Extending blocks PHP API

[nk-o]

Currently, we have an API for extending blocks using JS. It is ok. But we also need API on the backend to extend dynamic blocks.

For example, I have the plugin, that extends all core blocks and adds custom attribute and class in output using this filter blocks.getSaveContent.extraProps. Everything works fine with static blocks, but not with dynamic (for ex. Latest Posts block).

function addSaveProps( extraProps ) {
    extraProps.className = ( extraProps.className || '' ) + ' test-class';
    extraProps['data-test-attribure'] = 'test-attribute';
    return extraProps;
}

addFilter( 'blocks.getSaveContent.extraProps', 'test/extra-props', addSaveProps );

Tried to change attributes argument in blocks.getSaveElement filter, but also no luck.

Plus my plugin adds additional attributes to the blocks using filter blocks.registerBlockType:

function addAttribute( blockSettings ) {

    blockSettings.attributes.testAdditionalClass = {
        type: 'string',
        default: '',
    };

    return blockSettings;
}
addFilter( 'blocks.registerBlockType', 'test/additional-attributes', addAttribute );

p.s. all code example simplified just for demonstration. Full extensions code here - https://github.com/nk-o/ghostkit/blob/master/src/blocks/_extend/styles.jsx#L138-L9999

[mtias]

#8352

[mtias]

Consolidating with #6494.