RFC: Add predicate to createSlice reducers.

etc/redux-toolkit.api.md

@@ -62,8 +62,10 @@ export type CaseReducer<S = any, A extends Action = AnyAction> = (state: Draft<S
 // @public
 export type CaseReducerActions<CaseReducers extends SliceCaseReducers<any>> = {
     [Type in keyof CaseReducers]: CaseReducers[Type] extends {
-        prepare: any;
-    } ? ActionCreatorForCaseReducerWithPrepare<CaseReducers[Type]> : ActionCreatorForCaseReducer<CaseReducers[Type]>;
+        prepare: AnyFunction;
+    } ? ActionCreatorForCaseReducerWithPrepare<CaseReducers[Type]> : CaseReducers[Type] extends {
+        reducer: AnyFunction;
+    } ? ActionCreatorForCaseReducer<CaseReducers[Type]['reducer']> : ActionCreatorForCaseReducer<CaseReducers[Type]>;
 };
 
 // @public @deprecated
@@ -74,7 +76,8 @@ export type CaseReducers<S, AS extends Actions> = {
 // @public
 export type CaseReducerWithPrepare<State, Action extends PayloadAction> = {
     reducer: CaseReducer<State, Action>;
-    prepare: PrepareAction<Action['payload']>;
+    prepare?: PrepareAction<Action['payload']>;
+    predicate?: Predicate<State, Action>;
 };
 
 // @public
@@ -202,10 +205,18 @@ export { ThunkAction }
 // @public
 export type ValidateSliceCaseReducers<S, ACR extends SliceCaseReducers<S>> = ACR & {
     [T in keyof ACR]: ACR[T] extends {
+        prepare: AnyFunction;
         reducer(s: S, action?: infer A): any;
     } ? {
         prepare(...a: never[]): Omit<A, 'type'>;
     } : {};
+} & {
+    [T in keyof ACR]: ACR[T] extends {
+        predicate: AnyFunction;
+        reducer(s: S, action?: infer A): any;
+    } ? {
+        predicate: Predicate<S, A>;
+    } : {};
 };
 
 

src/createSlice.test.ts

@@ -202,3 +202,65 @@ describe('createSlice', () => {
     })
   })
 })
+
+test('createSlice with predicate for reducer', () => {
+  const slice = createSlice({
+    name: 'test',
+    initialState: {
+      status: 'idle'
+    } as { status: 'working' | 'idle'; workItem?: string },
+    reducers: {
+      startWork: {
+        predicate: state => state.status === 'idle',
+        reducer(state, action: PayloadAction<string>) {
+          state.status = 'working'
+          state.workItem = action.payload
+        }
+      },
+      finishWork: {
+        predicate: state => state.status === 'working',
+        reducer(state) {
+          state.status = 'idle'
+          state.workItem = undefined
+        }
+      }
+    }
+  })
+
+  expect(slice.reducer({ status: 'idle' }, slice.actions.startWork('workItem')))
+    .toMatchInlineSnapshot(`
+    Object {
+      "status": "working",
+      "workItem": "workItem",
+    }
+  `)
+  expect(
+    slice.reducer(
+      { status: 'working', workItem: 'shouldNotChange' },
+      slice.actions.startWork('workItem')
+    )
+  ).toMatchInlineSnapshot(`
+    Object {
+      "status": "working",
+      "workItem": "shouldNotChange",
+    }
+  `)
+  expect(
+    slice.reducer(
+      { status: 'idle', workItem: 'shouldNotChange' },
+      slice.actions.finishWork()
+    )
+  ).toMatchInlineSnapshot(`
+    Object {
+      "status": "idle",
+      "workItem": "shouldNotChange",
+    }
+  `)
+  expect(slice.reducer({ status: 'working' }, slice.actions.finishWork()))
+    .toMatchInlineSnapshot(`
+    Object {
+      "status": "idle",
+      "workItem": undefined,
+    }
+  `)
+})

src/createSlice.ts

@@ -12,7 +12,8 @@ import {
   ActionReducerMapBuilder,
   executeReducerBuilderCallback
 } from './mapBuilders'
-import { Omit } from './tsHelpers'
+import { Omit, AnyFunction } from './tsHelpers'
+import { Draft } from 'immer'
 
 /**
  * An action creator atttached to a slice.
@@ -100,9 +101,15 @@ export interface CreateSliceOptions<
  */
 export type CaseReducerWithPrepare<State, Action extends PayloadAction> = {
   reducer: CaseReducer<State, Action>
-  prepare: PrepareAction<Action['payload']>
+  prepare?: PrepareAction<Action['payload']>
+  predicate?: Predicate<State, Action>
 }
 
+export type Predicate<State, Action> = (
+  state: Draft<State>,
+  action: Action
+) => boolean
+
 /**
  * The type describing a slice's `reducers` option.
  *
@@ -120,8 +127,12 @@ export type SliceCaseReducers<State> = {
  * @public
  */
 export type CaseReducerActions<CaseReducers extends SliceCaseReducers<any>> = {
-  [Type in keyof CaseReducers]: CaseReducers[Type] extends { prepare: any }
+  [Type in keyof CaseReducers]: CaseReducers[Type] extends {
+    prepare: AnyFunction
+  }
     ? ActionCreatorForCaseReducerWithPrepare<CaseReducers[Type]>
+    : CaseReducers[Type] extends { reducer: AnyFunction }
+    ? ActionCreatorForCaseReducer<CaseReducers[Type]['reducer']>
     : ActionCreatorForCaseReducer<CaseReducers[Type]>
 }
 
@@ -189,12 +200,23 @@ export type ValidateSliceCaseReducers<
 > = ACR &
   {
     [T in keyof ACR]: ACR[T] extends {
+      prepare: AnyFunction
       reducer(s: S, action?: infer A): any
     }
       ? {
           prepare(...a: never[]): Omit<A, 'type'>
         }
       : {}
+  } &
+  {
+    [T in keyof ACR]: ACR[T] extends {
+      predicate: AnyFunction
+      reducer(s: S, action?: infer A): any
+    }
+      ? {
+          predicate: Predicate<S, A>
+        }
+      : {}
   }
 
 function getType(slice: string, actionKey: string): string {
@@ -243,8 +265,19 @@ export function createSlice<
     let prepareCallback: PrepareAction<any> | undefined
 
     if ('reducer' in maybeReducerWithPrepare) {
-      caseReducer = maybeReducerWithPrepare.reducer
-      prepareCallback = maybeReducerWithPrepare.prepare
+      const { reducer, predicate, prepare } = maybeReducerWithPrepare
+      if (predicate) {
+        caseReducer = (state, action) => {
+          if (predicate(state, action)) {
+            return reducer(state, action)
+          }
+        }
+      } else {
+        caseReducer = reducer
+      }
+      if (prepare) {
+        prepareCallback = prepare
+      }
     } else {
       caseReducer = maybeReducerWithPrepare
     }

src/tsHelpers.ts

@@ -89,3 +89,5 @@ type UnionToIntersection<U> = (U extends any
   : never
 
 export type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>
+
+export type AnyFunction = (...args: any[]) => any