Make search function provide a better API

This is a feature for instantsearch.js V2 – breaking change

Right now, we have a function that can be given to the instantsearch factory called searchFunction. This function is given a helper that can’t be modified, only the search function can be really used. Instead we could only provide a single function search as a parameter.

API proposal

instantsearch({
  searchFunction: (search, searchParameters) => {
     search(searchParameters.setQuery(''));
  };
});

In this example, the function will prevent the query to be set.

[bobylito]

After using this method a few times, using the fake helper while wanting to change the values before doing a search feels terribly clunky. However JS has prototypal inheritance and we could leverage it to provide a lightweight helper that actually triggers the search.

New proposal would be:

instantsearch({
  searchFunction: (helper) => {
     helper.setQuery('new value').search());
  };
});

This means that it will feel like the original helper with the same interface and we won’t need to actually get the helper instance on the instantsearch object.

[bobylito]

We’ve implemented the latest proposal in the latest beta of instantsearch.js v2. It works fine except that now it poses problem for custom widget that want to bind on the help lifecycle as well. This is because of the behavior of an event emitter behavior as a prototype (with the use of immutable objects which don’t update their values in place). see #2176