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Data: Avoid calling listeners on unchanging state #5339

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Mar 5, 2018
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Data: Avoid calling listeners on unchanging state #5339

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14 changes: 13 additions & 1 deletion data/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -72,7 +72,19 @@ export function registerReducer( reducerKey, reducer ) {
}
const store = createStore( reducer, flowRight( enhancers ) );
stores[ reducerKey ] = store;
store.subscribe( globalListener );

// Customize subscribe behavior to call listeners only on effective change,
// not on every dispatch.
let lastState = store.getState();
store.subscribe( () => {
const state = store.getState();
const hasChanged = state !== lastState;
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Per curiosity, do we have a lot of use-cases where a dispatch is triggered without any change in the state?

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@aduth aduth Mar 2, 2018 •
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Many of the actions which incur side-effects often don't do anything to state on their own, e.g.

  • SETUP_EDITOR
  • INSERT_DEFAULT_BLOCK
  • AUTOSAVE
  • FETCH_REUSABLE_BLOCKS

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For some of these, it could make sense to move the effect to be part of the action creator itself. We might not need a standalone INSERT_DEFAULT_BLOCK action type if the insertDefaultBlock action creator just returned the result of insertBlock.

lastState = state;

if ( hasChanged ) {
globalListener();
}
} );

return store;
}
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30 changes: 30 additions & 0 deletions data/test/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -253,6 +253,26 @@ describe( 'withSelect', () => {

store.dispatch( { type: 'increment' } );
} );

itWithExtraAssertions( 'should not rerun selection on unchanging state', () => {
const store = registerReducer( 'unchanging', ( state = {} ) => state );

registerSelectors( 'unchanging', {
getState: ( state ) => state,
} );

const mapSelectToProps = jest.fn();

const Component = compose( [
withSelectImplementation( mapSelectToProps ),
] )( () => <div /> );

wrapper = mount( <Component /> );

store.dispatch( { type: 'dummy' } );

expect( mapSelectToProps ).toHaveBeenCalledTimes( 1 );
} );
}

cases( withSelect );
Expand Down Expand Up @@ -380,6 +400,16 @@ describe( 'subscribe', () => {

expect( secondListener ).toHaveBeenCalled();
} );

it( 'does not call listeners if state has not changed', () => {
const store = registerReducer( 'unchanging', ( state = {} ) => state );
const listener = jest.fn();
subscribeWithUnsubscribe( listener );

store.dispatch( { type: 'dummy' } );

expect( listener ).not.toHaveBeenCalled();
} );
} );

describe( 'dispatch', () => {
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