Centered scroll view with zoomin

[typoman]

Hi all! I just figured out how to do zooming while keeping the view centered in the scroll view demo example. I don't know where the clip view should be implemented, but if you want I can send a PR for the new centered zoomable scroll view. Here is the code:

from AppKit import NSView, NSColor, NSRectFill, NSScrollView, NSClipView
from vanilla import Window, ScrollView

class CenteredClipView(NSClipView):
    """
    Thanks to:
    https://stackoverflow.com/a/28154936/4100686
    """

    def constrainBoundsRect_(self, proposedBounds):
        rect = super(CenteredClipView, self).constrainBoundsRect_(proposedBounds)
        containerViewRect = self.documentView().frame()
        if (rect.size.width > containerViewRect.size.width):
            rect.origin.x = (containerViewRect.size.width - rect.size.width) / 2
        if(rect.size.height > containerViewRect.size.height):
            rect.origin.y = (containerViewRect.size.height - rect.size.height) / 2
        return rect

class DemoView(NSView):

    def drawRect_(self, rect):
        NSColor.redColor().set()
        NSRectFill(self.bounds())

class ScrollViewDemo:

    def __init__(self):
        self.w = Window((400, 400), minSize=(100, 100))
        self.view = DemoView.alloc().init()
        self.view.setFrame_(((0, 0), (300, 300)))
        self.w.scrollView = ScrollView((10, 10, -10, -10), self.view, clipView=CenteredClipView.alloc().init())
        self.w.scrollView._nsObject.setAllowsMagnification_(True)
        self.w.open()

ScrollViewDemo()