UIVisualEffectView blur radius manipulation (new for iOS 9)

gistfile1.m

// iOS 9 allows you to animate between visual effects, which gives you the
// ability to manipulate the blur radius. this can be useful for animating
// a backdrop for a custom modal, and with a few tricks, can even be set
// indirectly, allowing you to "scrub" between them back and forth with
// a gesture, just like when you pull down Spotlight.

// these are the two effects you want to transition between
UIVisualEffect *startEffect = nil; // "nil" means no blur/tint/vibrancy (plain, fully-transparent view)
UIVisualEffect *endEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];

UIVisualEffectView *blurView = [[UIVisualEffectView alloc] initWithEffect:startEffect];
blurView.userInteractionEnabled = NO;
[self.view addSubview:blurView];

// transistion between the effects
[UIView animateWithDuration:1.0 animations:^{
    blurView.effect = endEffect;
}];

///////////////////////////////////////////

// or, "freeze" time so it doesn't actually animate...
blurView.layer.speed = 0;

[UIView animateWithDuration:1.0 animations:^{
    blurView.effect = endEffect;
}];

// ...then set timeOffset to desired "frozen in time" point of transition, from 0 to 1
blurView.layer.timeOffset = 0.5; // halfway blurred