Water animation using Particle System - http://iosrecipe.blogspot.com/2015/07/water-fall-animation-using-particle.html
Water falling is a cool piece of animation in iOS apps. I had to use particle system to create an animation where water hose is spraying water. Think of the real water hose.
- You have a continuous flow of water which are collection of water drops.
- The shape in which water falls (you may have seen the settings on the hose - shower, mist, full etc).
The above two concepts translate exactly to the concepts of emitter cell and an emitter layer.
There is one more thing:
In the real world, water will always fall down because of gravity. But in programming world water direction need to be controlled. This is controlled by a property called emissionLongitude.