", 600, 200)</script>
426<span id="diagramEventsMsg" style="color: red">(message)</span>
427
428
429<h2 id="InputEvents">Input Events</h2>
430<p>
431When a low-level HTML DOM event occurs, <b>GoJS</b> canonicalizes the keyboard/mouse/touch event information
432into a new <a>InputEvent</a> that can be passed to various event-handling methods and saved for later examination.
433</p>
434<p>
435An InputEvent keeps the <a>InputEvent.key</a> for keyboard events,
436the <a>InputEvent.button</a> for mouse events,
437the <a>InputEvent.viewPoint</a> for mouse and touch events,
438and <a>InputEvent.modifiers</a> for keyboard and mouse events.
439</p>
440<p>
441The diagram's event handlers also record the <a>InputEvent.documentPoint</a>,
442which is the <a>InputEvent.viewPoint</a> in document coordinates at the time of the mouse event,
443and the <a>InputEvent.timestamp</a>, which records the time that the event occurred in milliseconds.
444</p>
445<p>
446The InputEvent class also provides many handy properties for particular kinds of events.
447Examples include <a>InputEvent.control</a> (if the control key had been pressed) and
448<a>InputEvent.left</a> (if the left/primary mouse button was pressed).
449</p>
450<p>
451Some tools find the "current" <a>GraphObject</a> at the mouse point.
452This is remembered as the <a>InputEvent.targetObject</a>.
453</p>
454
455<h2 id="HigherLevelInputEvents">Higher-level input events</h2>
456<p>
457Some tools detect a sequence of input events to compose somewhat more abstract user events.
458Examples include "click" (mouse-down-and-up very close to each other) and "hover" (motionless mouse for some time).
459The tools will call an event handler (if there is any) for the current <a>GraphObject</a> at the mouse point.
460The event handler is held as the value of a property on the object.
461It then also "bubbles" the event up the chain of <a>GraphObject.panel</a>s until it ends with a <a>Part</a>.
462This allows a "click" event handler to be declared on a <a>Panel</a> and have it apply even if the click actually happens on an element deep inside the panel.
463If there is no object at the mouse point, the event occurs on the diagram.
464</p>
465<p>
466Click-like event properties include <a>GraphObject.click</a>, <a>GraphObject.doubleClick</a>, and <a>GraphObject.contextClick</a>.
467They also occur when there is no GraphObject -- the event happened in the diagram's background:
468<a>Diagram.click</a>, <a>Diagram.doubleClick</a>, and <a>Diagram.contextClick</a>.
469These are all properties that you can set to a function that is the event handler.
470These events are caused by both mouse events and touch events.
471</p>
472<p>
473Mouse-over-like event properties include <a>GraphObject.mouseEnter</a>, <a>GraphObject.mouseOver</a>, and <a>GraphObject.mouseLeave</a>.
474But only <a>Diagram.mouseOver</a> applies to the diagram.
475</p>
476<p>
477Hover-like event properties include <a>GraphObject.mouseHover</a> and <a>GraphObject.mouseHold</a>.
478The equivalent diagram properties are <a>Diagram.mouseHover</a> and <a>Diagram.mouseHold</a>.
479</p>
480<p>
481There are also event properties for dragging operations: <a>GraphObject.mouseDragEnter</a>, <a>GraphObject.mouseDragLeave</a>, and <a>GraphObject.mouseDrop</a>.
482These apply to stationary objects, not the objects being dragged.
483And they also occur when dragging by touch events, not just mouse events.
484</p>
485
486<p>
487This example demonstrates handling three higher-level input events:
488clicking on nodes and entering/leaving groups.
489</p>
490<pre class="lang-js" id="inputEvents"><code>
function showMessage(s) {
  document.getElementById("inputEventsMsg").textContent = s;
}
494
diagram.nodeTemplate =
  $(go.Node, "Auto",
    $(go.Shape, "Ellipse", { fill: "white" }),
    $(go.TextBlock,
      new go.Binding("text", "key")),
    { click: function(e, obj) { showMessage("Clicked on " + obj.part.data.key); } }
  );
502
diagram.groupTemplate =
  $(go.Group, "Vertical",
    $(go.TextBlock,
      { alignment: go.Spot.Left, font: "Bold 12pt Sans-Serif" },
      new go.Binding("text", "key")),
    $(go.Panel, "Auto",
      $(go.Shape, "RoundedRectangle",
        { name: "SHAPE",
          parameter1: 14,
          fill: "rgba(128,128,128,0.33)" }),
      $(go.Placeholder, { padding: 5 })
    ),
    { mouseEnter: function(e, obj, prev) {  // change group's background brush
          var shape = obj.part.findObject("SHAPE");
          if (shape) shape.fill = "red";
        },
      mouseLeave: function(e, obj, next) {  // restore to original brush
          var shape = obj.part.findObject("SHAPE");
          if (shape) shape.fill = "rgba(128,128,128,0.33)";
        } });
523
var nodeDataArray = [
  { key: "Alpha" },
  { key: "Beta", group: "Omega" },
  { key: "Gamma", group: "Omega" },
  { key: "Omega", isGroup: true },
  { key: "Delta" }
];
var linkDataArray = [
  { from: "Alpha", to: "Beta" },  // from outside the Group to inside it
  { from: "Beta", to: "Gamma" },  // this link is a member of the Group
  { from: "Omega", to: "Delta" }  // from the Group to a Node
];
diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
537</code></pre>
538<script>goCode("inputEvents", 600, 200)</script>
539<span id="inputEventsMsg" style="color: red">(message)</span>
540
541
542<h2 id="ClickingAndSelecting">Clicking and Selecting</h2>
543<p>
544This example demonstrates both the "click" and the "selectionChanged" events:
545</p>
546<pre class="lang-js" id="changeMethods"><code>
function showMessage(s) {
  document.getElementById("changeMethodsMsg").textContent = s;
}
550
diagram.nodeTemplate =
  $(go.Node, "Auto",
    { selectionAdorned: false },
    $(go.Shape, "Ellipse", { fill: "white" }),
    $(go.TextBlock,
      new go.Binding("text", "key")),
    {
      click: function(e, obj) { showMessage("Clicked on " + obj.part.data.key); },
      selectionChanged: function(part) {
          var shape = part.elt(0);
          shape.fill = part.isSelected ? "red" : "white";
        }
    }
  );
565
var nodeDataArray = [
  { key: "Alpha" }, { key: "Beta" }, { key: "Gamma" }
];
var linkDataArray = [
  { from: "Alpha", to: "Beta" },
  { from: "Beta", to: "Gamma" }
];
diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
574</code></pre>
575<script>goCode("changeMethods", 600, 200)</script>
576<span id="changeMethodsMsg" style="color: red">(message)</span>
577<p>
578Try Ctrl-A to select everything.
579Note the distinction between the <a>GraphObject.click</a> event property and the <a>Part.selectionChanged</a> event property.
580Both are methods that get called when something has happened to the node.
581The <a>GraphObject.click</a> occurs when the user clicks on the node, which happens to select the node.
582But the <a>Part.selectionChanged</a> occurs even when there is no click event or even any mouse event --
583it was due to a property change to the node.
584</p>
585
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