Dynamic Web Development with Seaside

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2.2What is a Smalltalk Image?

All Smalltalk objects live in something called an image. An image is a snapshot of memory containing all the objects at a given point in time. This means that your business objects, Seaside objects, all classes and their methods (since they are also objects), and development tools are all part of the image. The Seaside “One Click Image” includes a Smalltalk image with Seaside and a number of other tools pre-loaded to make it easier for you to get started using Seaside.

An image is loaded and executed by a virtual machine. When you ran Pharo in Section 2.1 you really were running the virtual machine on the pre-packaged “One Click Image” image. For the sake of brevity we call this “running the image.” The Smalltalk image includes active processes (Smalltalk processes are more akin to “threads” in other languages). When you load an image from a disk file you bring objects that were part of that image into RAM and resume execution of the active processes that were part of that image. If you were debugging when you saved the image, you’ll still be debugging when you load that image. We like to think of this as “picking up where you left off” (note that there are limits to this model: transient objects like a network connection that was active when the image was saved will not be available when the image is re-loaded). As an example, the Seaside “One Click Image” image was saved with the Comanche web server running so, when you load this image that web server process will be running. We’ll say more about that later.

Development in Pharo involves making incremental changes to the image (by creating classes, methods etc.). You will want to periodically save your Smalltalk image to disk so that your changes are saved. To save your image, select World | Save (i.e., raise the world menu by clicking somewhere in the background of the Pharo window, and click “Save” in that menu, as described previously). If you quit Pharo using World | Quit, you will also be prompted to save your image. You may resume your development efforts by running the image, as we did in Section 2.1.

Copyright © 20 December 2014 Stéphane Ducasse, Lukas Renggli, C. David Shaffer, Rick Zaccone
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