Gutenberg: Get All Attributes From «core/image» Block

As an answer to @RiddleMeThis, here is a working solution by parsing the default output from the core/image-Block with DOMDocument:

In the init-Hook, register your custom output function:

register_block_type( 'core/image', [
    'render_callback' => 'myImageOutput'
] );

Define your custom output function:

In this function render the default output ($content) which is passed as the second argument. Sadly, the first argument (being the block’s $attributes) lacks the required information (except the attachment’s ID and the link’s destination).

function myImageOutput( $attributes, $content ) {
    // $content contains e.g.
    // <!-- wp:image {"id":123,"linkDestination":"custom"} -->
    // <figure class="wp-block-image"><a href="https://www.example.com"><img src="path/to/my/image.jpg" alt="Alternative text here" class="wp-image-123"/></a><figcaption>Caption goes here</figcaption></figure>
    // <!-- /wp:image -->

    // prepare array for all info. Note: alignment and customized 
    // size are ignored here since it was not required in this case
    $info = [
        'title' => '',
        'imagUrl' => '',
        'blank' => FALSE,
        'url' => '',
        'caption' => '',
    ];

    // Fortunately, the attachment id is saved in $attributes, so 
    // we can get the image's url
    $infos[ 'imageUrl' ] = wp_get_attachment_image_src( $attributes[ 'id' ], 'your-size' )[ 0 ];

    // we get the remaining info by loading the html via DOMDocument
    libxml_use_internal_errors( TRUE );
    $dom = new DOMDocument();
    $dom->preserveWhiteSpace = FALSE;
    $dom->loadHtml( mb_convert_encoding( $content, 'HTML-ENTITIES', 'UTF-8' ) );

    // get the figure element       
    $figure = $dom->getElementsByTagName( 'figure' )[ 0 ];

    // alternatively, get the image title or description etc. 
    // by querying it from the database
    $infos[ 'title' ] = $figure->getElementsByTagName( 'img' )[ 0 ]->getAttribute( 'alt' );

    // if we have a custom url on the image
    if ( isset( $attributes[ 'linkDestination' ] ) && $attributes[ 'linkDestination' ] == 'custom' ) {
        $a = $figure->getElementsByTagName( 'a' )[ 0 ];
        $infos[ 'url' ] = $a->getAttribute( 'href' );
        $infos[ 'blank' ] = strpos( $infos[ 'url' ], get_home_url() ) !== 0;
    }

    // caption, also see https://stackoverflow.com/a/2087136/1107529
    // because the caption can contain html
    $figCaption = $figure->getElementsByTagName( 'figcaption' );
    if ( count( $figCaption ) ) {
        $caption = '';
        foreach ( $figCaption[ 0 ]->childNodes as $child ) {
            $caption .= $dom->saveHTML( $child );
        }
        $infos[ 'caption' ] = $caption;
    }

    // create your custom html output here. In my case, I passed the 
    // info to a vue component
    $html = sprintf( '<my-custom-vue-component :info="%s"></my-custom-vue-component>', 
                esc_attr( json_encode( $info ) ) );

    return $html;
}

This solutions works for me. I am sure, there will be a better way to do this one day, possibly when the gutenberg ecosystem gets more mature. But for the moments, it works without problems.

Hope this helps.