I am playing with TodayExtension in iOS 8 and I wondered how to apply that blur effect to the Text or to buttons. I already figured out that it has something to do with UIVi
Updated Answer for iOS 10
In iOS 10, you can use widgetPrimaryVibrancyEffect and widgetSecondaryVibrancyEffect to automatically return a UIVibrancyEffect object.
Check out the documentation here and here.
Answer for iOS 9
Use this code to apply a vibrancy effect to your whole widget:
UIVisualEffectView *effectView = [[UIVisualEffectView alloc] initWithEffect:[UIVibrancyEffect notificationCenterVibrancyEffect]];
effectView.frame = self.view.bounds
effectView.autoresizingMask = self.view.autoresizingMask;
__strong UIView *oldView = self.view;
self.view = effectView;
[effectView.contentView addSubview:oldView];
self.view.tintColor = [UIColor clearColor];