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--- blog:databinding_chaining_observablevalues_together [2012/08/17 22:59]
djo created
+++ blog:databinding_chaining_observablevalues_together [2014/10/20 15:44]
djo Added comment form
@@ Line 1: / Line 1: @@
+====== Databinding Chaining ObservableValues together ======
+Sometimes the standard data binding primitives are perfect: data gets converted, validated, and reported nicely.  But sometimes one's validation requirements are more than fit nicely into a standard IValidator.  And sometimes the data bindingp pipeline that Binding implements for you isn't quite as flexible as one might want.
+What I realized after reading quite a bit about monads is that if we treat an IObservableValue as immutable for a given moment in time (e.g.: it only gets changed by an external actor to the system, like the user or a clock or...) then we can chain IObservable values together using a function that maps from one IObservableValue to another in much the same way one would use the map function to transform one functional collection to another.  Implementing this idea lets us write code like this:
+<code>
+IObservableValue textFieldTarget = new WritableValue("", String.class);  // Bound to a UI field somewhere
+IObservableValue model = new WritableValue(new Integer(1), Integer.class);
+ValuePindingPipeline pipeline = new ValueBindingPipeline(bindingContext)
+   .from(model)
+      .map(new IntToSringConversion())
+      .to(textFieldTarget);
+pipeline.getReversePipeline()
+   .from(textFieldTarget)
+      .map(new IsValidIntegerValidation())
+      .map(new ConvertToInteger())
+      .map(new RangeCheckValidation())
+      .map(new LessThanValidation(someOtherObservable))
+      .to(model);
+</code>
+As you can see, the resulting code is nice and explicit about what happens everywhere in the pipeline.  This is really similar to some [[http://fire-change-event.blogspot.com/|ideas that Matt Hall had]] , just expressed in a more monadic form.
+~~LINKBACK~~
+~~DISQUS~~

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