Binding handlers should be able to use the init function to manage updates

[mbest]

Currently a binding handler that uses a computed observable created in its init function to manage updates won't work in all cases.

It will work if the observable value is directly available through the valueAccessor function and can be "unwrapped" in the computed observable.

It will not work if an observable is unwrapped when the binding is parsed or if an observable view model is used.

An advantage of using this pattern is that one can set up persistent values in the init function that will be available during updates via closure.

In order to fully support this pattern, the valueAccessor function must provide notification for all observables that may cause the binding to update.

[mbest]

Wow, we're at 500 issues!

[rniemeyer]

This particular issue causes me problems frequently.

[mbest]

Once #321 is in, this will be easy.

[mbest]

I'd also like to add that observables accessed directly in init (not wrapped in a computed) shouldn't set any dependencies for the binding (or other bindings). @SteveSanderson mentioned in #326 that this is a breaking change, but it's likely a change that provides more good than bad (in my mind we'd be fixing a bug).

[rniemeyer]

I am on board with that change too. I always thought it was odd for the init to register what amounts to a one-time dependency.

[mbest]

Another related issue is that when an element has multiple bindings, the init functions for all bindings are run before all the update functions. This needs to be changed so that for each binding, init is run first and then update (and then on to the next binding). Without this change, it's impossible for a binding that uses init to manage updates to run after a binding that uses update.

[mbest]

In order to fully support this pattern, the valueAccessor function must provide notification for all observables that may cause the binding to update.

This will have a side-effect that if the value of a binding is the result of a function call, that function will be run each time valueAccessor is called. Currently such a function is run once for each update of the element's bindings. It's probably not a big deal but just something to keep in mind.

[rniemeyer]

That side-effect sounds reasonable to me. Does not sound like a problematic change.

[mbest]

Completed with #946.