re-frame.std-interceptors — re-frame 0.9.1

contains re-frame supplied, standard interceptors

after clj/s

(after f)

Interceptor factory which runs a given function f in the "after" position, presumably for side effects.

f is called with two arguments: the effects value of :db (or the coeffect value of db if no db effect is returned) and the event. Its return value is ignored so f can only side-effect.

Example use:

• f runs schema validation (reporting any errors found)
• f writes some aspect of db to localstorage.

ctx-handler->interceptor clj/s

(ctx-handler->interceptor handler-fn)

Returns an interceptor which wraps the kind of event handler given to reg-event-ctx. These advanced handlers take one argument: context and they return a modified context. Example: (fn [context] (enqueue context [more interceptors]))

db-handler->interceptor clj/s

(db-handler->interceptor handler-fn)

Returns an interceptor which wraps the kind of event handler given to reg-event-db.

These handlers take two arguments; db and event, and they return db.

(fn [db event] ....)

So, the interceptor wraps the given handler:

1. extracts two :coeffects keys: db and event
2. calls handler-fn
3. stores the db result back into context's :effects

debug clj/s

An interceptor which logs data about the handling of an event.

Includes a clojure.data/diff of the db, before vs after, showing the changes caused by the event handler.

You'd typically want this interceptor after (to the right of) any path interceptor.

Warning: calling clojure.data/diff on large, complex data structures can be slow. So, you won't want this interceptor present in production code. See the todomvc example to see how to exclude interceptors from production code.

enrich clj/s

(enrich f)

Interceptor factory which runs the given function f in the after handler position. f is called with two arguments: db and v, and is expected to return a modified db.

Unlike the after inteceptor which is only about side effects, enrich expects f to process and alter the given db coeffect in some useful way, contributing to the derived data, flowing vibe.

Example Use:

Imagine that todomvc needed to do duplicate detection - if any two todos had the same text, then highlight their background, and report them in a warning down the bottom of the panel.

Almost any action (edit text, add new todo, remove a todo) requires a complete reassesment of duplication errors and warnings. Eg: that edit update might have introduced a new duplicate or removed one. Same with a todo removal.

And to perform this enrichment, a function has to inspect all the todos, possibly set flags on each, and set some overall list of duplicates. And this duplication check might just be one check among many.

f would need to be both adding and removing the duplicate warnings. By applying f in an :after interceptor, we keep the handlers simple and yet we ensure this important step is not missed.

fx-handler->interceptor clj/s

(fx-handler->interceptor handler-fn)

Returns an interceptor which wraps the kind of event handler given to reg-event-fx.

These handlers take two arguments; coeffects and event, and they return effects.

(fn [coeffects event] {:db ... :dispatch ...})

Wrap handler in an interceptor so it can be added to (the RHS) of a chain:

1. extracts :coeffects
2. call handler-fn giving coeffects
3. stores the result back into the :effects

on-changes clj/s

(on-changes f out-path & in-paths)

Interceptor factory which acts a bit like reaction (but it flows into db, rather than out) It observes N paths in db and if any of them test not indentical? to their previous value (as a result of a handler being run) then it runs f to compute a new value, which is then assoced into the given out-path within db.

Usage:

(defn my-f [a-val b-val] ... some computation on a and b in here)

(on-changes my-f [:c] [:a] [:b])

Put this Interceptor on the right handlers (ones which might change :a or :b). It will:

• call f each time the value at path [:a] or [:b] changes
• call f with the values extracted from [:a] [:b]
• assoc the return value from f into the path [:c]

path clj/s

(path & args)

An interceptor factory which supplies a sub-path of :db to the handler. It's action is somewhat analogous to update-in. It grafts the return value from the handler back into db.

Usage: (path :some :path) (path [:some :path]) (path [:some :path] :to :here) (path [:some :path] [:to] :here)

Notes:

1. cater for path appearing more than once in an interceptor chain.
2. :effects may not contain :db effect. Which means no change to :db should be made.

trim-v clj/s

An interceptor which removes the first element of the event vector, allowing you to write more aesthetically pleasing db handlers. No leading underscore on the event-v! Your event handlers will look like this:

(defn my-handler
  [db [x y z]]    ;; <-- instead of [_ x y z]
  ....)