GEF/GEF4/Graphics

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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 GEF 4 Graphics component provides a level of abstraction over different drawing toolkits. At the moment, SWT and AWT backends are in development. Its goal is to be as complete as possible, i.e. you should have access to all the functionality that you have access to when working directly with one of the underlying drawing toolkits. Of course, this is not always viable, but rather the greatest common divisor of the underlying drawing toolkits is accessible via the abstraction layer.

IGraphics

The heart of the GEF 4 Graphics component is the IGraphics interface. Normally, you will be handed over an IGraphics to accomplish your displaying tasks by a surrounding framework, but it can be used stand-alone, too. Thereto, you have to create a specific IGraphics implementation. Let us consider the following SWT example:

package org.eclipse.gef4.graphics.examples;
 
import org.eclipse.gef4.geometry.planar.Ellipse;
import org.eclipse.gef4.graphics.Color;
import org.eclipse.gef4.graphics.IGraphics;
import org.eclipse.gef4.graphics.swt.DisplayGraphics;
import org.eclipse.swt.SWT;
import org.eclipse.swt.events.PaintEvent;
import org.eclipse.swt.events.PaintListener;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;
 
public class SWTGraphicsExample implements PaintListener {
 
	public static void main(String[] args) {
		new SWTGraphicsExample("GEF 4 Graphics - SWT");
	}
 
	public SWTGraphicsExample(String title) {
		Display display = new Display();
 
		Shell shell = new Shell(display, SWT.SHELL_TRIM | SWT.DOUBLE_BUFFERED);
		shell.setText(title);
		shell.setBounds(0, 0, 640, 480);
		shell.open();
 
		shell.addPaintListener(this);
 
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}
	}
 
	public void paintControl(PaintEvent e) {
		DisplayGraphics g = new DisplayGraphics(e.gc);
		renderScene(g);
		g.cleanUp();
	}
 
	public void renderScene(IGraphics g) {
		g.getFillProperties().setColor(new Color(255, 0, 0, 255));
		g.fill(new Ellipse(50, 50, 350, 200));
	}
 
}

As you can see in this example, the DisplayGraphics is an IGraphics implementation. It can be constructed via a SWT GC which is passed-in to the paintControl() event handler. The renderScene() method does not know anything about the actual IGraphics implemention that is used. That's why we do not need to adjust our rendering code when switching to AWT, for example. You may wonder why we have to call the cleanUp() method on the DisplayGraphics. In the case of SWT, this is necessary to fully free the system resources that will be allocated during the rendering process. But let us concentrate on the renderScene() method.

There are two important things to notice:

  1. A set of property groups exists which control the state of the IGraphics. In the example, the color attribute of the fillProperties is set to a fully opaque red. This will affect any fill() operation on our IGraphics.
  2. The GEF 4 Geometry component is integrated into the IGraphics interface, so that we can easily display geometry objects.