Photo_view: Callbacks

[renancaraujo]

Right now PhotoView is very short on callbacks. I'm sure that with the exposition of the internal state through the controller, most of the internal events will be accessible. But I'm sure that some callbacks would make things easier. React native's implementation of PhotoView has a lot of them.
So I want to hear from you which callbacks you would like to be included in PhotoView's options?

[renancaraujo]

I will start with some I have on mind:

• onDoubleTap
  When user double-taps PhotoView, before subsequent scale state changes

• onScalestart
  When a scale gesture or animation starts

• onscalefinish
  when a scale gesture or animation finishes

[ElTopo]

Please also consider adding

• onPanStart
  When user starts dragging to move the photo (usually by tap-hold-move)

• onPanFinish
  When user finishes dragging to move the photo

and

• onScaling
  When user has changd the scale of the photo (and continues changing)

• onPanning
  When user has moved the photo (and continues moving)

Thanks.

[pxc0215]

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