Replacing elements

How to replace elements in a design.

Apps can detect the selection of certain types of elements and then replace the content of those elements. This unlocks a range of powerful features, such as image effects and text manipulation.

Check out the Selection guidelines

Our design guidelines help you create a high-quality app that easily passes app review.

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In the Developer Portal, enable the following permissions:

  • canva:design:content:read
  • canva:design:content:write
  • canva:asset:private:read
  • canva:asset:private:write

In the future, the Apps SDK will throw an error if the required permissions are not enabled.

To learn more, see Configuring permissions.

Use the useSelection hook or register a callback with the selection.registerOnChange method:

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("image");
const isElementSelected = currentSelection.count > 0;
async function replaceImage() {
if (!isElementSelected) {
return;
}
const draft = await currentSelection.read();
console.log(draft.contents); // => [{ ref: "..." }]
}
return (
<div className={styles.scrollContainer}>
<Button disabled={!isElementSelected} onClick={replaceImage} stretch>
Replace image
</Button>
</div>
);
}
tsx

The selection event has a read method that returns an array of the selected images. Each image is represented as an object with a ref property. The ref contains a unique identifier that points to an asset in Canva's backend:

const draft = await currentSelection.read();
console.log(draft.contents); // => [{ ref: "..." }]
ts

To learn more, see Reading elements.

To access the ref property of each image, loop through the selected images:

const draft = await currentSelection.read();
for (const content of draft.contents) {
console.log(content.ref); // => "..."
}
ts

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

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

Once the app has access to the URL of one or more images:

  1. Download the images.
  2. Transform the images.
  3. Upload the transformed images to Canva's backend.

In some cases, this process can take place entirely via the frontend. In other cases, such as when integrating with AI models, it makes more sense to handle the transformation via the app's backend.

To transform images via the app's frontend:

  1. Download each image.
  2. Draw each image into an HTMLCanvasElement.
  3. Apply some sort of transformation to each image.
  4. Get the data URL of the transformed images.

The following code sample contains a reusable function that handles this logic for you:

import { getTemporaryUrl, ImageMimeType, ImageRef } from "@canva/asset";
/**
* Downloads and transforms a raster image.
* @param ref - A unique identifier that points to an image asset in Canva's backend.
* @param transformer - A function that transforms the image.
* @returns The data URL and MIME type of the transformed image.
*/
async function transformRasterImage(
ref: ImageRef,
transformer: (ctx: CanvasRenderingContext2D, imageData: ImageData) => void
): Promise<{ dataUrl: string; mimeType: ImageMimeType }> {
// Get a temporary URL for the asset
const { url } = await getTemporaryUrl({
type: "IMAGE",
ref,
});
// Download the image
const response = await fetch(url, { mode: "cors" });
const imageBlob = await response.blob();
// Extract MIME type from the downloaded image
const mimeType = imageBlob.type;
if (!isSupportedMimeType(mimeType)) {
throw new Error(`Unsupported mime type: ${mimeType}`);
}
// Create an object URL for the image
const objectURL = URL.createObjectURL(imageBlob);
// Define an image element and load image from the object URL
const image = new Image();
image.crossOrigin = "Anonymous";
await new Promise((resolve, reject) => {
image.onload = resolve;
image.onerror = () => reject(new Error("Image could not be loaded"));
image.src = objectURL;
});
// Create a canvas and draw the image onto it
const canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
const ctx = canvas.getContext("2d");
if (!ctx) {
throw new Error("CanvasRenderingContext2D is not available");
}
ctx.drawImage(image, 0, 0);
// Get the image data from the canvas to manipulate pixels
const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
transformer(ctx, imageData);
// Put the transformed image data back onto the canvas
ctx.putImageData(imageData, 0, 0);
// Clean up: Revoke the object URL to free up memory
URL.revokeObjectURL(objectURL);
// Convert the canvas content to a data URL with the original MIME type
const dataUrl = canvas.toDataURL(mimeType);
return { dataUrl, mimeType };
}
function isSupportedMimeType(
input: string
): input is "image/jpeg" | "image/heic" | "image/png" | "image/webp" {
// This does not include "image/svg+xml"
const mimeTypes = ["image/jpeg", "image/heic", "image/png", "image/webp"];
return mimeTypes.includes(input);
}
ts

To use the transformRasterImage, function, pass an image reference in as the first argument and a function for transforming the image as the second argument. The following usage inverts the colors of an image:

const { dataUrl, mimeType } = await transformRasterImage(
content.ref,
(_, { data }) => {
// Invert the colors of each pixel
for (let i = 0; i < data.length; i += 4) {
data[i] = 255 - data[i];
data[i + 1] = 255 - data[i + 1];
data[i + 2] = 255 - data[i + 2];
}
}
);
console.log("The data URL of the transformed image is:", dataUrl);
ts

Once the app's frontend has a URL or data URL for the new image, upload the image to Canva's backend.

To upload the image:

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

    import { getTemporaryUrl, upload } from "@canva/asset";
    ts
  2. Call the method, passing in the required properties:

    // Upload the replaced image
    const uploadedImage = await upload({
    type: "IMAGE",
    id: "uniqueIdGoesHere",
    mimeType: "image/png",
    url: "URL OR DATA URL GOES HERE",
    thumbnailUrl: "URL OR DATA URL GOES HERE",
    parentRef: content.ref,
    });
    ts

    To learn more about the required properties, see upload.

    The upload method accepts a parentRef property. This property doesn't have a visible impact on the app, but it must contain the ref of the originally selected image.

    Here's why:

    Canva licenses assets from a number of creators. By setting the parentRef property, Canva can keep track of the original asset and ensure that any licensing requirements are met.

    A side-effect of this requirement is that apps are not allowed to combine multiple assets into a single asset. This is because the tracking mechanism doesn't account for assets derived from multiple assets.

In the loop, replace the current ref of the image with the new ref:

for (const content of draft.contents) {
// Get a temporary URL for the asset
// Download and transform the image
// Upload the transformed image
// ...
// Replace the image
content.ref = uploadedImage.ref;
}
ts

By default, any changes to the content are not reflected in the user's design. To persist the changes and update the user's design, call the save method that's returned by the read method:

await draft.save();
ts

This method should be called after the loop.

Loading…

In the Developer Portal, enable the following permissions:

  • canva:design:content:read
  • canva:design:content:write

In the future, the Apps SDK will throw an error if the required permissions are not enabled.

To learn more, see Configuring permissions.

Use the useSelection hook or register a callback with the selection.registerOnChange method:

import React from "react";
import { useSelection } from "utils/use_selection_hook";
import { Button } from "@canva/app-ui-kit";
import styles from "styles/components.css";
export function App() {
const currentSelection = useSelection("plaintext");
const isElementSelected = currentSelection.count > 0;
async function replaceText() {
if (!isElementSelected) {
return;
}
const draft = await currentSelection.read();
console.log(draft.contents); // => [{ text: "Some text" }]
}
return (
<div className={styles.scrollContainer}>
<Button disabled={!isElementSelected} onClick={replaceText} stretch>
Replace text
</Button>
</div>
);
}
tsx

The selection event has a read method for accessing the content of each element. In the case of text elements, the plain text of each element is available via the text property:

const draft = await currentSelection.read();
console.log(draft.contents); // => [{ text: "Some text" }]
ts

To learn more, see Reading elements.

Loop through the contents of the selected elements and replace the text property with a new value:

const draft = await currentSelection.read();
for (const content of draft.contents) {
content.text = `${content.text} was modified!`;
}
ts

By default, any changes to the content are not reflected in the user's design. To persist the changes and update the user's design, call the save method that's returned by the read method:

await draft.save();
ts

This method should be called after the loop.

  • You can't replace one type of element with a different type of element.
  • If multiple elements are selected, the ordering of the elements is not stable and should not be relied upon.
  • If something is selected when selection.registerOnChange is called, the callback fires immediately.
import React from "react";
import { Button } from "@canva/app-ui-kit";
import { getTemporaryUrl, upload, ImageMimeType, ImageRef } from "@canva/asset";
import { useSelection } from "utils/use_selection_hook";
import styles from "styles/components.css";
export function App() {
const currentSelection = useSelection("image");
const isElementSelected = currentSelection.count > 0;
async function replaceImage() {
if (!isElementSelected) {
return;
}
const draft = await currentSelection.read();
for (const content of draft.contents) {
// Download and transform the image
const newImage = await transformRasterImage(
content.ref,
(_, { data }) => {
for (let i = 0; i < data.length; i += 4) {
data[i] = 255 - data[i];
data[i + 1] = 255 - data[i + 1];
data[i + 2] = 255 - data[i + 2];
}
}
);
// Upload the transformed image
const uploadedImage = await upload({
type: "IMAGE",
id: createRandomId(), // The ID must be a unique, alphanumeric string
url: newImage.dataUrl,
mimeType: newImage.mimeType,
thumbnailUrl: newImage.dataUrl,
parentRef: content.ref,
});
// Replace the image
content.ref = uploadedImage.ref;
}
await draft.save();
}
return (
<div className={styles.scrollContainer}>
<Button disabled={!isElementSelected} onClick={replaceImage} stretch>
Replace image
</Button>
</div>
);
}
/**
* Downloads and transforms a raster image.
* @param ref - A unique identifier that points to an image asset in Canva's backend.
* @param transformer - A function that transforms the image.
* @returns The data URL and MIME type of the transformed image.
*/
async function transformRasterImage(
ref: ImageRef,
transformer: (ctx: CanvasRenderingContext2D, imageData: ImageData) => void
): Promise<{ dataUrl: string; mimeType: ImageMimeType }> {
// Get a temporary URL for the asset
const { url } = await getTemporaryUrl({
type: "IMAGE",
ref,
});
// Download the image
const response = await fetch(url, { mode: "cors" });
const imageBlob = await response.blob();
// Extract MIME type from the downloaded image
const mimeType = imageBlob.type;
if (!isSupportedMimeType(mimeType)) {
throw new Error(`Unsupported mime type: ${mimeType}`);
}
// Create an object URL for the image
const objectURL = URL.createObjectURL(imageBlob);
// Define an image element and load image from the object URL
const image = new Image();
image.crossOrigin = "Anonymous";
await new Promise((resolve, reject) => {
image.onload = resolve;
image.onerror = () => reject(new Error("Image could not be loaded"));
image.src = objectURL;
});
// Create a canvas and draw the image onto it
const canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
const ctx = canvas.getContext("2d");
if (!ctx) {
throw new Error("CanvasRenderingContext2D is not available");
}
ctx.drawImage(image, 0, 0);
// Get the image data from the canvas to manipulate pixels
const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
transformer(ctx, imageData);
// Put the transformed image data back onto the canvas
ctx.putImageData(imageData, 0, 0);
// Clean up: Revoke the object URL to free up memory
URL.revokeObjectURL(objectURL);
// Convert the canvas content to a data URL with the original MIME type
const dataUrl = canvas.toDataURL(mimeType);
return { dataUrl, mimeType };
}
function isSupportedMimeType(
input: string
): input is "image/jpeg" | "image/heic" | "image/png" | "image/webp" {
// This does not include "image/svg+xml"
const mimeTypes = ["image/jpeg", "image/heic", "image/png", "image/webp"];
return mimeTypes.includes(input);
}
function createRandomId(length: number = 10): string {
return Array.from({ length }, () => Math.random().toString(36)[2]).join("");
}
tsx
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 replaceText() {
if (!isElementSelected) {
return;
}
const draft = await currentSelection.read();
for (const content of draft.contents) {
content.text = `${content.text} was modified!`;
}
await draft.save();
}
return (
<div className={styles.scrollContainer}>
<Button disabled={!isElementSelected} onClick={replaceText} stretch>
Replace text
</Button>
</div>
);
}
tsx