[Seaside] seaside by example RPN example

[Sven Van Caekenberghe]

Hi Nicolai,

This article of mine: https://medium.com/concerning-pharo/rediscovering-the-ux-of-the-legendary-hp-35-scientific-pocket-calculator-d1d497ece999 contains two Seaside RPN calculator web apps based on a pure domain model. You can find the web stuff towards the end, you will probably have to skip some intro stuff in between. The appendix explains how to get the code. I hope it all still works.

Sven

> On 18 Dec 2016, at 13:27, Nicolai Hess <nicolaihess at gmail.com> wrote:
> 
> 
> 
> 2016-12-16 22:06 GMT+01:00 Nicolai Hess <nicolaihess at gmail.com>:
> 
> 
> 2016-12-15 14:46 GMT+01:00 Nicolai Hess <nicolaihess at gmail.com>:
> Hi,
> 
> anyone knows the seaside example from the pharo by example book:
> 
> SBE2-SeasideRPN-on.1
> 
> MCSqueaksourceRepository
>     location: 'http://www.squeaksource.com/SqueakByExample'
>     user: ''
>     password: ''
> 
> I can not get it to work. On Seaside 3.0 you need to change the class initialization,
> because the application registration had changed.
> But despite that, the calculator does not work. Everytime I push on a number
> I get a empty-collection-error.
> I don't understand why, because the rendering of the calculator stack 
> calls a method ensureStackMachineNotEmpty, that puts a 0 onto the stack.
> Now, pushing a keypad button for a number, will pop this element and convert both
> the existing number and the new number to a new number.
> But somehow the stack is empty, eventhough we call ensureStackMachineNotEmpty.
> 
> Anyone has experience with this example or can find out what is wrong ?
> I don't have much experience with seaside.
> 
> thanks in advance.
> 
> Ah, on seaside 3.0 the current (initial state) of the StackMachine is saved once before the rendering (WASnapshot), and if we modify the stack machine state (the OrderedCollection)
> during #renderContentsOn:, the prior state (with the empty stack) is restored before executing the callbacks. 
> This had changed, in seaside 2.8, saving the state snapshots happens first after the initial rendering.
> Was this change on purpose ? Is there some information about, when it is  allowed to modify the state during rendering ?
> 
> I guess we may have to make sure that any state changes are happening outside of the rendering, is there some ~pattern~ to follow, when it is save to change states ?
> 
> thanks in advance
> 
> nicolai
> 
> 
> 
> Ok I found a hint in http://book.seaside.st/book/fundamentals/anchors/about-callbacks
> "Important : Do not change state while rendering....."
> 
> Ok, so the old RPNCalculator example does not work anymore if we try to "ensure the stack is not empty" during then renderContentsOn: method.
> Now I need to find a way to ensure the stackmachine non-emptyness before doing the rendering.
> 
> Is there a hook, called before the rendering, that *can* change the component state ?
> 
> 
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