问题
Following the style of this Facebook app sample using Redux and Flow together, I made an action type in this manner:
type Action =
| { type: 'ADD_FILES', files: Array<{ id: number, file: File }> }
| { type: 'HANDLE_IMAGE_PUBLISHED', id: number, name: string }
| { type: 'SET_IMAGE_UPLOAD_PROGRESS', id: number, progress: number }
;
But I've found that when I try to process my actions with a reducer, Flow complains if I try to access the name or progress properties, saying "Property not found in object type".
That is, in my reducer, if I check that action.type === 'HANDLE_IMAGE_PUBLISHED' and then access action.name, Flow complains. And the same thing goes for for accessing action.progress property when action.type === 'SET_IMAGE_UPLOAD_PROGRESS'. Both these property accesses should be legit under their respective circumstances, as far as I can tell, but Flow complains.
Yet for some reason it's OK for me to access action.id anywhere, even though one of the types in my union doesn't specify an id property. I'm very confused.
Here is a live demo in the Flow REPL. What am I doing wrong?
回答1:
This is simply a case of a type refinement invalidation:
https://flow.org/en/docs/lang/refinements/#toc-refinement-invalidations
Because you are using the value in a callback, Flow pessimistically assumes that you could have re-assigned action before the callback runs (it does not know that the map callback is called immediately). It also does not do the analysis to see that there is no place, in fact, that you re-assign it.
All that's needed is to pull the action out as a const:
export default (state: Array<ImageRecordModel> = [], action_: Action): Array<ImageRecordModel> => {
const action = action_;
(tryflow link)
You may also want to consider enabling const params in your .flowconfig. This does basically what you expect: treats all params as const:
[options]
experimental.const_params=true
来源：https://stackoverflow.com/questions/43376241/using-flow-union-types-for-redux-actions