05.useReducer应用案例

import { useReducer } from 'react';
function reducer(state, action) {
  if (action.type === 'incremented_age') {
    return {
      age: state.age + 1
    };
  }
  throw Error('Unknown action.');
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, { age: 42 });

  return (<div>
        <button onClick={() => {
          dispatch({ type: 'incremented_age' })
        }}>
          Increment age
        </button>
        <p>Hello! You are {state.age}.</p>
      </div>);
}

ReactDOM.render(<Counter />, document.getElementById('root'));