Dynamic Web Development with Seaside

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11.9Show/Answer Explained

This section explains the method show: in WAComponent. show: is a variation of call:. You may want to skip this section if you are new to Seaside. You will find it helpful later on if you need to have more control on how components replace each other.

The method show: passes the control from the receiving component to the component given as the first argument. As with call: the receiver will be temporarily replaced by aComponent. However, as opposed to call:, show: does not block the flow of control and immediately returns.

If we replace the call: in the method editContact: with show: the application does not behave the same way as before anymore:

ContactListView>>editContact: aContact
| view |
view := ContactView new.
view contact: aContact.
self show: view.
self inform: 'Thanks for editing ' , aContact name

The reason is that show: does not block and the confirmation is displayed immediately, effectively replacing the ContactView. Clicking on the Ok then reveals the ContactView. Of course this is not the intended behavior. We can fix this issue by assigning an answer handler to the view that displays the confirmation:

ContactListView>>editContact: aContact
| view |
view := ContactView new.
view contact: aContact.
view onAnswer: [ :answer |
self inform: 'Thanks for editing ' , aContact name ].
self show: view

This solves our problem, but is arguably not very readable. Luckily there is show:onAnswer: that combines the two method calls:

ContactListView>>editContact: aContact
| view |
view := ContactView new.
view contact: aContact.
self show: view onAnswer: [ :answer |
self inform: 'Thanks for editing ' , aContact name ]

In fact, what we did above is continuation-passing style. Like this we can emulate the blocking behavior of call: by using show: and a block that defines what happens afterwards. Any code that uses call: can be transformed like this, however in case of loops that can become quite complicated (see Section 11.9.1).

Copyright © 26 September 2017 Stéphane Ducasse, Lukas Renggli, C. David Shaffer, Rick Zaccone
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