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GEF/GEF4/Layout

< GEF‎ | GEF4

Note to non-wiki readers: This documentation is generated from the Eclipse wiki - if you have corrections or additions it would be awesome if you added them in the original wiki page.


Introduction

The GEF4 Layout component provides basic abstractions for layout algorithms and related listeners, as well as a set of layout algorithm implementations. It is internally decomposed into the single Layout module. There are also a couple of undeployed Layout Examples.

GEF4-Components-Layout.png


Layout

  • feature: org.eclipse.gef4.layout
  • bundle: org.eclipse.gef4.layout

The Layout module of GEF4 Layout provides abstractions and related to auto-layout ({Root}) as well as concrete layout algorithm implementations(Algorithms).


{Root}

  • package: org.eclipse.gef4.layout

The {Root} package provides support for automatic layout. Automatic layout is performed by ILayoutAlgorithm implementations. Each ILayoutAlgorithm is bound to a LayoutContext, which provides all necessary information about what is to be layouted and how. The what is described in terms of a GEF4 Graph input model. An ILayoutFilter can be used to sort out those graph elements that are (currently) not of relevance for layouting. Details about the what can be inferred from properties (LayoutProperties) that can be attributed to all layout abstractions. These can also be used to influence how the layout algorithm performs the layout.

GEF4-Layout-layout.png

ILayoutAlgorithm

ILayoutAlgorithm is the base abstraction for all layout algorithm implementations. An ILayoutAlgorithm needs a reference to a LayoutContext, which provides information about what is to be layouted in terms of a Graph, which will have to be attributed in addition with respective properties that are required by the algorithm.

LayoutContext

A LayoutContext provides the necessary context information needed by an ILayoutAlgorithm. The LayoutContext is the central entry point for clients to trigger layout. It provides the input Graph model that indicates what is to be layouted, and references the layout algorithm that is to be applied. The LayoutContext further supports scheduling pre-layout-passes and post-layout-passes which are executed before or after applying a layout, respectively. These can, for example, be used to update the information which the LayoutContext provides, prior to applying a layout, or transfering information back from the LayoutContext to the underlying data model after applying a layout.

When applying a layout, the configured algorithm will evaluate all Graph model returned by the LayoutContext when computing the layout. If not all model elements of the input model are relevant for layout, an ILayoutFilter can be applied to filter input model elements.

ILayoutFilter

An ILayoutFilter is used to filter out GEF4 Graph input elements, so that they are not returned by the context, and therefore, are not layed out. For example, this is how GEF4 Zest filters out hidden nodes and edges:

  // add layout filter for hidden/layout irrelevant elements
  final HidingModel hidingModel = getHost().getRoot().getViewer().getAdapter(HidingModel.class);
  if (hidingModel != null) {
    getLayoutContext().addLayoutFilter(new ILayoutFilter() {
      @Override
      public boolean isLayoutIrrelevant(Edge edge) {
        return Boolean.TRUE.equals(ZestProperties.getLayoutIrrelevant(edge))
               || isLayoutIrrelevant(edge.getSource()) || isLayoutIrrelevant(edge.getTarget());
      }
 
      @Override
      public boolean isLayoutIrrelevant(org.eclipse.gef4.graph.Node node) {
        return Boolean.TRUE.equals(ZestProperties.getLayoutIrrelevant(node)) || hidingModel.isHidden(node);
      }
    });
  }

LayoutProperties

The LayoutProperties class defines the layout properties for Graph, Node, and Edge elements processed by the ILayoutAlgorithm. The algorithms access these properties to read/write node locations and sizes, among other things.

The following properties are supported for graphs (g), nodes (n), and edges (e):

The following code snippet demonstrates how to use LayoutProperties:

public static void main(String[] args) {
 
  // create graph
  Graph graph = new Graph();
  Node n1 = new Node();
  n1.setGraph(graph);
  Node n2 = new Node();
  n2.setGraph(graph);
  graph.getNodes().addAll(Arrays.asList(new Node[] { n1, n2 }));
  Edge e = new Edge(n1, n2);
  e.setGraph(graph);
 
  // create layout context
  LayoutContext context = new LayoutContext();
 
  // provide the input model
  context.setGraph(graph);
 
  // set layout bounds
  LayoutProperties.setBounds(graph, new Rectangle(0, 0, 100, 100));
 
  // set node layout sizes
  LayoutProperties.setSize(n1, 25, 25);
  LayoutProperties.setSize(n2, 25, 25);
 
  // set layout algorithm
  context.setLayoutAlgorithm(new SpringLayoutAlgorithm());
 
  // apply layout
  context.applyLayout(true);
 
  // read locations after layout
  System.out.println("n1 at " + LayoutProperties.getLocation(n1));
  System.out.println("n2 at " + LayoutProperties.getLocation(n2));
 
}

Algorithms

  • package: org.eclipse.gef4.layout.algorithms

The Algorithms package provides different implementations of concrete layout algorithms. There are a couple of simple algorithms that arrange nodes in a grid (GridLayoutAlgorithm), a single column or row (BoxLayoutAlgorithm), or which simply prevent overlapping of nodes (HorizontalShiftLayoutAlgorithm).

GEF4-Layout-algorithms-simple.png

There are also tree-based algorithms that arrange nodes in a classical (TreeLayoutAlgorithm) or radial tree (RadialLayoutAlgorithm), as well as a dedicated algorithm that optimizes folding and unfolding of sub-trees based on the available size (SpaceTreeLayoutAlgorithm).

GEF4-Layout-algorithms-treebased.png

Last, there is the implementation (SugiyamaLayoutAlgorithm) of a rank-based algorithm as published by Kozo Sugiyama, Shojiro Tagawa, and Mitsuhiko Toda in their paper about Methods for Visual Understanding of Hierarchical System Structures.

GEF4-Layout-algorithms-sugiyama.png

GridLayoutAlgorithm

A GridLayoutAlgorithm is a simple layout algorithm that places all elements into a grid, where the number of columns and rows is computed by the algorithm.

BoxLayoutAlgorithm

A BoxLayoutAlgorithm is a simple layout algorithm that places all elements in a single column or row, depending on a specifiable orientation.

SpringLayoutAlgorithm

GEF4-Layout-spring.png

HorizontalShiftLayoutAlgorithm

A HorizontalShiftLayoutAlgorithm is a simple layout algorithm that shifts overlapping nodes to the right.

TreeLayoutAlgorithm

The TreeLayoutAlgorithm is a simple algorithm to arrange nodes in a layered, tree-like layout.

Zest-tree-layout-dag.png

RadialLayoutAlgorithm

The RadialLayoutAlgorithm lays out a tree in a circular fashion, where the roots are located in the center.

GEF4-Layout-radial.png

SpaceTreeLayoutAlgorithm

The SpaceTreeLayoutAlgorithm keeps track of node positions all the time, always trying to form a nice tree structure. This means movement of nodes with the mouse is somehow restricted (you can move a node within its current layer, but only if it doesn't cause nodes to be pushed out of the graph area.

Zest-tree-layout-spacetree.png

SugiyamaLayoutAlgorithm

The SugiyamaLayoutAlgorithm lays out nodes in ranks. Its an implementation of the algorithm published by Kozo Sugiyama, Shojiro Tagawa, and Mitsuhiko Today in their paper about Methods for Visual Understanding of Hierarchical System Structures.

GEF4-Layout-sugiyama.png


Migration from Zest 1.x to GEF4 Layout

The layout API that was part of Zest 1.x has been migrated into the GEF4 Layout component, where it has been reworked (see #372365, #469472, #484421, and #491097 for details).

Custom Layouts

To define custom layouts, instead of extending AbstractLayoutAlgorithm implement ILayoutAlgorithm (see example below and full code in CustomLayoutExample.java in the examples bundle).

  ILayoutAlgorithm layoutAlgorithm = new ILayoutAlgorithm() {
    private LayoutContext context;
 
    @Override
    public void applyLayout(boolean clean) {
      Node[] entitiesToLayout = context.getNodes();
      int totalSteps = entitiesToLayout.length;
      double distance = LayoutProperties.getBounds(context.getGraph()).getWidth() / totalSteps;
      int xLocation = 0;
 
      for (int currentStep = 0; currentStep < entitiesToLayout.length; currentStep++) {
        Node layoutEntity = entitiesToLayout[currentStep];
        LayoutProperties.setLocation(layoutEntity, new Point(xLocation,0));
        xLocation += distance;
      }
    }
 
    @Override
    public LayoutContext getLayoutContext() {
      return context;
    }
 
    @Override
    public void setLayoutContext(LayoutContext context) {
      this.context = context;
    }
  };