# Groups

Use the Group class to treat a collection of Nodes and Links as if they were a single Node. Those nodes and links are members of the group; together they constitute a subgraph.

A subgraph is not another Diagram, so there is no separate HTML Div element for the subgraph of a group. All of the Parts that are members of a Group belong to the same Diagram as the Group. There can be links between member nodes and nodes outside of the group as well as links between the group itself and other nodes. There can even be links between member nodes and the containing group itself.

Groups can also be collapsed and expanded, to hide or show the member parts. This is distinct from collapsing and expanding trees.

The member parts of a group are available via the Group.memberParts property. Conversely, the Part.containingGroup property refers to the group, if the part belongs to one. A part can be member of at most one group at a time. You can set that property in order to add that part to a group. However you must make sure that no group contains itself, either directly or indirectly through other groups.

Because every Group is a Node, you can have nested groups. Although member Nodes and Links belong to the Group that contains them, they are not in the visual tree of the group -- their GraphObject.panel is null and no member part is in the group's Panel.elements collection. No Part can be in the visual tree of another Part. Parts normally do belong directly to one Layer.

See samples that make use of Groups in the samples index.

## Simple Groups

In a GraphLinksModel the Model.nodeDataArray holds node data, each of which might be represented by a Group rather than by a regular Node. You can declare that it should be a group by setting the isGroup data property to true. You can declare that a node data be a member of a group by referring to the group's key as the group data property value.

Here is a group containing two nested groups as well as two regular nodes. If you move a group, its member parts move along. If you copy a group, its member parts are copied too. If you delete a group, its member parts are deleted too. If you move a member node, its containing group inflates or shrinks to cover the area occupied by all of the members.

    diagram.model.nodeDataArray = [
      { key: 1, text: "Alpha", isGroup: true },
      { key: 2, text: "Beta", group: 1 },
      { key: 3, text: "Gamma", group: 1, isGroup: true },
      { key: 4, text: "Delta", group: 3 },
      { key: 5, text: "Epsilon", group: 3 },
      { key: 6, text: "Zeta", group: 1 },
      { key: 7, text: "Eta", group: 1, isGroup: true},
      { key: 8, text: "Theta", group: 7 }
    ];

### Groups and Links

Because Groups are Nodes, a Link may connect with a group as well as with a plain node.

Here is a simple example of four regular nodes and one group node. In this example the link from "Alpha" goes directly to the "Beta" node, but the link to "Delta" actually comes from the "Omega" group rather than from any particular member of the group.

    const nodeDataArray = [
      { key: 1, text: "Alpha" },
      { key: 2, text: "Beta", group: 4 },
      { key: 3, text: "Gamma", group: 4 },
      { key: 4, text: "Omega", isGroup: true },
      { key: 5, text: "Delta" }
    ];
    const linkDataArray = [
      { from: 1, to: 2 },  // from outside the Group to inside it
      { from: 2, to: 3 },  // this link is a member of the Group
      { from: 4, to: 5 }  // from the Group to a Node
    ];
    diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);

If you drag the "Delta" node around you can see how the link from the "Omega" group appears to come from the center of the group and start at the group's edge rather than at any member node. This is different than for the link from "Alpha" to "Beta".

Note also how the link from "Beta" to "Gamma" is effectively owned by the "Omega" group because both of the nodes are owned by that group. Copying the group automatically copies the link too.

This example did not set any of the following properties: Diagram.nodeTemplate, Diagram.groupTemplate, and Diagram.linkTemplate, in order to demonstrate the default templates for Nodes, Groups, and Links.

## Group Templates

Here is an example of how one might define templates for nodes and for groups. The node template is very simple: some text inside an ellipse. The group template is different from a node template in several aspects.

First, the group template builds a go.Group, not a go.Node or go.Part. The group can use a number of the panel types, just as a node may use various panel types.

Second, the group template includes a Placeholder object. This object, of which you may have at most one within the visual tree of a Group, gets the size and position of the union of the bounds of the member parts, plus some padding. The use of a Placeholder results in the Group surrounding the collection of group members, no matter where the member nodes are placed.

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

    diagram.groupTemplate =
      new go.Group("Vertical")
        .add(
          new go.Panel("Auto")
            .add(
              new go.Shape("RoundedRectangle", {  // surrounds the Placeholder
                  parameter1: 14,
                  fill: "rgba(128,128,128,0.33)"
                }),
              new go.Placeholder(    // represents the area of all member parts,
                  { padding: 5})  // with some extra padding around them
            ),
          new go.TextBlock({         // group title
              alignment: go.Spot.Right, font: "Bold 12pt Sans-Serif"
            })
            .bind("text")
        );

    const nodeDataArray = [
      { key: 1, text: "Alpha" },
      { key: 2, text: "Beta", group: 4 },
      { key: 3, text: "Gamma", group: 4 },
      { key: 4, text: "Omega", isGroup: true },
      { key: 5, text: "Delta" }
    ];
    const linkDataArray = [
      { from: 1, to: 2 },  // from outside the Group to inside it
      { from: 2, to: 3 },  // this link is a member of the Group
      { from: 4, to: 5 }  // from the Group to a Node
    ];
    diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);

Note how when you move the "Beta" or "Gamma" nodes the "Omega" group automatically resizes so that the TextBlock on the group stays below and on the right side of the "RoundedRectangle" shape.

Just as a Diagram can have its own Layout, a Group can have its own Group.layout. This is discussed in the next page about SubGraphs.