Add transform start support

Author: thame

src/Gallery.js

@@ -27,6 +27,7 @@ export default class Gallery extends PureComponent {
         errorComponent: PropTypes.func,
         flatListProps: PropTypes.object,
 				maxScale: PropTypes.number,
+        onTransformStart: PropTypes.func,
     };
 
     static defaultProps = {
@@ -227,7 +228,7 @@ export default class Gallery extends PureComponent {
     }
 
     renderPage (pageData, pageId) {
-        const { onViewTransformed, onTransformGestureReleased, errorComponent, imageComponent, maxScale } = this.props;
+        const { onViewTransformed, onTransformStart, onTransformGestureReleased, errorComponent, imageComponent, maxScale } = this.props;
         return (
             <TransformableImage
               onViewTransformed={((transform) => {
@@ -237,6 +238,9 @@ export default class Gallery extends PureComponent {
                   // need the 'return' here because the return value is checked in ViewTransformer
                   return onTransformGestureReleased && onTransformGestureReleased(transform, pageId);
               })}
+              onTransformStart={((transform) => {
+                  return onTransformStart && onTransformStart(transform, pageId);
+              })}
               ref={((ref) => { this.imageRefs.set(pageId, ref); })}
               key={'innerImage#' + pageId}
 							pageId={pageId}

src/libraries/TransformableImage/index.js

@@ -20,6 +20,7 @@ export default class TransformableImage extends PureComponent {
         enableTranslate: PropTypes.bool,
         onTransformGestureReleased: PropTypes.func,
         onViewTransformed: PropTypes.func,
+        onTransformStart: PropTypes.func,
         imageComponent: PropTypes.func,
         resizeMode: PropTypes.string,
         errorComponent: PropTypes.func,
@@ -145,7 +146,7 @@ export default class TransformableImage extends PureComponent {
 
     render () {
         const { imageDimensions, viewWidth, viewHeight, error, keyAccumulator, imageLoaded } = this.state;
-        const { style, image, imageComponent, resizeMode, enableTransform, enableScale, enableTranslate, onTransformGestureReleased, onViewTransformed, pageId, maxScale } = this.props;
+        const { style, image, imageComponent, resizeMode, enableTransform, enableScale, enableTranslate, onTransformGestureReleased, onViewTransformed, pageId, maxScale, onTransformStart } = this.props;
 
         let contentAspectRatio;
         let width, height; // imageDimensions
@@ -183,6 +184,7 @@ export default class TransformableImage extends PureComponent {
               enableResistance={true}
               onTransformGestureReleased={onTransformGestureReleased}
               onViewTransformed={onViewTransformed}
+              onTransformStart={onTransformStart}
               maxScale={maxScale}
 							pageId={pageId}
               contentAspectRatio={contentAspectRatio}