While I don't agree that push/pull is not relevant, I totally agree on the
glitches of the observer pattern you described here, which IMO make the
observer pattern only really practical in a model/view setting, where the
view is observer and the model is the subject.
2009/6/9 Daniel Bünzli
I don't think the push/pull issue is relevant to the discussion about frp and the OO observer pattern. The push/pull issue is an (important) implementation detail of FRP.
Le 9 juin 09 à 19:09, Álvaro García Pérez a écrit :
This has similarities to the OO Observer pattern (in fact, you can
implement it using the pattern) and is also supported in some new scripting languages as JavaFX.
One thing the observer pattern doesn't give you is any guarantee on the order of updates: when the observed value changes it updates all the observers in no specified order. However if a value observes more than one value this may result in glitches (i.e. values you actually don't want to see).
For example suppose your value dependencies are as follows :
a = b + c b = c + 1
i.e. a observes b and c, and b observes c and initially we have :
c = 0 b = 1 a = 1
If c updates from 0 to 2 any of the following two sequences of updates may be seen with the OO observer pattern :
c = 2; a = 3; b = 2; a = 4; c = 2; b = 2; a = 4;
But usually you don't want to see the a = 3, it's a glitch. FRP systems update the graph of dependencies in topological order (i.e. they ensure before updating a value that each value it depends on has been updated) and you are guaranteed you'll only see the second sequence of updates.
FRP can be seen as a form of value observation in the sense that changes in a value get eventually propagated to other values that depend on it. But it is clearly not the same as the OO observer pattern as usually understood/implemented because of this ordering issue. FRP is more subtle and powerful in the management of the value's dependency graph.
Best,
Daniel_______________________________________________
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