translationAddedTo specification
This is the engineering specification for the MotionObservable operator translationAddedTo that operates
on TranslationGestureRecognizer value types.
Overview
Adds the current drag gesture translation to the provided initial position and emits the result.
Example usage:
gesture.translation(addedTo: propertyOf(target).center, in: view)
MVP
Expose translationAddedTo API
This API should only be available for streams emitting TranslationGestureRecognizer values. It should
accept an initialPosition readable property and an element within which the translations should be
calculated.
extension MotionObservable where T: TranslationGestureRecognizer {
func translation(addedTo initialPosition: MotionObservable<Point>, in element: Element) -> MotionObservable<Point>
}
Cache the initial position on gesture activation
Update the cache on gesture began or gesture changed if no cache currently exists.
func translation(addedTo initialPosition: MotionObservable<Point>, in element: Element) -> MotionObservable<Point> {
var cachedInitialPosition: Point?
return _nextOperator { value, next in
if value.state == .began || (value.state == .changed && cachedInitialPosition == nil) {
cachedInitialPosition = clone(initialPosition._read())
}
...
}
}
Blow away the cache when the gesture recognizer is inactive
func translation(addedTo initialPosition: MotionObservable<Point>, in element: Element) -> MotionObservable<Point> {
...
return _nextOperator { value, next in
if value.state == .began || (value.state == .changed && cachedInitialPosition == nil) {
...
} else if value.state != .began && value.state != .changed {
cachedInitialPosition = nil
}
...
}
}
Emit the translationAddedTo initial position
Emit the sum of the cached initial position and the gesture’s translation.
func translation(addedTo initialPosition: MotionObservable<Point>, in element: Element) -> MotionObservable<Point> {
...
return _nextOperator { value, next in
...
if let cachedInitialPosition = cachedInitialPosition {
let translation = value.translation(in: view)
next(Point(x: cachedInitialPosition.x + translation.x,
y: cachedInitialPosition.y + translation.y))
}
}
}