feat: Add a new headerPosition prop (#59)

Added a new headerPosition prop that returns the reanimated node to keep track of the current location of the header (in dp). The reason to add this is in reference to issue #54 where I needed to find the current location to implement the said UI functionality.

Author: itsyogesh

README.md

@@ -67,6 +67,7 @@ class Example extends React.Component {
 | enabledInnerScrolling     | no       | `true`  | Defines whether it's possible to scroll inner content of bottom sheet. |
 | callbackNode              | no       |         | `reanimated` node which holds position of bottom sheet, where `0` it the highest snap point and `1` is the lowest. |
 | contentPosition           | no       |         | `reanimated` node which holds position of bottom sheet's content (in dp) |
+| headerPosition           | no       |         | `reanimated` node which holds position of bottom sheet's header (in dp) |
 | overdragResistanceFactor  | no       |   0     | `Defines how violently sheet has to stopped while overdragging. 0 means no overdrag |
 | springConfig              | no       | `{ }`   | Overrides config for spring animation |
 | innerGestureHandlerRefs   | no       |         | Refs for gesture handlers used for building bottom sheet. The array consists fo three refs. The first for PanGH used for inner content scrolling. The second for PanGH used for header. The third for TapGH used for stopping scrolling the content.   |

src/index.tsx

@@ -54,6 +54,11 @@ type Props = {
    */
   contentPosition?: Animated.Value<number>
 
+  /**
+   * Reanimated node which holds position of bottom sheet's header (in dp).
+   */
+  headerPosition?: Animated.Value<number>
+
   /**
    * Defines how violently sheet has to stopped while overdragging. 0 means no overdrag. Defaults to 0.
    */
@@ -777,6 +782,14 @@ export default class BottomSheetBehavior extends React.Component<Props, State> {
                 )}
               />
             )}
+            {this.props.headerPosition && (
+              <Animated.Code
+                exec={onChange(
+                  this.translateMaster,
+                  set(this.props.headerPosition, this.translateMaster)
+                )}
+              />
+            )}
           </View>
         </Animated.View>
       </React.Fragment>