"""Classes for class-based forward declaration."""


class Forward(object):
    """Base class for autocomplete forward declaration."""

    @property
    def type(self):
        """Forward type. Should be implemented in subclasses."""
        raise NotImplementedError("Please use one of my subclasses")

    def to_dict(self):
        """Convert to dictionary which will be rendered as JSON."""
        return {
            "type": self.type
        }


class Field(Forward):
    """Forward field value.

    .. py:attribute:: src

        The name of the form field whose value will be forwarded to a view.

    .. py:attribute:: dst

        The name of the key of the forwarded value from the src field in the
        forwarded dictionary. If this value is ``None``, then the key is
        ``src``.
    """

    type = "field"

    def __init__(self, src, dst=None):
        """Instantiate a forwarded field value."""
        self.src = src
        self.dst = dst

    def to_dict(self):
        """Convert to dictionary which will be rendered as JSON."""
        d = super(Field, self).to_dict()

        d.update(src=self.src)
        if self.dst is not None:
            d.update(dst=self.dst)

        return d


class Const(Forward):
    """Forward arbitrary constant value.

    .. py:attribute:: val

        The value to forward. Must be JSON-serializable.

    .. py:attribute:: dst

        The name of the key of the forwarded value.
    """

    type = "const"

    def __init__(self, val, dst):
        """Instantiate a forwarded constant value."""
        self.val = val
        self.dst = dst

    def to_dict(self):
        """Convert to dictionary which will be rendered as JSON."""
        d = super(Const, self).to_dict()

        d.update(val=self.val)
        d.update(dst=self.dst)

        return d