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guideCustom image upload adapter

In this guide you will learn the basic concepts of the file upload architecture in CKEditor 5 WYSIWYG editor which will help you implement your own custom upload adapter.

While this guide is mainly focused on the image upload (the most common kind of upload), keep in mind that the presented concepts and the API allow developing all sorts of file upload adapters for different file types like PDFs, movies, etc.

Check out the comprehensive Image upload overview to learn about other ways to upload images into CKEditor 5.

# Glossary of terms

Before we start, let’s make sure all terms used in this guide are clear.

Term Description
Upload adapter

A piece of code (a class) that handles the image upload from the moment it is requested by the user (e.g. when the file is dropped into the content) to the moment the server returns a response to the requested upload. A bridge between the feature and the server.

Upload adapters are used by other plugins like image upload to connect to the server and fetch the response. For every user action (e.g. when a file is dropped into the content), a new upload adapter instance is created.

CKEditor 5 comes with some official upload adapters but you can also implement your own adapters.

See the "How does the image upload work?" section to learn more.

UploadAdapter interface

An interface defining the minimal API required to create an upload adapter. In other words, it tells you what methods your upload adapter class must have in order to work.

See "The anatomy of the adapter" section to learn more.

File repository plugin

A central point for managing file upload in CKEditor 5. It glues upload adapters and features using them:

  • Upload adapters are enabled in the editor by defining the FileRepository.createUploadAdapter() factory method.
  • Features like image upload use the FileRepository API to create a new upload adapter instance each time an upload is requested by the user.
Image upload plugin

A top–level plugin that responds to actions of the users (e.g. when a file is dropped into the content) by uploading files to the server and updating the edited content once the upload finishes. This particular plugin handles user actions related to uploading images.

It uses the FileRepository API to spawn upload adapter instances, triggers the image upload (UploadAdapter.upload()) and finally uses the data returned by the adapter's upload promise to update the image in the editor content.

See the "How does the image upload work?" section to learn more.

# How does the image upload work?

Before you can implement your own custom upload adapter, you should learn about the image upload process in CKEditor 5. The whole process boils down to the following steps:

  1. First, an image (or images) need to get into the rich-text editor content. There are many ways to do that, for instance:

    • pasting an image from clipboard,
    • dragging a file from the file system,
    • selecting an image through a file system dialog.

    The images are intercepted by the image upload plugin (which is enabled in all official editor builds).

  2. For every image, the image upload plugin creates an instance of a file loader.

    • The role of the file loader is to read the file from the disk and upload it to the server by using the upload adapter.
    • The role of the upload adapter is, therefore, to securely send the file to the server and pass the response from the server (e.g. the URL to the saved file) back to the file loader (or handle an error, if there was one).
  3. While the images are being uploaded, the image upload plugin:

    • Creates placeholders of these images.
    • Inserts them into the editor.
    • Displays the progress bar for each of them.
    • When an image is deleted from the editor content before the upload finishes, it aborts the upload process.
  4. Once the file is uploaded, the upload adapter notifies the editor about this fact by resolving its Promise. It passes the URL (or URLs in case of responsive images) to the image upload plugin which replaces the src and srcset attributes of the image placeholder in the editor content.

This is just an overview of the image upload process. Actually, the whole thing is more complicated. For instance, images can be copied and pasted within the WYSIWYG editor (while the upload takes place) and all potential upload errors must be handled, too. The good news is these tasks are handled transparently by the image upload plugin so you do not have to worry about them.

To sum up, for the image upload to work in the rich-text editor, two conditions must be true:

# The anatomy of the adapter

A custom upload adapter allows you to take the full control over the process of sending the files to the server as well as passing the response from the server back to the rich-text editor.

Any upload adapter, whether an image upload adapter or a generic file upload adapter, must implement the UploadAdapter interface in order to work, i.e. it must bring its own upload() and abort() methods.

  • The upload() method must return a promise:
    • resolved by a successful upload with an object containing information about the uploaded file (see the section about responsive images to learn more),
    • rejected because of an error, in which case nothing is inserted into the content.
  • The abort() method must allow the editor to abort the upload process. It is necessary, for instance, when the image was removed from the content by the user before the upload finished or the editor instance was destroyed.

In its simplest form, a custom adapter implementing the UploadAdapter interface will look as follows. Note that server.upload(), server.onUploadProgress() and server.abortUpload() should be replaced by specific implementations (dedicated for your application) and only demonstrate the minimal communication necessary for the upload to work:

class MyUploadAdapter {
    constructor( loader ) {
        // The file loader instance to use during the upload.
        this.loader = loader;
    }

    // Starts the upload process.
    upload() {
        // Update the loader's progress.
        server.onUploadProgress( data => {
            loader.uploadTotal = data.total;
            loader.uploaded = data.uploaded;
        } );

        // Return a promise that will be resolved when the file is uploaded.
        return loader.file
            .then( file => server.upload( file ) );
    }

    // Aborts the upload process.
    abort() {
        // Reject the promise returned from the upload() method.
        server.abortUpload();
    }
}

Define the FileRepository.createUploadAdapter() factory method which uses the MyUploadAdapter class to enable the upload adapter in the editor:

editor.plugins.get( 'FileRepository' ).createUploadAdapter = ( loader ) => {
    return new MyUploadAdapter( loader );
};

# Implementing a custom upload adapter

In this section, you are going to implement and enable a custom upload adapter. The adapter will use the native XMLHttpRequest to send files returned by the loader to a pre–configured URL on the server, handling the error, abort, load, and progress events fired by the request.

Note that this is just an example implementation and XMLHttpRequest might not necessarily be the best solution for your application.

Use the provided code snippets as an inspiration for your own custom upload adapter — it is up to you to choose the technologies and APIs to use. For instance, you may want to check out the native fetch() API that works with Promises out of the box.

If you do not want to read through this entire guide, you can simply go and check out the full source code of an XMLHttpRequest–based upload adapter that you can use in your application.

To start off, define the MyUploadAdapter class with its constructor.

class MyUploadAdapter {
    constructor( loader ) {
        // The file loader instance to use during the upload. It sounds scary but do not
        // worry — the loader will be passed into the adapter later on in this guide.
        this.loader = loader;
    }

    // ...
}

Implement the minimal UploadAdapter adapter interface as explained in “The anatomy of the adapter” section. The details of the implementation are explained in the following chapters of this guide.

class MyUploadAdapter {
    // ...

    // Starts the upload process.
    upload() {
        return this.loader.file
            .then( file => new Promise( ( resolve, reject ) => {
                this._initRequest();
                this._initListeners( resolve, reject, file );
                this._sendRequest( file );
            } ) );
    }

    // Aborts the upload process.
    abort() {
        if ( this.xhr ) {
            this.xhr.abort();
        }
    }

    // ...
}

# Using XMLHttpRequest in an adapter

Let’s see what the _initRequest() method looks like in your custom upload adapter. It should prepare the XMLHttpRequest object before it can be used to upload an image.

Note that for the sake of keeping the code simple, in this example implementation no particular security mechanism is used that would prevent your application and services from being abused.

We strongly recommend using both authentication and CSRF protection mechanisms (i.e. CSRF tokens) in your application. For instance, they can be implemented as XMLHttpRequest headers.

class MyUploadAdapter {
    // ...

    // Initializes the XMLHttpRequest object using the URL passed to the constructor.
    _initRequest() {
        const xhr = this.xhr = new XMLHttpRequest();

        // Note that your request may look different. It is up to you and your editor
        // integration to choose the right communication channel. This example uses
        // a POST request with JSON as a data structure but your configuration
        // could be different.
        xhr.open( 'POST', 'http://example.com/image/upload/path', true );
        xhr.responseType = 'json';
    }
}

Now let’s focus on the _initListeners() method which attaches the error, abort, load, and progress event listeners to the XMLHttpRequest object created in the last step.

A successful image upload will finish when the upload promise is resolved upon the load event fired by the XMLHttpRequest request. The promise must be resolved with an object containing information about the image. See the documentation of the upload() method to learn more.

class MyUploadAdapter {
    // ...

    // Initializes XMLHttpRequest listeners.
    _initListeners( resolve, reject, file ) {
        const xhr = this.xhr;
        const loader = this.loader;
        const genericErrorText = `Couldn't upload file: ${ file.name }.`;

        xhr.addEventListener( 'error', () => reject( genericErrorText ) );
        xhr.addEventListener( 'abort', () => reject() );
        xhr.addEventListener( 'load', () => {
            const response = xhr.response;

            // This example assumes the XHR server's "response" object will come with
            // an "error" which has its own "message" that can be passed to reject()
            // in the upload promise.
            //
            // Your integration may handle upload errors in a different way so make sure
            // it is done properly. The reject() function must be called when the upload fails.
            if ( !response || response.error ) {
                return reject( response && response.error ? response.error.message : genericErrorText );
            }

            // If the upload is successful, resolve the upload promise with an object containing
            // at least the "default" URL, pointing to the image on the server.
            // This URL will be used to display the image in the content. Learn more in the
            // UploadAdapter#upload documentation.
            resolve( {
                default: response.url
            } );
        } );

        // Upload progress when it is supported. The file loader has the #uploadTotal and #uploaded
        // properties which are used e.g. to display the upload progress bar in the editor
        // user interface.
        if ( xhr.upload ) {
            xhr.upload.addEventListener( 'progress', evt => {
                if ( evt.lengthComputable ) {
                    loader.uploadTotal = evt.total;
                    loader.uploaded = evt.loaded;
                }
            } );
        }
    }
}

Last but not least, the _sendRequest() method sends the XMLHttpRequest. In this example, the FormData interface is used to pass the file provided by the file loader.

Note that both the data format and actual data passed to XMLHttpRequest.send() in this example implementation are arbitrary. Your implementation could be different and it will depend on the back–end of your application and interfaces it provides.

class MyUploadAdapter {
    // ...

    // Prepares the data and sends the request.
    _sendRequest( file ) {
        // Prepare the form data.
        const data = new FormData();

        data.append( 'upload', file );

        // Important note: This is the right place to implement security mechanisms
        // like authentication and CSRF protection. For instance, you can use
        // XMLHttpRequest.setRequestHeader() to set the request headers containing
        // the CSRF token generated earlier by your application.

        // Send the request.
        this.xhr.send( data );
    }
}

# Responsive images and srcset attribute

If the upload is successful, a Promise returned by the MyUploadAdapter.upload() method can resolve with more than just a default path to the uploaded image (see the implementation of MyUploadAdapter._initListeners()), which usually looks like this:

{
    default: 'http://example.com/images/image–default-size.png'
}

Other image sizes can also be provided in the response, allowing responsive images in the editor. The response containing more than just one image size could look like this:

{
    default: 'http://example.com/images/image–default-size.png',
    '160': 'http://example.com/images/image–size-160.image.png',
    '500': 'http://example.com/images/image–size-500.image.png',
    '1000': 'http://example.com/images/image–size-1000.image.png',
    '1052': 'http://example.com/images/image–default-size.png'
}

When returning multiple images, the widest returned one should be the default one. It is essential to correctly set the width attribute of the image in the rich-text editor content.

The image upload plugin, which is capable of handling multiple image sizes returned by the upload adapter, will automatically add the URLs to other images sizes to the srcset attribute of the image in the content.

The Easy Image feature provides responsive image support out of the box.

Knowing that, you can implement the XMLHttpRequest#load listener that resolves the upload promise in the previous section so that it passes the entire urls property of the server response to the image upload plugin:

// ...

xhr.addEventListener( 'load', () => {
    const response = xhr.response;

    // ...

    // response.urls = {
    // 	default: 'http://example.com/images/image–default-size.png',
    // 	'160': '...',
    // 	'500': '...',
    // 	// ...
    // 	'1052': 'http://example.com/images/image–default-size.png'
    // }
    resolve( response.urls );
} );

# Activating a custom upload adapter

Having implemented the adapter, you must figure out how to enable it in the WYSIWYG editor. The good news is that it is pretty easy, and you do not need to rebuild the editor to do that!

You are going to extend the basic implementation presented in “The anatomy of the adapter” section of this guide so your custom adapter becomes an editor plugin. To do that, create a simple standalone plugin (MyCustomUploadAdapterPlugin) that will create an instance of the file loader and glue it with your custom MyUploadAdapter.

import ClassicEditor from '@ckeditor/ckeditor5-build-classic';

class MyUploadAdapter {
    // ...
}

function MyCustomUploadAdapterPlugin( editor ) {
    editor.plugins.get( 'FileRepository' ).createUploadAdapter = ( loader ) => {
        // Configure the URL to the upload script in your back-end here!
        return new MyUploadAdapter( loader );
    };
}

Enable the MyCustomUploadAdapterPlugin in the editor by using the config.extraPlugins option:

ClassicEditor
    .create( document.querySelector( '#editor' ), {
        extraPlugins: [ MyCustomUploadAdapterPlugin ],

        // ...
    } )
    .catch( error => {
        console.log( error );
    } );

Run the editor and see if your implementation works. Drop an image into the WYSIWYG editor content and it should be uploaded to the server thanks to the MyUploadAdapter.

# The complete implementation

Here is what the complete implementation of an XMLHttpRequest–based upload adapter looks like. You can use this code as a foundation to build custom upload adapters for your applications.

class MyUploadAdapter {
    constructor( loader ) {
        // The file loader instance to use during the upload.
        this.loader = loader;
    }

    // Starts the upload process.
    upload() {
        return this.loader.file
            .then( file => new Promise( ( resolve, reject ) => {
                this._initRequest();
                this._initListeners( resolve, reject, file );
                this._sendRequest( file );
            } ) );
    }

    // Aborts the upload process.
    abort() {
        if ( this.xhr ) {
            this.xhr.abort();
        }
    }

    // Initializes the XMLHttpRequest object using the URL passed to the constructor.
    _initRequest() {
        const xhr = this.xhr = new XMLHttpRequest();

        // Note that your request may look different. It is up to you and your editor
        // integration to choose the right communication channel. This example uses
        // a POST request with JSON as a data structure but your configuration
        // could be different.
        xhr.open( 'POST', 'http://example.com/image/upload/path', true );
        xhr.responseType = 'json';
    }

    // Initializes XMLHttpRequest listeners.
    _initListeners( resolve, reject, file ) {
        const xhr = this.xhr;
        const loader = this.loader;
        const genericErrorText = `Couldn't upload file: ${ file.name }.`;

        xhr.addEventListener( 'error', () => reject( genericErrorText ) );
        xhr.addEventListener( 'abort', () => reject() );
        xhr.addEventListener( 'load', () => {
            const response = xhr.response;

            // This example assumes the XHR server's "response" object will come with
            // an "error" which has its own "message" that can be passed to reject()
            // in the upload promise.
            //
            // Your integration may handle upload errors in a different way so make sure
            // it is done properly. The reject() function must be called when the upload fails.
            if ( !response || response.error ) {
                return reject( response && response.error ? response.error.message : genericErrorText );
            }

            // If the upload is successful, resolve the upload promise with an object containing
            // at least the "default" URL, pointing to the image on the server.
            // This URL will be used to display the image in the content. Learn more in the
            // UploadAdapter#upload documentation.
            resolve( {
                default: response.url
            } );
        } );

        // Upload progress when it is supported. The file loader has the #uploadTotal and #uploaded
        // properties which are used e.g. to display the upload progress bar in the editor
        // user interface.
        if ( xhr.upload ) {
            xhr.upload.addEventListener( 'progress', evt => {
                if ( evt.lengthComputable ) {
                    loader.uploadTotal = evt.total;
                    loader.uploaded = evt.loaded;
                }
            } );
        }
    }

    // Prepares the data and sends the request.
    _sendRequest( file ) {
        // Prepare the form data.
        const data = new FormData();

        data.append( 'upload', file );

        // Important note: This is the right place to implement security mechanisms
        // like authentication and CSRF protection. For instance, you can use
        // XMLHttpRequest.setRequestHeader() to set the request headers containing
        // the CSRF token generated earlier by your application.

        // Send the request.
        this.xhr.send( data );
    }
}

// ...

function MyCustomUploadAdapterPlugin( editor ) {
    editor.plugins.get( 'FileRepository' ).createUploadAdapter = ( loader ) => {
        // Configure the URL to the upload script in your back-end here!
        return new MyUploadAdapter( loader );
    };
}

// ...

ClassicEditor
    .create( document.querySelector( '#editor' ), {
        extraPlugins: [ MyCustomUploadAdapterPlugin ],

        // ...
    } )
    .catch( error => {
        console.log( error );
    } );

# What’s next?

Check out the comprehensive Image upload overview to learn more about different ways of uploading images in CKEditor 5. See the Image feature guide to find out more about handling images in CKEditor 5.