feat(switchReduce): Added type safety way to reduce state

Author: gionkunz

README.md

@@ -134,6 +134,40 @@ class MyAppComponent {
 }
 ```
 
+Using typed actions in conjunction with `switchReduce` allow you to write reducers in a more type safety way.
+
+```ts
+// counter.ts
+import { ActionReducer, TypedAction, switchReduce } from '@ngrx/store';
+
+export class AddAction implements TypedAction<number> {
+  readonly type = 'ADD';
+  constructor(public readonly payload: number) {}
+}
+export class SubtractAction implements TypedAction<number> {
+  readonly type = 'SUBTRACT';
+  constructor(public readonly payload: number) {}
+}
+export class ResetAction implements TypedAction<any> {
+  readonly type = 'RESET';
+  readonly payload: any;
+  constructor() {}
+}
+
+export const counterReducer: ActionReducer<number> =
+  (state: number = 0, action: TypedAction<any>) =>
+    switchReduce(state, action)
+      .byClass(AddAction, (num: number) => {
+        return state + num;
+      })
+      .byClass(SubtractAction, (num: number) => {
+        return state - num;
+      })
+      .byClass(ResetAction, () => {
+        return 0;
+      })
+      .reduce();
+```
 
 ## Contributing
 Please read [contributing guidelines here](https://github.com/ngrx/store/blob/master/CONTRIBUTING.md).

spec/switch-reduce.spec.ts

@@ -0,0 +1,88 @@
+import {switchReduce} from '../src/utils';
+import {TypedAction} from '../src/dispatcher';
+interface TestState {
+  num: number;
+}
+
+class AddNumberAction implements TypedAction<number> {
+  type: 'ADD_NUMBER';
+  constructor(public payload: number) {}
+}
+
+class SubtractNumberAction implements TypedAction<number> {
+  type: 'SUBTRACT_NUMBER';
+  constructor(public payload: number) {}
+}
+
+let testState: TestState;
+describe('switchReduce', () => {
+  beforeEach(() => {
+    testState = {
+      num: 1
+    };
+  });
+
+  it('should return initial state with no cases and no default', () => {
+    const runSpy = jasmine.createSpy('spy');
+    const payload = 1;
+
+    switchReduce(testState, new AddNumberAction(payload)).reduce();
+
+    expect(runSpy).not.toHaveBeenCalled();
+  });
+
+  it('should take default if nothing else specified', () => {
+    const newState = switchReduce(testState, new AddNumberAction(1))
+      .reduce(() => ({
+        num: 5
+      }));
+
+    expect(newState.num).toBe(5);
+  });
+
+  it('should take default if nothing else matches', () => {
+    const runSpy = jasmine.createSpy('spy');
+
+    const newState = switchReduce(testState, new AddNumberAction(1))
+      .byClass(SubtractNumberAction, runSpy)
+      .byType('NOT_EXISTING', runSpy)
+      .reduce(() => ({
+        num: 5
+      }));
+
+    expect(newState.num).toBe(5);
+    expect(runSpy).not.toHaveBeenCalled();
+  });
+
+  it('should execute run function only once', () => {
+    const runSpy = jasmine.createSpy('spy');
+    const payload = 1;
+
+    switchReduce(testState, new AddNumberAction(payload))
+      .byClass(AddNumberAction, runSpy)
+      .byClasses([AddNumberAction, SubtractNumberAction], runSpy)
+      .byType('ADD_NUMBER', runSpy)
+      .byTypes(['ADD_NUMBER', 'SUBTRACT_NUMBER'], runSpy)
+      .reduce(runSpy);
+
+    expect(runSpy).toHaveBeenCalledWith(payload, jasmine.any(AddNumberAction), jasmine.anything());
+    expect(runSpy.calls.count()).toBe(1);
+  });
+
+  it('should execute same byClasses for each action', () => {
+    const applySwitchReduce = (state: TestState, action: TypedAction<number>) =>
+      switchReduce(state, action)
+        .byClasses([AddNumberAction, SubtractNumberAction], (payload: number, innerAction: TypedAction<number>) => {
+          const addend: number = innerAction instanceof AddNumberAction ? payload : -payload;
+          return Object.assign({}, state, {
+            num: state.num + addend
+          });
+        })
+        .reduce();
+
+    const newState1 = applySwitchReduce(testState, new AddNumberAction(1));
+    const newState2 = applySwitchReduce(newState1, new SubtractNumberAction(2));
+    expect(newState1.num).toBe(2);
+    expect(newState2.num).toBe(0);
+  });
+});

src/dispatcher.ts

@@ -5,6 +5,10 @@ export interface Action {
   payload?: any;
 }
 
+export interface TypedAction<T> extends Action {
+  payload: T;
+}
+
 export class Dispatcher extends BehaviorSubject<Action> {
   static INIT = '@ngrx/store/init';
 

src/utils.ts

@@ -1,4 +1,5 @@
 import {ActionReducer} from './reducer';
+import {TypedAction} from './dispatcher';
 
 export function combineReducers(reducers: any): ActionReducer<any> {
   const reducerKeys = Object.keys(reducers);
@@ -28,3 +29,50 @@ export function combineReducers(reducers: any): ActionReducer<any> {
     return hasChanged ? nextState : state;
   };
 }
+
+export type SwitchReduceRun<P, S> = (payload: P, action?: TypedAction<P>, state?: S) => S;
+
+export class SwitchReduceBuilder<S> {
+  private newState: S;
+
+  constructor(private state: S, private action: TypedAction<any>) {
+    this.newState = state;
+  }
+
+  byClass<P>(actionConstructor: {new(P): TypedAction<P>; }, run: SwitchReduceRun<P, S>): SwitchReduceBuilder<S> {
+    if (this.newState === this.state && this.action instanceof actionConstructor) {
+      this.newState = run(this.action.payload, this.action, this.state);
+    }
+    return this;
+  }
+
+  byClasses(actionConstructors: {new(a: any): TypedAction<any>; }[], run: SwitchReduceRun<any, S>): SwitchReduceBuilder<S> {
+    actionConstructors.forEach((actionConstructor) =>
+      this.byClass(actionConstructor, run));
+    return this;
+  }
+
+  byType(type: string, run: SwitchReduceRun<any, S>): SwitchReduceBuilder<S> {
+    if (this.newState === this.state && type === this.action.type) {
+      this.newState = run(this.action.payload, this.action, this.state);
+    }
+    return this;
+  }
+
+  byTypes(types: string[], run: SwitchReduceRun<any, S>): SwitchReduceBuilder<S> {
+    types.forEach((type) =>
+      this.byType(type, run));
+    return this;
+  }
+
+  reduce(defaultRun?: SwitchReduceRun<any, S>): S {
+    if (defaultRun instanceof Function && this.newState === this.state) {
+      this.newState = defaultRun(this.action.payload, null, this.state);
+    }
+    return this.newState;
+  }
+}
+
+export function switchReduce<S>(state: S, action: TypedAction<any>): SwitchReduceBuilder<S> {
+  return new SwitchReduceBuilder(state, action);
+}