Reading elements

When a user selects certain type of elements, an app can detect the selection of those elements and read content from them. This unlocks a number of powerful features, such as tools for replacing elements.

For the time being, apps cannot arbitrarily read data from a design. They can only read certain types of content from the user's current selection.

Supported content types

Apps can read the following types of content from selected elements:

• Images, including raster images and vector images
• Text, as plain text

Be aware that different types of elements may contain the same types of content. For example, both image elements and page backgrounds contain image content that can be read by an app.

In the future, we intend to support more types of content.

How to read elements

Step 1: Enable the required permissions

In the Developer Portal, enable the canva:design:content:read permission. In the future, the Apps SDK will throw an error if the required permissions are not enabled. To learn more, see Configuring permissions.

Step 2: Listen for selection events

When a user selects one or more elements, Canva emits a selection event. This event contains information about the selected elements. You can listen for selection events with a React hook (recommended) or manually.

You can then access the selection event via the currentSelection variable:

console.log(currentSelection);

Step 3: Check if an element is selected

The selection event contains a count property that contains the number of selected elements. When an element isn't selected, this value is 0. You can use the count property to check if an element is currently selected:

const isElementSelected = currentSelection.count > 0;

This is useful for updating the user interface in response to the user's selection — for example, by disabling a button if an element isn't selected:

<Button variant="primary" disabled={!isElementSelected}>
  Click me
</Button>

Step 4: Read the contents of the element

The selection event has a read method that returns an object with a contents property. This property contains an array of objects. Each object represents an individual piece of content, such as the image used for a page background.

Images

If the selection event contains image content and the change handler is scoped to "image", each object contains a ref property. This property contains a unique identifier that points to an asset in Canva's backend:

const draft = await currentSelection.read();

for (const content of draft.contents) {
  console.log(content.ref);
}

The value of the ref property is an opaque string. This means it's not intended to be read or manipulated. You can, however, convert the ref into a URL and then download the image data from that URL.

To convert the ref into a URL:

1. Import the getTemporaryUrl method from the @canva/asset package:

   import { getTemporaryUrl } from "@canva/asset";

   ts

   Copy

2. Call the method, passing in the ref and the type of asset:

   const { url } = await getTemporaryUrl({
     type: "IMAGE",
     ref: content.ref,
   });
   
   console.log(url);

   ts

   Copy

Text

If the selection event contains text content, each object contains the plain text of the element:

const draft = await currentSelection.read();

for (const content of draft.contents) {
  console.log(content.text);
}

All formatting is stripped out, but the text may contain line breaks.

API reference

• getTemporaryUrl
• selection.registerOnChange

Code sample

Image

import React from "react";
import { Button } from "@canva/app-ui-kit";
import { useSelection } from "utils/use_selection_hook";
import { getTemporaryUrl } from "@canva/asset";
import styles from "styles/components.css";

export function App() {
  const currentSelection = useSelection("image");
  const isElementSelected = currentSelection.count > 0;

  async function readImages() {
    if (!isElementSelected) {
      return;
    }

    const draft = await currentSelection.read();

    for (const content of draft.contents) {
      const { url } = await getTemporaryUrl({
        type: "IMAGE",
        ref: content.ref,
      });
      console.log(url);
    }
  }

  return (
    <div className={styles.scrollContainer}>
      <Button
        variant="primary"
        disabled={!isElementSelected}
        onClick={readImages}
      >
        Read images
      </Button>
    </div>
  );
}

Text

import React from "react";
import { Button } from "@canva/app-ui-kit";
import { useSelection } from "utils/use_selection_hook";
import styles from "styles/components.css";

export function App() {
  const currentSelection = useSelection("plaintext");
  const isElementSelected = currentSelection.count > 0;

  async function readText() {
    if (!isElementSelected) {
      return;
    }

    const draft = await currentSelection.read();

    for (const content of draft.contents) {
      console.log(content.text);
    }
  }

  return (
    <div className={styles.scrollContainer}>
      <Button
        variant="primary"
        disabled={!isElementSelected}
        onClick={readText}
      >
        Read text
      </Button>
    </div>
  );
}