from ..Qt import QtGui, QtCore
from ..Point import Point


class GraphicsWidgetAnchor:
    """
    Class used to allow GraphicsWidgets to anchor to a specific position on their
    parent. 

    """

    def __init__(self):
        self.__parent = None
        self.__parentAnchor = None
        self.__itemAnchor = None
        self.__offset = (0,0)
        if hasattr(self, 'geometryChanged'):
            self.geometryChanged.connect(self.__geometryChanged)

    def anchor(self, itemPos, parentPos, offset=(0,0)):
        """
        Anchors the item at its local itemPos to the item's parent at parentPos.
        Both positions are expressed in values relative to the size of the item or parent;
        a value of 0 indicates left or top edge, while 1 indicates right or bottom edge.
        
        Optionally, offset may be specified to introduce an absolute offset. 
        
        Example: anchor a box such that its upper-right corner is fixed 10px left
        and 10px down from its parent's upper-right corner::
        
            box.anchor(itemPos=(1,0), parentPos=(1,0), offset=(-10,10))
        """
        parent = self.parentItem()
        if parent is None:
            raise Exception("Cannot anchor; parent is not set.")
        
        if self.__parent is not parent:
            if self.__parent is not None:
                self.__parent.geometryChanged.disconnect(self.__geometryChanged)
                
            self.__parent = parent
            parent.geometryChanged.connect(self.__geometryChanged)
        
        self.__itemAnchor = itemPos
        self.__parentAnchor = parentPos
        self.__offset = offset
        self.__geometryChanged()
        
    def __geometryChanged(self):
        if self.__parent is None:
            return
        if self.__itemAnchor is None:
            return
            
        o = self.mapToParent(Point(0,0))
        a = self.boundingRect().bottomRight() * Point(self.__itemAnchor)
        a = self.mapToParent(a)
        p = self.__parent.boundingRect().bottomRight() * Point(self.__parentAnchor)
        off = Point(self.__offset)
        pos = p + (o-a) + off
        self.setPos(pos)