redux - Why `mapDispatchToProps` does not see `stateProps`? -


intro

the signature of mapdispatchtoprops is:

mapdispatchtoprops(dispatch, [ownprops]): dispatchprops

one wonder, why mapdispatchtoprops not provided stateprops i.e. result of mapstatetoprops call. alternative syntax such as:

mapdispatchtoprops(dispatch, [ownprops], [stateprops]): dispatchprops

it seems me not break (not performance) , can beneficial in situations.

question

is there reason why not implemented? missing something? how solve use case presented below without feature?

why not break anything

such change allow user customize dispatchprops functions stateprops. when stateprops change, react-redux can recompute dispatchprops actual stateprops , rerender component. such logic happens ownprops

performance

if user not need read stateprops, can omit argument in mapdispatchtoprops implementation. react-redux can detect , assume, dispatchprops not dependent on stateprops. such performance tweak implemented ownprops.

use case

say have 10 action(creator)s deal specific todo item. these 10 actions looks like:

changetitle(todoid, title) {   return {     type: 'changetitle',     todoid,     title,   } } // etc...

if 10 action(creator)s need todoid 1 might make sense make partial instantiation of action(creator)s (it can written more succinctly that's not relevant now):

actionsfortodo(todoid) {   return {     changetitle: (title) => ({       type: 'changetitle',       todoid,       title,     }),     // more action(creator)s here   } }

and provide todoid action(creator)s:

mapdispatchtoprops((dispatch, ownprops, stateprops) =>    bindactioncreators(actionsfortodo(stateprops.todoid)))

this would negatively affect performance quite bit. since result of mapstate function of store state , possibly component's own incoming props, require mapdispatch re-run every time output of mapstate changes, , result in new function objects being allocated every single time. waste of cpu cycles in cases. it's simpler pass in item-specific values arguments when call these functions component.

there is supported workaround: third argument connect() mergeprops function, does the output of mapstate , mapdispatch, , can customize how they're merged together. if use case matters you, can implement logic providing mergeprops function.

that said, upcoming react-redux@5.0.0 prerelease version major rewrite of implementation, does have new connectadvanced function supports type of scenario better. see https://github.com/reactjs/react-redux/pull/416 .


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