I have a start and end property which stores a date in my state.Anytime other part of the state get changed my subscription for start and end date get triggered.
this is my subscription
this.subs.sink = this.store
.select(fromTransactionReducer.selectStartAndEndDate)
.subscribe((date) => {
console.log("range", date);
this.range.setValue({
start: date.start,
end: date.end,
});
});
this is the selector
export const selectStartAndEndDate = createSelector(
selectTransactionState,
(state) => ({ start: state.start, end: state.end })
);
this is the dateRange reducer
on(transactionActions.UpdateDateRange, (state, { start, end }) => ({
...state,
start,
end,
})),
this is the date range action
export const UpdateDateRange = createAction(
"[Transaction Page] Update Date Range",
props<{ start: Date; end: Date }>()
);
this is my state
export interface State {
transaction: Transaction[];
cursors: Cursor;
totalTransactions: number;
loading: boolean;
errorMessage: string;
formErrorMessage: string;
items_per_page: number;
pageSizeOptions: number[];
pageIndex: number;
searchKey: string;
formMessage: string;
start: Date;
end: Date;
trans_type: string;
base_type: string;
}
export const initialState: State = {
transaction: [],
cursors: {
after: "",
before: "",
hasNext: false,
hasPrevious: false,
},
totalTransactions: 0,
loading: false,
errorMessage: null,
formErrorMessage: null,
items_per_page: 5,
pageSizeOptions: [2, 3, 5, 10, 15],
pageIndex: 0,
searchKey: "",
formMessage: "",
start: null,
end: null,
trans_type: null,
base_type: null,
};
Anytime i dispatch a different action for example this.store.dispatch(transactionActions.ResetPageIndex());, my subscription for the date get triggered.
why is that ?
In my reducer i only update start and end date when UpdateDateRange action is dispatched.
The ngrx select operator utilizes the rxjs distinctUntilChanged operator to ensure that a selector only emits if its result has changed. Refer to the source code here: https://github.com/ngrx/platform/blob/a6fe92df3134b7a589a2e95abe0aea183b411f03/modules/store/src/store.ts#L249
By default distinctUntilChanged uses an equality comparer (refer to https://www.learnrxjs.io/learn-rxjs/operators/filtering/distinctuntilchanged) and note 'object references must match!'
Now to your selectStartAndEndDate selector - it returns an object.
Furthermore, it returns a new instance of an object each time it executes. Because of this, the comparison in distinctUntilChanged always fails so the selector always emits.
How to resolve it:
// returned date will be considered equal next time so selector won't fire
export const selectStartDate = createSelector(
selectTransactionState,
(state) => state.start
);
// same here
export const selectEndDate = createSelector(
selectTransactionState,
(state) => state.end
);
// as input selectors didn't change, this won't fire either
export const selectStartAndEndDate = createSelector(
selectStartDate,
selectEndDate ,
(start, end) => ({ start, end })
);
Stackblitz: https://codesandbox.io/s/so-ngrx-selector-emit-1zxg1?file=/src/app/store/selectors.ts