Dynamic Web Development with Seaside

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7.1Rendering Hello World

Our first Seaside component will simply display Hello world. Begin by creating a category called ’SeasideBook-Hello’ and then create the class ScrapBook as a subclass of WAComponent as shown below.

WAComponent subclass: #ScrapBook
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'SeasideBook-Hello'

When we use the term component in this text we generally mean an instance of a subclass of WAComponent. For now, just think of subclasses of WAComponent as “visual components”. When it is time for a component to be displayed, Seaside sends it the message WAComponent>>renderContentOn: with a single argument (by convention called html) which is an instance of the class WARenderCanvas (the “canvas”). Think of the canvas as the medium on which you will paint your component. It provides a transparent interface to XHTML which makes it easy to produce text, anchors, images etc., in a modular way (i.e., attached to each component of your application). To start, we just want to show a simple text message. Fortunately the canvas supports a text: message for just this purpose, which we can use as shown below.

Note that all the classes in Seaside are prefixed with WA which acts as a namespace. Do not use this prefix for your components. WA is intended for Seaside framework classes.

ScrapBook>>renderContentOn: html
html text: 'Hello world'

Great, we have a component but how do we get Seaside to serve it? For now, evaluate the following code in a workspace:

WAAdmin register: ScrapBook asApplicationAt: 'hello'

Now open your web browser and go to http://localhost:8080/hello, and you should see something very like Figure 53.

Hello World in Seaside

Seaside added XHTML markup for the skeletal structure of an XHTML document (html, head and body tags). OK, so what is happening here? Grossly simplified: When we request this URI, Seaside creates a new instance of our class for us and then sends it WAComponent>>renderContentOn:. After being placed inside a skeleton XHTML document, the XHTML painted onto the canvas is then returned to the web browser to be displayed.

Never invoke the method renderContentOn: directly, Seaside will do it for you.

You will never need to send your component the message WAComponent>>renderContentOn: since the Seaside framework takes care of that for you. When it is time to paint your component, Seaside sends it renderContentOn:. This is very similar to models used in most GUI frameworks where a component (or window) is told to paint itself whenever the windowing system deems necessary. Also, keep this in mind as you work with Seaside: a rendering method is just for displaying a component not changing its state.

Your rendering method is just for painting the current state of your component, it shouldn’t be concerned with changing that state.

Copyright © 23 February 2017 Stéphane Ducasse, Lukas Renggli, C. David Shaffer, Rick Zaccone
This book is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license.

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