Data: Introduce countSelectorsByStatus redux metadata selector

packages/data/package.json

@@ -41,6 +41,7 @@
 		"is-plain-object": "^5.0.0",
 		"is-promise": "^4.0.0",
 		"redux": "^4.1.2",
+		"rememo": "^4.0.2",
 		"turbo-combine-reducers": "^1.0.2",
 		"use-memo-one": "^1.1.1"
 	},

packages/data/src/redux-store/index.js

@@ -430,6 +430,7 @@ function mapResolveSelectors( selectors, store ) {
 		getResolutionState,
 		getResolutionError,
 		hasResolvingSelectors,
+		countSelectorsByStatus,
 		...storeSelectors
 	} = selectors;
 

packages/data/src/redux-store/metadata/selectors.js

@@ -1,3 +1,8 @@
+/**
+ * External dependencies
+ */
+import createSelector from 'rememo';
+
 /**
  * Internal dependencies
  */
@@ -153,3 +158,33 @@ export function hasResolvingSelectors( state ) {
 		)
 	);
 }
+
+/**
+ * Retrieves the total number of selectors, grouped per status.
+ *
+ * @param {State} state Data state.
+ *
+ * @return {Object} Object, containing selector totals by status.
+ */
+export const countSelectorsByStatus = createSelector( ( state ) => {
+	const selectorsByStatus = {};
+
+	Object.values( state ).forEach( ( selectorState ) =>
+		/**
+		 * This uses the internal `_map` property of `EquivalentKeyMap` for
+		 * optimization purposes, since the `EquivalentKeyMap` implementation
+		 * does not support a `.values()` implementation.
+		 *
+		 * @see https://github.com/aduth/equivalent-key-map
+		 */
+		Array.from( selectorState._map.values() ).forEach( ( resolution ) => {
+			const currentStatus = resolution[ 1 ]?.status ?? 'error';
+			if ( ! selectorsByStatus[ currentStatus ] ) {
+				selectorsByStatus[ currentStatus ] = 0;
+			}
+			selectorsByStatus[ currentStatus ]++;
+		} )
+	);
+
+	return selectorsByStatus;
+} );

packages/data/src/redux-store/metadata/test/selectors.js

@@ -356,3 +356,90 @@ describe( 'hasResolvingSelectors', () => {
 		expect( result ).toBe( true );
 	} );
 } );
+
+describe( 'countSelectorsByStatus', () => {
+	let registry;
+
+	beforeEach( () => {
+		registry = createRegistry();
+		registry.registerStore( 'store', {
+			reducer: ( state = null, action ) => {
+				if ( action.type === 'RECEIVE' ) {
+					return action.items;
+				}
+
+				return state;
+			},
+			selectors: {
+				getFoo: ( state ) => state,
+				getBar: ( state ) => state,
+				getBaz: ( state ) => state,
+				getFailingFoo: ( state ) => state,
+				getFailingBar: ( state ) => state,
+			},
+			resolvers: {
+				getFailingFoo: () => {
+					throw new Error( 'error fetching' );
+				},
+				getFailingBar: () => {
+					throw new Error( 'error fetching' );
+				},
+			},
+		} );
+	} );
+
+	it( 'counts selectors properly by status, excluding missing statuses', () => {
+		registry.dispatch( 'store' ).startResolution( 'getFoo', [] );
+		registry.dispatch( 'store' ).startResolution( 'getBar', [] );
+		registry.dispatch( 'store' ).startResolution( 'getBaz', [] );
+		registry.dispatch( 'store' ).finishResolution( 'getFoo', [] );
+		registry.dispatch( 'store' ).finishResolution( 'getBaz', [] );
+
+		const { countSelectorsByStatus } = registry.select( 'store' );
+		const result = countSelectorsByStatus();
+
+		expect( result ).toEqual( {
+			finished: 2,
+			resolving: 1,
+		} );
+	} );
+
+	it( 'counts errors properly', async () => {
+		registry.dispatch( 'store' ).startResolution( 'getFoo', [] );
+		await resolve( registry, 'getFailingFoo' );
+		await resolve( registry, 'getFailingBar' );
+		registry.dispatch( 'store' ).finishResolution( 'getFoo', [] );
+
+		const { countSelectorsByStatus } = registry.select( 'store' );
+		const result = countSelectorsByStatus();
+
+		expect( result ).toEqual( {
+			finished: 1,
+			error: 2,
+		} );
+	} );
+
+	it( 'applies memoization and returns the same object for the same state', () => {
+		const { countSelectorsByStatus } = registry.select( 'store' );
+
+		expect( countSelectorsByStatus() ).toBe( countSelectorsByStatus() );
+
+		registry.dispatch( 'store' ).startResolution( 'getFoo', [] );
+		registry.dispatch( 'store' ).finishResolution( 'getFoo', [] );
+
+		expect( countSelectorsByStatus() ).toBe( countSelectorsByStatus() );
+	} );
+
+	it( 'returns a new object when different state is provided', () => {
+		const { countSelectorsByStatus } = registry.select( 'store' );
+
+		const result1 = countSelectorsByStatus();
+
+		registry.dispatch( 'store' ).startResolution( 'getFoo', [] );
+		registry.dispatch( 'store' ).finishResolution( 'getFoo', [] );
+
+		const result2 = countSelectorsByStatus();
+
+		expect( result1 ).not.toBe( result2 );
+	} );
+} );