Fix for hasFocus / checked binding if not applied via DataBindProvider

[mcselle]

Problem

An observable on hasFocus binding would not fire on focus changes if the binding was not applied via the dataBindProvider (e.g. LegacyBinding)

Testcase

[...]
const createElementWithHasFocusBinding  = {
  init: (element: HTMLElement, valueAccessor: () => any) => {
    let parent = $(element);
    let $hasFocus = valueAccessor();
    
    applyBindingsToNode(parent[0], { hasFocus: $hasFocus })
    }
  };
options.bindingProviderInstance.bindingHandlers.set("customBinding",  createElementWithHasFocusBinding)

testNode.innerHTML = `<input data-bind='customBinding: customProps' />`

var $hasFocus= observable(false)
applyBindings({customProps: $hasFocus}, testNode );
[...]

Fix

//  in hasFocus.ts
[...]
var modelValue = valueAccessor(isFocused, {onlyIfChanged: true});
if (isWriteableObservable(modelValue) && (modelValue.peek() !== isFocused)) {
  modelValue(isFocused);
}
[...]

Found this bug while migrating from ko to tko, we have a custom LegacyBinding that applies bindings itself.
So its not a DataBindProvider which would otherwise trigger Parser.ConvertToAccessor() and therefore override the value function.

// in parser.ts
  convertToAccessors (result, context, globals, node) {
    objectForEach(result, (name, value) => {
      if (value instanceof Identifier) {
        Object.defineProperty(result, name, {
          value: function (optionalValue, options) {
            const currentValue = value.get_value(undefined, context, globals, node)
            if (arguments.length === 0) { return currentValue }
            const unchanged = optionalValue === currentValue
            if (options && options.onlyIfChanged && unchanged) { return }
            return value.set_value(optionalValue, context, globals) // <------------------------ THIS is missing
          }
        [...]

I guess after #202 is merged it would be best to migrate the hasFocus / checked / etc. Bindings like the value Binding which already inherites from BindingHandler that basically contains the same logic as my quick fix

// in BindingHandler.ts
get value () { return this.valueAccessor() }
set value (v) {
  const va = this.valueAccessor()
  if (isWriteableObservable(va)) {
    va(v)
  } else {
    this.valueAccessor(v)
  }
}

[phillipc]

Good work! I know it was not easy to find.

[phillipc]

@brianmhunt: I was involved in developing this solution. If this PR is okay for you, we can merge it.

[brianmhunt]

This looks great, thank you. A few stylistic changes, and I think we want to avoid a jQuery dependency here (unless we're testing positively for things using jQuery, but that doesn't seem to be the case here). It'd be good to not introduce more var statements, since they hoist and collide, introducing the possibility of unexpected side effects.

Please share if you have any comments/questions/thoughts, and I'll keep an eye out for updates. Thank you!

[mcselle]

Thanks @brianmhunt ! i changed the mentioned code

[phillipc]

@brianmhunt mcselle made all requested changes. Can I merge it now?