How to create a signature pad in React

How to create a signature pad in React
Learn how to build a signature pad for your forms in React.

I was recently asked by a client to build a form that allows users to sign using a wireless pen mouse. The goal of the client was to obtain user signatures for a report that would be used for a future audit.

At first I was considering building my own signature pad from scratch. However, I didn't have the time to do that. This feature needed to be live in a week. This unfortunately is the reality when working in real life situations. This is why I wanted to write about this specific case. You won't always have the time to build every single feature in an app from scratch. Sometimes you have to rely on libraries created by other developers. This article will help beginners see how you can combine different libraries to create a working product.

OK let start building the signature pad. First I wrote a user story. Below is the user story for what we will be building:

  • User sees a button that can be clicked to open a popup
  • User can see and draw on a canvas when the popup is opened
  • User has the option to exit popup via an 'Close' button, clear the canvas via a 'Clear' button, or save the signature as an image via the 'Save' button.

Now that we know what we are building lets start coding it. We can use the user story list items as our milestones.

User sees a button that can be clicked to open a popup

I will use React for this task, so lets bootstrap the project by using create-react-app.

To do that all we need to do is type the following command on our terminal:

npx create-react-app signature-pad

For more information on create-react-app check out this Github Repo

Once our project is ready, we navigate into our folder and start up our code editor. In my case I am using VS Code.

# navigate to the project folder in your terminal
cd signature-pad

# command to start VS Code once you are in root folder of the project
code .

Now that we have everything set lets start up our project by typing the following command:

# start project
npm start

This is what we should see once the project starts:

If you open up your App.js file this is what we should see:

Lets get rid of all this code and add the one thing we need. A button that when clicked should open up a popup with a signature pad, per our user story.

This is what our app should look like now:

Our final step for this part of the project is to show a popup when the button is clicked. A good library that can help with this is reactjs-popup. To install it we type

npm install --save reactjs-popup

This library can be used to create Popup Models, Tool-tips, and Nested Menus. In our case we will focus on using it to create Popup Models. For more information on it check out their very helpful documentation page

Once we have the library installed we import it into our App.js file. Next we add the Popup component to our render method. This component takes in many props. In our case we focus on two: modal props (by default this is false which results in getting a tool-tip) and the trigger props which takes in a component that will trigger the popup. In our case this is our button that we already created.

Now when we click on the button we should get a popup. the content of the popup will go inside our Popup component.

This is what we should have now:

We have successfully completed the first part of our User story. Now lets more to the next one!

User can see and draw on a canvas when the popup is opened

To build the canvas inside our popup we will be using react-signature-canvas. This is a powerful library that will make working with a signature pad very easy. Specifically when we start thinking about adding features such as clear and save to our pad.

To install it we type:

npm install --save react-signature-canvas

Once we have that installed we import the SignatureCanvas component and place it inside our Popup component. That is all!

We now have a canvas inside our popup. This is what the code looks like:

and this is what our app should be showing:

Before we move to the next item in the user story lets add some styling to our canvas. All I will do is give give our canvas a border in order for us to easily identify it and give it a width and height.

the SignaturePad component has a prop called canvasProps which allows us to pass attributes to the canvas. In this case I will give the canvas a class called "signatureCanvas".

You probably noticed that I am also importing a css file called sigCanvas.css. This is where I will put all my canvas related styles.

The result should look like this:

OK ready for our final step.

User has the option to exit popup via an 'Close' button, clear the canvas via a 'Clear' button, or save the signature as an image via the 'Save' button.

lets start with the close button. lets add a button with an onClick, inside that onClick will be a way to close the popup. The reactjs-popup docs recommend providing a function as a child to the Popup component. this function has a parameter close that will handle our close event and will return the components that we want inside the popup.

In addition we should not allow users to close the popup except via this button. To do that we need to pass the closeOnDocumentClick prop and pass it a true value.

This is what our app looks like now:

Next we need a button to clear the canvas. react-signature-canvas provides several methods that help with manipulating our canvas. In order to use this method we need to get an instant of our canvas. This can be done via React refs.

Let use React Hooks for this. First we create a ref called sigCanvas using useRef hook, the initial value is an empty object. Next we pass this into a SignaturePad component using a prop called ref. finally we create a function called clear that will be called anytime the clear button is clicked.

inside the clear function, we simply invoke the method clear() via our canvas ref sigCanvas. I realize these are a lot of steps, so I wrote comments in the code shown below:

This is what our app should look like now:

Now for the final button, the save button. This button should simply output a base64 string representing the image. You can use this to store it in a database or instead you can upload the image into your server. In our case we won't be connecting to any servers, so we will simply log the base64 code to the console.

react-signature-canvas has a method called getTrimmedCanvas() that can trim the white space from the canvas (to optimize the image) and give back a base64 string representing the image. All this via our sigCanvas ref.

This is what our app should look like now:

Alright! all basic functionalities are complete!

I do want to add one more part to this project. Lets output the saved signature on our screen instead of just logging it.

We can simply add an image tag below our popup button and pass it our image URL. Of course this image URL will be what we logged into the console, so instead of doing that we can store it in state.

This should be the result:

That's all folks! we successfully completed our user story. Of course there is room to improve but all the basic functionalities are there.

You can play around with the code and add more features to it by following the link

I hope this tutorial will surely help and you if you liked this tutorial, please consider sharing it with others.

Further reading

☞ React - The Complete Guide (incl Hooks, React Router, Redux)

☞ Modern React with Redux [2019 Update]

☞ Best 50 React Interview Questions for Frontend Developers in 2019

Originally published on dev.to

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