Sky Hooks

[Martin Glow]

Reacts "hooks" APIs give function components the ability to use local component state, execute side effects, and more. React also lets us write custom hooks, which let us extract reusable hooks to add our own behavior on top of React's built-in hooks.

The selector will be called with the entire Redux store state as its only argument. The selector may return any value as a result, including directly returning a value that was nested inside state, or deriving new values. The return value of the selector will be used as the return value of the useSelector() hook.

The selector will be run whenever the function component renders (unless its reference hasn't changed since a previous render of the component so that a cached result can be returned by the hook without re-running the selector). useSelector() will also subscribe to the Redux store, and run your selector whenever an action is dispatched.

When an action is dispatched, useSelector() will do a reference comparison of the previous selector result value and the current result value. If they are different, the component will be forced to re-render. If they are the same, the component will not re-render. useSelector() uses strict === reference equality checks by default, not shallow equality (see the following section for more details).

You may call useSelector() multiple times within a single function component. Each call to useSelector() creates an individual subscription to the Redux store. Because of the React update batching behavior used in React Redux v7, a dispatched action that causes multiple useSelector()s in the same component to return new values should only result in a single re-render.

When the function component renders, the provided selector function will be called and its result will be returnedfrom the useSelector() hook. (A cached result may be returned by the hook without re-running the selector if it's the same function reference as on a previous render of the component.)

However, when an action is dispatched to the Redux store, useSelector() only forces a re-render if the selector resultappears to be different than the last result. The default comparison is a strict === referencecomparison. This is different than connect(), which uses shallow equality checks on the results of mapState callsto determine if re-rendering is needed. This has several implications on how you should use useSelector().

With mapState, all individual fields were returned in a combined object. It didn't matter if the return object wasa new reference or not - connect() just compared the individual fields. With useSelector(), returning a new objectevery time will always force a re-render by default. If you want to retrieve multiple values from the store, you can:

When using useSelector with an inline selector as shown above, a new instance of the selector is created whenever the component is rendered. This works as long as the selector does not maintain any state. However, memoizing selectors (e.g. created via createSelector from reselect) do have internal state, and therefore care must be taken when using them. Below you can find typical usage scenarios for memoizing selectors.

However, when the selector is used in multiple component instances and depends on the component's props, you need to ensure that selector's memoization behavior is properly configured (see here for details).

In development, the provided selector function is run an extra time with the same parameter during the first call to useSelector, and warns in the console if the selector returns a different result (based on the equalityFn provided).

A useSelector call returning the entire root state is almost always a mistake, as it means the component will rerender whenever anything in state changes. Selectors should be as granular as possible, like state => state.some.nested.field.

When passing a callback using dispatch to a child component, you may sometimes want to memoize it with useCallback. If the child component is trying to optimize render behavior using React.memo() or similar, this avoids unnecessary rendering of child components due to the changed callback reference.

However, the React hooks lint rules do not know that dispatch should be stable, and will warn that the dispatch variableshould be added to dependency arrays for useEffect and useCallback. The simplest solution is to do just that:

This hook should probably not be used frequently. Prefer useSelector() as your primary choice. However, this may be useful for less common scenarios that do require access to the store, such as replacing reducers.

The component allows you to specify an alternate context via the context prop. This is useful if you're building a complex reusable component, and you don't want your store to collide with any Redux store your consumers' applications might use.

The React-Redux hooks API has been production-ready since we released it in v7.1.0, and we recommend using the hooks API as the default approach in your components. However, there are a couple of edge cases that can occur, and we're documenting those so that you can be aware of them.

One of the most difficult aspects of React Redux's implementation is ensuring that if your mapStateToProps function is defined as (state, ownProps), it will be called with the "latest" props every time. Up through version 4, there were recurring bugs reported involving edge case situations, such as errors thrown from a mapState function for a list item whose data had just been deleted.

Starting with version 5, React Redux has attempted to guarantee that consistency with ownProps. In version 7, that is implemented using a custom Subscription class internally in connect(), which forms a nested hierarchy. This ensures that connected components lower in the tree will only receive store update notifications once the nearest connected ancestor has been updated. However, this relies on each connect() instance overriding part of the internal React context, supplying its own unique Subscription instance to form that nesting, and rendering the with that new context value.

With hooks, there is no way to render a context provider, which means there's also no nested hierarchy of subscriptions. Because of this, the "stale props" and "zombie child" issues may potentially re-occur in an app that relies on using hooks instead of connect().

useSelector() tries to deal with this by catching all errors that are thrown when the selector is executed due to a store update (but not when it is executed during rendering). When an error occurs, the component will be forced to render, at which point the selector is executed again. This works as long as the selector is a pure function and you do not depend on the selector throwing errors.

As mentioned earlier, by default useSelector() will do a reference equality comparison of the selected value when running the selector function after an action is dispatched, and will only cause the component to re-render if the selected value changed. However, unlike connect(), useSelector() does not prevent the component from re-rendering due to its parent re-rendering, even if the component's props did not change.

We've pared down our hooks API from the original alpha release, focusing on a more minimal set of API primitives.However, you may still wish to use some of the approaches we tried in your own apps. These examples should be readyto copy and paste into your own codebase.

This hook was in our original alpha release, but removed in v7.1.0-alpha.4, based on Dan Abramov's suggestion.That suggestion was based on "binding action creators" not being as useful in a hooks-based use case, and causing toomuch conceptual overhead and syntactic complexity.

You should probably prefer to call the useDispatch hook in your components to retrieve a reference to dispatch,and manually call dispatch(someActionCreator()) in callbacks and effects as needed. You may also use the ReduxbindActionCreators function in your own code to bind action creators,or "manually" bind them like const boundAddTodo = (text) => dispatch(addTodo(text)).

There are some architectural trade offs to take into consideration when deciding whether to use hooks or not. Mark Erikson summarizes these nicely in his two blog posts Thoughts on React Hooks, Redux, and Separation of Concerns and Hooks, HOCs, and Tradeoffs.

The tanago hooks are specifically designed for fish thathave very tiny mouths. Thesize of the tanago hooks, but even more the shape of the hooks, makes themmuch more effective for extremely small fish - particularly when fishing the way they do for tanago - with a small float, a very small weight and an even smaller bait.

I have caught shiners, dace, chubs, fallfish, trout, bass and sunfish on tanago hooks and on midge hooks. The tanago hooks are better if you plan to fish with bait and the midge hooks are better if you plan to fly fish for micros.

I am often asked which hook is the smallest. It can be a bit confusing, since both Owner and Gamakatsu have hooks named "Smallest" (rough translation). In a sense, though the question misses the point. The point is the point (the hook point, that is). People just assume that a fish takes the whole hook into it's mouth, so they want to use a small hook to catch small fish.

The unusual shapes for tanago hooks, though, are because the fish doesn't take the whole hook into its mouth! The only part of the tanago hook that matters is the point. That is the only part of the hook that tanago (which have very small mouths) take. The extreme bend is to keep the hook shank out of the way!

The key, then, is how long is the point. A shorter hook point can be taken by a smaller fish. The Owner Migen has a hook point that is even shorter than the Gamakatsu "Ultimate" - short enough that it really is for the smallest of fish.

Midge hooks were designed for small flies, but they were also designedfor large fish. The gape is small because it is a small hook overall,but the length of the hook point is too great for the smallest fish. This is particularly true if the hook is hanging vertically, as it would if you are fishing under a float.

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