Smooth pinch with ResumableZoom

[rrebase]

Description

Hey, first of all wanted to say that you've done a fantastic job with this library 🙇‍♂️

Tried the ResumableZoom component and it works well, there's only two places where it "jitters" on iOS device. Did some investigation and here's what I found:

• When pinch is active and panning just a tiny bit then it "stutters". Traced it down to inaccurate focalX & focalY values on RNGH side Pinch gesture focalX & focalY truncated on iOS software-mansion/react-native-gesture-handler#2833
• When releasing a pinch, then it often jumps. Apparently there's a few updates that happen with one touch / finger instead of two. This leads to a significant change in focal point. Possible solution is to add a check here to only handle the update if e.numberOfPointers === 2

[Glazzes]

Hello there 👋, thank you for your kind comments and this awesome contribution, here's what I come up with about the issues you've described.

The Jittering

I tried simulating the problem with the focal point lack of decimal places by using Math.round, floor and ceil and they are responsible for the jittering as you mentioned, for this issue I'm tied to whatever Software Mansion decides to do about it, there's nothing I can do about it, however I can probably come up with a workaround using the pan gesture in the meantime.

Could you please create a pan gesture which uses minPointers property set to two, then print x and y event properties to see if they have decimal places? I currently don't have access to a mac and Iphone, so let me know if they have them.

const pan = Gesture.Pan()
  .minPointers(2)
  .maxPointer(2)
  .onPanUpdate(e => {
    console.log(e.x, e.y);
  });

Jump at the end of the pinch gesture

Observations

You're right about assuming its caused by a single pointer triggering undesired updates, however the gesture triggering such updates is actually the pan gesture, however this activation's behaviour is abnormal because of the conditions that trigger it, from my perspective a couple of onPanChange callback updates appear to outrun onPanEnd callback, I actually fixed a way more severe error related to this one, by mixing some code that should be in onPanEnd to onPanchange, this line right here, I assumed it was because I do not have a decent phone.

What is the root cause of this issue?

All gestures are disabled automatically at the very beginning of onPinchEnd callback, it seems the pan gesture manages to activate as it's getting disabled and at the middle of the snapback animation of ScaleMode.BOUNCE, then it gets clamped right away by the onPanChange.

Solution

Disabling pan gesture at the beginning of pinch gesture so they can not collide with each other when pinch gesture ends, disabling pan gesture makes no effect to "pan with pinch feature" as pan gesture is not responsible for it. following this action seems to fix the jump issue as I have not been able to replicate it anymore.

I'm not able to obtain a single pointer with the pinch gesture, however I will add the pointer check to prevent any strange and undesired issues.

I'll work on it tomorrow morning to ensure it works properly this time.

[Glazzes]

After many unsuccessful attempts to implement a workaround for this feature I could not make it work.

• Using a pan based approach with two pointers outputs the same focal point a pinch gesture does, however its behaviour is not reliable at all, produces sudden jumps and jittering.
• Using pan gesture's translationX and translationY value result in a displacement of the image that prevents it from being aligned with the user's interaction.
• Manual gestures can probably be an option, I managed to simulate the focal points position, however for the scale value the end result is an image that looks to be a two different places at the same time.

Best I can do at this point is to make panWithPinch an opt in feature for iOS users and warn them in the documentation until a release of Gesture Handler with your fix comes out.

You can test again whether the jump issue has gone away with the new version 1.0.2.