React：在 useEffect 中调用上下文函数时防止无限循环

Question

In my react app, I am rendering different instances of <Item> Components and I want them to register/unregister in a Context depending if they are currently mounted or not.

I am doing this with two Contexts ( ItemContext provides the registered items, ItemContextSetters provides the functions to register/unregister).

const ItemContext = React.createContext({});
const ItemContextSetters = React.createContext({
  registerItem: (id, data) => undefined,
  unregisterItem: (id) => undefined
});

function ContextController(props) {
  const [items, setItems] = useState({});

  const unregisterItem = useCallback(
    (id) => {
      const itemsUpdate = { ...items };
      delete itemsUpdate[id];
      setItems(itemsUpdate);
    },
    [items]
  );

  const registerItem = useCallback(
    (id, data) => {
      if (items.hasOwnProperty(id) && items[id] === data) {
        return;
      }

      const itemsUpdate = { ...items, [id]: data };
      setItems(itemsUpdate);
    },
    [items]
  );

  return (
    <ItemContext.Provider value={{ items }}>
      <ItemContextSetters.Provider value={{ registerItem, unregisterItem }}>
        {props.children}
      </ItemContextSetters.Provider>
    </ItemContext.Provider>
  );
} 

The <Item> Components should register themselves when they are mounted or their props.data changes and unregister when they are unmounted. So I thought that could be done very cleanly with useEffect :

function Item(props) {
  const itemContextSetters = useContext(ItemContextSetters);

  useEffect(() => {
    itemContextSetters.registerItem(props.id, props.data);

    return () => {
      itemContextSetters.unregisterItem(props.id);
    };
  }, [itemContextSetters, props.id, props.data]);

  ...
}

Full example see this codesandbox

Now, the problem is that this gives me an infinite loop and I don't know how to do it better. The loop is happening like this (I believe):

• An <Item> calls registerItem
• In the Context, items is changed and therefore registerItem is re-built (because it depends on [items]
• This triggers a change in <Item> because itemContextSetters has changed and useEffect is executed again.
• Also the cleanup effect from the previous render is executed! (As stated here : "React also cleans up effects from the previous render before running the effects next time")
• This again changes items in the context
• And so on ...

I really can't think of a clean solution that avoids this problem. Am I misusing any hook or the context api? Can you help me with any general pattern on how write such a register/unregister Context that is called by Components in their useEffect -body and useEffect -cleanup?

Things I'd prefer not to do:

• Getting rid of the context altogether. In my real App, the structure is more complicated and different components throughout the App need this information so I believe I want to stick to a context
• Avoiding the hard removal of the <Item> components from the dom (with {renderFirstBlock && ) and use something like a state isHidden instead. In my real App this is currently nothing I can change. My goal is to track data of all existing component instances.

Thank you!

Answer 1

You can make your setters have stable references, as they don't really need to be dependant on items :

const ItemContext = React.createContext({});
const ItemContextSetters = React.createContext({
  registerItem: (id, data) => undefined,
  unregisterItem: (id) => undefined
});

function ContextController(props) {
  const [items, setItems] = useState({});

  const unregisterItem = useCallback(
    (id) => {
      setItems(currentItems => {
        const itemsUpdate = { ...currentItems };
        delete itemsUpdate[id];
        return itemsUpdate
      });
    },
    []
  );

  const registerItem = useCallback(
    (id, data) => {

      setItems(currentItems => {
        if (currentItems.hasOwnProperty(id) && currentItems[id] === data) {
          return currentItems;
        }
        return { ...currentItems, [id]: data }
      } );
    },
    []
  );
  
  const itemsSetters = useMemo(() => ({ registerItem, unregisterItem }), [registerItem, unregisterItem])

  return (
    <ItemContext.Provider value={{ items }}>
      <ItemContextSetters.Provider value={itemsSetters}>
        {props.children}
      </ItemContextSetters.Provider>
    </ItemContext.Provider>
  );
}

Now your effect should work as expected