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guideClipboard

The clipboard feature (implemented by the Clipboard plugin) is responsible for the integration with the native clipboard — a feature of the operating system and the browser used when the user copies, cuts, pastes or drags and drops content within the editor or from/to the “outside”.

CKEditor 5 intercepts all native events like copy, cut or drop and handles them on its side. The goal is to not allow the browser to touch the content in the rich text editor which would lead to the browser messing it up.

There are two directions in which the content is processed:

  • When the content is being pasted or dropped into the editor, it goes through the input pipeline.
  • When the content is being copied, cut or dragged from the editor it goes through the output pipeline.

Both pipelines allow the features to process the content to be inserted or set to the clipboard as well as override the default mechanisms at different stages of those processes.

# Input pipeline

When the user pastes or drops content into the editor, the browser fires an event which is intercepted by the clipboard feature and which kickstarts the following mechanism:

  1. ClipboardObserver turns that event into a synthetic view.Document#paste or view.Document#drop.
  2. Since the content to be inserted by both actions (paste and drop) should usually be processed in the same way and both actions have a very simillar effect, both events are turned into a single view.Document#clipboardInput event for easier handling.
  3. Next, the clipboard feature listens to the view.Document#clipboardInput event, retrieves and pre-processes the text/html or text/plain content which it finds in the event’s dataTransfer and fires the Clipboard#inputTransformation event with the retrieved content.
  4. Finally, the clipboard feature listens to the Clipboard#inputTransformation event, takes the processed content and inserts it into the editor.

The clipboard feature listens to the view.Document#clipboardInput and Clipboard#inputTransformation events using low priority listeners. This means that adding a normal listener and calling evt.stop() allows overriding the behavior implemented by the clipboard feature. It is a similar mechanism to DOM’s evt.preventDefault() that lets you override the default browser behavior.

# Handling clipboard input differently

By default the clipboard feature retrieves text/html or text/plain from the clipboard, normalizes that data a bit (e.g. cleans up the mess with whitespaces), converts that to a view DocumentFragment and fires the Clipboard#inputTransformation event with that document fragment for further processing.

The view.Document#clipboardInput event can be used to override this behavior. For example, you can use it to:

  • Handle pasted or droppped files (that you can retrieve from the dataTransfer).

    Handling file upload requires, however, a lot more than reading dataTransfer.files so for a complete code example we recommend checking the source code of plugins like ImageUploadEditing.

  • Change the type of data that the clipboard feature reads from the clipboard. For instance, you may want to use application/rtf if it is present in the dataTransfer (and ignore text/html in that case).

    editor.editing.view.document.on( 'clipboardInput', ( evt, data ) => {
        const dataTransfer = data.dataTransfer;
        const rtfContent = dataTransfer.getData( 'application/rtf' );
    
        // If no RTF was pasted, abort and let the clipboard feature handle the input.
        if ( !rtfContent ) {
            return;
        }
    
        // Convert an RTF raw string to a view document fragment.
        const viewContent = convertRtfStringToView( rtfContent );
    
        // Just like the clipboard feature, trigger the inputTransformation event
        // to allow further processing of the content.
        this.fire( 'inputTransformation', { content: viewContent, dataTransfer } );
    
        editor.editing.view.scrollToTheSelection();
        evt.stop();
    } );
    

# Processing input content

The view.Document#inputTransformation event lets you process the content which is going to be inserted into the editor.

The default action is to insert the content (data.content, represented by a DocumentFragment) to the editor if the data is not empty.

At this stage the pasted content can be processed by the features. For example, a feature that wants to transform the pasted text into a link can be implemented in this way:

const writer = new UpcastWriter();

editor.plugins.get( 'Clipboard' ).on( 'inputTransformation', ( evt, data ) => {
    if ( data.content.childCount == 1 && isUrlText( data.content.getChild( 0 ) ) ) {
        const linkUrl = data.content.getChild( 0 ).data;

        data.content = writer.createDocumentFragment( [
            writer.createElement(
                'a',
                { href: linkUrl },
                [ writer.createText( linkUrl ) ]
            )
        ] );
    }
} );

The default action (inserting the content into the editor) is performed by a low priority listener, so it can be overridden by a normal one. With the lowest priority you can also execute actions after the content was already inserted.

editor.plugins.get( 'Clipboard' ).on( 'inputTransformation', ( evt, data ) => {
    console.log( 'Content was inserted.' );
}, { priority: 'lowest' } );

# Output pipeline

The output pipeline is the equivalent of the input pipeline but for the copy and cut operations.
It allows to process the content that will be then put into the clipboard or to override the whole process.

The default action is to:

  1. Get the selected content from the editor.
  2. Prevent the default action of the native copy or cut event.
  3. Fire view.Document#clipboardOutput with a clone of the selected content converted to a view document fragment.

The default action is to put the content (data.content, represented by a DocumentFragment) to the clipboard as HTML. In case of the cut operation, the selected content is also deleted from the editor.

This action is performed by a low priority listener, so it can be overridden by a normal one.

At this stage the copied or cut content can be processed by other features.