Understanding CAMediaTiming Protocol and Hierarchical Time Structure in iOS Animations

In iOS development, animation timing can be controlled through the duration , speed , beginTime , and timeOffset properties defined by the CAMediaTiming protocol.

A simple example shows a red square moving from left to right over 1 s with no repeat. After modifying the timing properties, the animation runs twice as fast, starts after a delay, and begins from the centre of the view.

CFTimeInterval currentTime = CACurrentMediaTime();
CFTimeInterval currentTimeInLayer = [self.testLayer convertTime:currentTime fromLayer:nil];
CFTimeInterval addTime = currentTimeInLayer;
anim.beginTime = 0.3 + addTime;
[anim setTimeOffset:0.5];
[anim setSpeed:2];

The CAMediaTiming protocol builds a hierarchical time system that coordinates the timing of every CALayer and CAAnimation object. Each object that conforms to the protocol owns a “time space”. Time spaces are nested: a sub‑layer’s time space is a child of its parent layer’s time space.

To determine the time in a sub‑layer, three steps are required, analogous to determining a layer’s screen position:

Obtain the parent layer’s time (window layer time).

Combine the parent time with the child layer’s beginTime and offset to compute the child’s time.

Apply the child’s beginTime , offset , and speed to obtain the final local time.

The conversion from parent time to active local time follows the formula:

t = (tp - begin) * speed + offset

Active local time is used to handle the relationship between a child’s time line and its parent’s time line, as well as the relative speed of time flow.

After the active local time is calculated, a second conversion produces the basic local time, which accounts for repeat behavior and optional auto‑reversing. For example, if an animation has a duration of 1 s, an active local time of 5.5 s, and a repeatCount of 10, the basic local time becomes 0.5 s, meaning the animation is displayed halfway through its current repeat.

Key properties involved in these conversions are:

beginTime : the time offset at which a child time space starts relative to its parent.

speed : the multiplier that determines how fast the child’s time advances compared to the parent.

timeOffset : an additional offset added to the child’s local time.

Understanding these concepts enables a variety of practical techniques:

Pause an animation by setting its layer’s speed to 0.

Create a door‑opening‑and‑closing effect by enabling autoreverses .

Stagger multiple animations on the same layer by assigning different beginTime values.

Manually scrub an animation’s progress by setting speed to 0 and adjusting timeOffset .

These capabilities are provided by Apple’s Core Animation framework; the only limit is the developer’s imagination.

References:

Controlling Animation Timing – http://ronnqvi.st/controlling-animation-timing/

Time Warp in Animation – http://wangling.me/2011/06/time-warp-in-animation.html#fn-1