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Text Field Content Proposals in JFace

At 12:38 AM on Oct 26, 2006, R.J. Lorimer wrote:

A particularly nice feature of GUI applications is the ability to provide context-sensitive rich field assist for text-fields. As an example, consider this picture of the 'new class' dialog in the Eclipse Java support:

In the past, I think a lot of applications have avoided this user-friendly feature in many cases because of (lack of) support from their widget toolkit. It is so easy to implement with JFace, however, you'd be mistaken not to (I will say, however, that too much of any good thing is in turn a bad thing).

The heart of content proposals is the org.eclipse.jface.fieldassist.ContentProposalAdapter class. Binding content proposals to a text field is trivial:

Text t = new Text(parent, SWT.BORDER); // your SWT text field
ContentProposalAdapter adapter = new ContentProposalAdapter(
	t, 
	new TextContentAdapter(), 
	new SimpleContentProposalProvider(new String [] {"A", "B", "C"}),
	null, 
	null);

Amazingly, this is all you have to do to provide simple content proposals to a text field. Now, the configuration above obviously uses a fairly spartan content proposal provider; you can implement a more functional version that checks a list of local file names, looks at the user's last 100 entries, or even connects via web-service to a dictionary website; Eclipse uses a list of class names that can be subclassed in the dialog above, but the options are only bound by the intended input in the text field. In addition, your proposals can be sensitive to the location of the caret as well as the data already entered in to the field (this simple proposal provider will never consider any of those more subtle details).

The two null values represent the activation keystroke and the 'auto-activation' characters respectively. The activation keystroke is typically Ctrl+Space in Eclipse, but you could make it whatever you want in your application. The auto-activation characters in this case is left empty. If you supply values, ONLY those characters would trigger auto-assistance. If you don't, but you have a key-stroke, only the key-stroke will trigger assistance. If neither are supplied (as in the example above), all character input will trigger auto-activation (meaning the content proposal will always be there when you type).

Here is a screenshot of the content proposal in action:

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