9 March, 2019
We were discussing with @erusev what we can do with async operation when using useReducer()
in our application. Our app is simple and we don't want to use a state management library. All our requirements are satisfied with using one root useReducer()
. The problem we are facing and don't know how to solve is async operations.
In a discussion with Dan Abramov he recommends Solution 3 but points out that things are fresh with hooks and there could be better ways of handling the problem.
Doing asynchronous operations in a useReducer
reducer is not possible. We have thought of three possible solutions and can't figure which one is better or if there is an even better solution.
We are searching for a solution where a single action will be used multiple times in multiple places all over the application.
Just manually call the async function and after it completes call the dispatch.
- No additional abstraction
- Doesn't introduce additional learning curve because it uses already existing ideas
- Now calling
dispatch({ type: 'DELETE_FILE' })
have an invisible dependency/requirement. If you don't execute the required code before thedispatch
call you are calling for a strange bug that can be missed depending on the app architecture - For larger async operations we need to extract the code in a global place
function App() {
const [state, dispatch] = useReducer((state, action) => {
switch (action.type) {
case 'DELETE_FILE':
let index = state.files.indexOf(action.file);
return {
...state,
files: [...state.files.slice(0, index), ...state.file.slice(index + 1)]
};
}
}, {
files: ['a', 'b', 'c'],
});
return (
<DispatchContext.Provider value={dispatchMiddleware(dispatch)}>
<Component files={state.files} />
</DispatchContext.Provider>
);
}
function Component({ files }) {
const dispatch = useContext(DispatchContext);
function deleteFile(file) {
unlink(file, () => {
dispatch({ type: 'DELETE_FILE', file: file });
});
}
return (
<>
{files.map(file =>
<button onClick={() => deleteFile(file)}>Delete File</button>
)}
</>
);
}
Use useEffect()
hook to delete the file.
- The only location where a side effect like writing to a file or fetching data can be is in a
useEffect()
hook. This improves the cognitive load of understanding the code.
- An additional state property
- State property which may not be used in the UI. Not sure if that is a problem. Maybe state properties that are not part of the UI are normal.
- If while deleting the file we don't want to show a UI indication the code goes through the component twice which is a small inefficiency and a confusion when you imagine it. Goes through the component logic just to execute a
useEffect()
call.
function App() {
const [state, dispatch] = useReducer((state, action) => {
switch (action.type) {
case 'REQUEST_DELETE_FILE':
return {
...state,
deleteFile: action.file
};
case 'DELETE_FILE':
const index = state.files.indexOf(action.file);
return {
...state,
files: [...state.files.slice(0, index), ...state.file.slice(index + 1)]
};
}
}, {
deleteFile: null,
files: ['a', 'b', 'c'],
});
useEffect(() => {
if (!state.deleteFile) {
return;
}
unlink(state.deleteFile, () => {
dispatch({ type: 'DELETE_FILE', file: state.deleteFile });
});
}, [state.deleteFile]);
return (
<DispatchContext.Provider value={dispatchMiddleware(dispatch)}>
<Component files={state.files} />
</DispatchContext.Provider>
);
}
function Component({ files }) {
const dispatch = useContext(DispatchContext);
return (
<>
{files.map(file =>
<button onClick={dispatch({ type: 'REQUEST_DELETE_FILE', file: file })}>Delete File</button>
)}
</>
);
}
Use a middleware for dispatch which performs the async operation and then calls the actual dispatch.
- Doesn't have the disadvantages of Solution 1 and Solution 2
- A more complicated architecture. Two places where actions are handled.
function dispatchMiddleware(dispatch) {
return (action) => {
switch (action.type) {
case 'DELETE_FILE':
unlink(action.file, () => dispatch(action));
break;
default:
return dispatch(action);
}
};
}
function App() {
const [state, dispatch] = useReducer((state, action) => {
switch (action.type) {
case 'DELETE_FILE':
let index = state.files.indexOf(action.file);
return {
...state,
files: [...state.files.slice(0, index), ...state.file.slice(index + 1)]
};
}
}, {
files: ['a', 'b', 'c']
});
return (
<DispatchContext.Provider value={dispatchMiddleware(dispatch)}>
<Component files={state.files} />
</DispatchContext.Provider>
);
}
function Component({ files }) {
const dispatch = useContext(DispatchContext);
return (
<>
{files.map(file =>
<button onClick={dispatch({ type: 'DELETE_FILE', file: file })}>Delete File</button>
)}
</>
);
}
While discussing we raised two questions we are not sure the answers to. They can help us in deciding the right pattern:
- Is it normal to do side effects like writing a file or fetching data in an event handler or it should be only in
useEffect()
? - Is it normal to have properties in the state object which are not used in the view at all?
Solution 3 has a few issues. Below I list the issues and proposed ways to tackle them.
Solution 3 - Issue 1
React guarantees
dispatch
to be the same function instance, which helps with reducing the number of re-renders. With the middleware in place, you now return the middleware function instead of the dispatch function, which is a new function every time. This will increase the number of re-renders and make the reducer less useful and less efficient. You'll no longer benefit form thedispatch
being an unchangeable reference to the same function instance.The solution is to
useMemo
:Solution 3 - Issue 2
unlink(action.file, () => dispatch(action));
This is prone to race conditions! While the file is being deleted, some other component can request its deletion again.
In order to avoid this, you need a state variable that stores current api/ajax status, i.e.
currentAjaxId = null
.Before you start deleting the file:
After the api is complete:
Bonus: you can now disable the "Delete" button based on the
currentAjaxId