There are a lot of posts about crossfade of UIImages using an animation. But I can't find how to control the crossfade without animation, but instead by myself; for example with a parameter between 0.0 and 1.0 that controls the amount of crossfade.
I could put two UIImageViews behind each other and control the alpha (top one going from 1.0 to 0.0, and the bottom one at 1.0), but that's not a proper crossfade AFAIK.
To make it more graphic: On screen there's a slider that gives a value between 0.0 and 1.0, above it is an image. What I want is to crossfade between the initially shown image (when slider is 0.0) and another image (which if fully visible when slider is 1.0). So, when you'd slowly move the slider, you'd see the other image slowly appear.
If you use two UIImageViews with clear backgrounds and vary the alpha channels from 0-1, 1-0, you should get very close to the effect you're looking for. I've found it works better if there's a black background behind both of them. It's perhaps not a true crossfade, but comes close enough for most usages.