GenericArrayObject.php

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<?php

abstract class GenericArrayObject extends ArrayObject {
    abstract public function getObjectType();

    protected $indexOffset = 0;

    protected function getNewOffset() {
        while ( $this->offsetExists( $this->indexOffset ) ) {
            $this->indexOffset++;
        }

        return $this->indexOffset;
    }

    public function __construct( $input = null, $flags = 0, $iterator_class = 'ArrayIterator' ) {
        parent::__construct( [], $flags, $iterator_class );

        if ( !is_null( $input ) ) {
            foreach ( $input as $offset => $value ) {
                $this->offsetSet( $offset, $value );
            }
        }
    }

    public function append( $value ) {
        $this->setElement( null, $value );
    }

    public function offsetSet( $index, $value ) {
        $this->setElement( $index, $value );
    }

    protected function hasValidType( $value ) {
        $class = $this->getObjectType();
        return $value instanceof $class;
    }

    protected function setElement( $index, $value ) {
        if ( !$this->hasValidType( $value ) ) {
            throw new InvalidArgumentException(
                'Can only add ' . $this->getObjectType() . ' implementing objects to '
                . static::class . '.'
            );
        }

        if ( is_null( $index ) ) {
            $index = $this->getNewOffset();
        }

        if ( $this->preSetElement( $index, $value ) ) {
            parent::offsetSet( $index, $value );
        }
    }

    protected function preSetElement( $index, $value ) {
        return true;
    }

    public function serialize() {
        return serialize( $this->getSerializationData() );
    }

    protected function getSerializationData() {
        return [
            'data' => $this->getArrayCopy(),
            'index' => $this->indexOffset,
        ];
    }

    public function unserialize( $serialization ) {
        $serializationData = unserialize( $serialization );

        foreach ( $serializationData['data'] as $offset => $value ) {
            // Just set the element, bypassing checks and offset resolving,
            // as these elements have already gone through this.
            parent::offsetSet( $offset, $value );
        }

        $this->indexOffset = $serializationData['index'];

        return $serializationData;
    }

    public function isEmpty() {
        return $this->count() === 0;
    }
}