Custom Hooks allow you to extract commonly used functionality away to be used in multiple parts of your app. In the last post, we talked about what it looks like to leverage hooks and the Amplify library with TypeScript to fetch some data. There were some common elements involved with this, so let's make our data fetching more useful by extracting a custom hook we can use whenever we want to query our API.
The Hook:
Here is the code we left with from the previous post:
const [gameData, setGameData] = React.useState(undefined);
React.useEffect(() => {
const fetchGames = async () => {
try {
const response: any = await API.graphql(graphqlOperation(popularGames));
setGameData(response.data);
} catch (error) {
console.log(error);
}
};
fetchGames();
}, []);
Let's take this and put it inside a custom hook called useQuery. We need to rename the return values from our useState call because we're not specifically talking about game stuff, we're making a re-usable hook.
The result is below:
const useQuery = () => {
const [data, setData] = React.useState(undefined);
React.useEffect(() => {
const fetchQuery = async () => {
try {
const response: any = await API.graphql(graphqlOperation(popularGames));
setData(response.data);
} catch (error) {
console.log(error);
}
};
fetchQuery();
}, []);
};
Wait, that doesn't look like anything. It's just a function.
Exactly.
Hooks are functions that we can use to encapsulate and compose other hooks that we want to leverage in our code.
Passing arguments
There are some things that we need to generalize to be able to re-use this function elsewhere. Namely, the popularGames query and any possible variables (this example doesn't have any, but we want to accommodate other queries that may).
We can accomplish this by passing arguments into the hook just like any other function.
const useQuery = (query: string, variables?: any) => {
const [data, setData] = React.useState(undefined);
React.useEffect(() => {
const fetchQuery = async () => {
try {
const response: any = await API.graphql(graphqlOperation(query));
setData(response.data);
} catch (error) {
console.log(error);
}
};
fetchQuery();
}, [query, variables]);
};
We've done a few things here, including adding two new arguments (query and variables), adding them to the dependency array for the useEffect declaration, and updating the graphqlOperation to take the query.
However, in doing this we've lost any help provided to us by the types that we were using in the previous post's example. We need to fix that - but how?
Generics
Generics allow us to pass types to functions, in a similar way to passing arguments to functions. In this case, we want to pass generic type arguments to our useQuery hook to give it information about what types our query and variables are.
It looks like this:
const useQuery = <ResultType extends {}, VariablesType extends {} = {}>(
query: string,
variables?: VariablesType
) => {
const [data, setData] = React.useState({} as ResultType);
React.useEffect(() => {
const fetchQuery = async () => {
try {
const { data } = (await API.graphql(
graphqlOperation(query, variables)
)) as {
data: ResultType;
};
setData(data);
} catch (error) {
console.log(error);
}
};
fetchQuery();
}, [query]);
};
Here, we are saying that we can pass two types to our useQuery hook. The first type is the ResultsType we are going to have returned from our network call. The second is the VariablesType that we can use if we want to pass variables to our Amplify query call. We default this to an empty object by putting = {} after the declaration. We use this type after the optional variables declaration for our function.
Returning Data
You may have noticed that we are not returning anything from this hook. We are fetching data and that data is set to the state within the hook - but that's it. We can return whatever we want from this hook, like our data, and we can also return helpful information like whether we're currently loading the data or have encountered an error. We can add some state to keep track of these and also return an object holding our different pieces of state. It looks like this:
type UseQueryType<ResultType> = {
loading: boolean;
error: any;
data: ResultType;
refetch: () => void;
};
export const useQuery = <ResultType extends {}, VariablesType extends {} = {}>(
query: string,
variables?: VariablesType
): UseQueryType<ResultType> => {
const [loading, setLoading] = React.useState(true);
const [error, setError] = React.useState('');
const [data, setData] = React.useState({} as ResultType);
const fetchQuery = async (query: string, variables?: VariablesType) => {
try {
const { data } = (await API.graphql(
graphqlOperation(query, variables)
)) as {
data: ResultType;
};
setData(data);
} catch (error) {
console.log(error);
setError(error);
} finally {
setLoading(false);
}
};
React.useEffect(() => {
fetchQuery(query, variables);
}, [query]);
return {
loading,
data,
error,
};
};
We're going to initialize the loading state to true, set the error to an empty string, and then return all our stateful parts in an object to the client. The last thing we'd like to do is set the return type for the function, and we can make use of generics again to create another type that encapsulates the different parts of the state.
The type will look like this:
type UseQueryType<ResultType> = {
loading: boolean;
error: any;
data: ResultType;
};
We can use it by declaring the return type after the hook, like this:
export const useQuery = <ResultType extends {}, VariablesType extends {} = {}>(
query: string,
variables?: VariablesType
): UseQueryType<ResultType> => {
...
Adding Convenience
Just as a convenience, we're going to add a function to our hook that we can pass back to the consumers of our hook that will let us re-fetch the data (maybe to refresh a list manually if we choose). I've added that ability here:
type UseQueryType<ResultType> = {
loading: boolean;
error: any;
data: ResultType;
refetch: () => void;
};
export const useQuery = <ResultType extends {}, VariablesType extends {} = {}>(
query: string,
variables?: VariablesType
): UseQueryType<ResultType> => {
const [loading, setLoading] = React.useState(true);
const [error, setError] = React.useState('');
const [data, setData] = React.useState({} as ResultType);
const fetchQuery = async (query: string, variables?: VariablesType) => {
try {
const { data } = (await API.graphql(
graphqlOperation(query, variables)
)) as {
data: ResultType;
};
setData(data);
} catch (error) {
console.log(error);
setError(error);
} finally {
setLoading(false);
}
};
const refetch = () => {
fetchQuery(query, variables);
};
React.useEffect(() => {
fetchQuery(query, variables);
}, [query]);
return {
loading,
data,
error,
refetch,
};
};
Nice, mmkay - this is a pretty fun hook that takes a lot of boilerplate off our tray. You can give this a try in an app, and it will fetch the data just fine. But, if you run it in the latest version of create-react-app, you will see a warning about hook dependencies. The problem is that for our useEffect hook, we're providing the query argument, but not the variables that we also use within the hook. What this means is that if the variables change, the effect will not re-run and we will have incorrectly run the graphqlOperation with stale variables.
We can fix this by adding variables to the dependency array.
React.useEffect(() => {
fetchQuery(query, variables);
}, [query, variables]);
But if you run this, you will end up with the hooks infinitely looping over and over! Why is this? variables is an object, and for dependencies that are an object, we need to compare their properties, which React does not do by default. We could write this ability ourselves. But luckily, Kent Dodds already has.
Install this by running npm i use-deep-compare-effect (or yarn add use-deep-compare-effect).
Next, we need to update our useEffect call to use this instead.
First, import it:
import useDeepCompareEffect from 'use-deep-compare-effect';
and replace the call to useEffect:
useDeepCompareEffect(() => {
fetchQuery(query, variables);
}, [query, variables]);
Now, your objects will be compared using deep equality and the infinite loops will be a thing of the past.
Wrapping Up
The final iteration of this particular demonstration is below. I hope this helps explain some different aspects of hooks when working with Amplify, GraphQL, and TypeScript. Follow me on twitter and let me know if you found it helpful or if you had any problems!
type UseQueryType<ResultType> = {
loading: boolean;
error: any;
data: ResultType;
refetch: () => void;
};
export const useQuery = <ResultType extends {}, VariablesType extends {} = {}>(
query: string,
variables?: VariablesType
): UseQueryType<ResultType> => {
const [loading, setLoading] = React.useState(true);
const [error, setError] = React.useState('');
const [data, setData] = React.useState({} as ResultType);
const fetchQuery = async (query: string, variables?: VariablesType) => {
try {
const { data } = (await API.graphql(
graphqlOperation(query, variables)
)) as {
data: ResultType;
};
setData(data);
} catch (error) {
console.log(error);
setError(error);
} finally {
setLoading(false);
}
};
const refetch = () => {
fetchQuery(query, variables);
};
useDeepCompareEffect(() => {
fetchQuery(query, variables);
}, [query, variables]);
return {
loading,
data,
error,
refetch,
};
};
This post was originally posted on my website. You can see this and other articles like it, as well as what I'm all about. Follow me on @twitter and reach out if you have any questions!
Top comments (4)
Fantastic post. Thanks for taking the time to write this.
Great post, already helped me out a lot! It'd be awesome to also see how you'd do the same with subscriptions.
That's coming next!
Here you are! 😉
dev.to/mwarger/aws-amplify-graphql...