Implement setNativeSelectedIndex command for Picker component

Summary:
This is required so we can switch to using commands instead of `setNativeProps`.
`setNativeProps` are not supported in Fabric, we are moving away from it.

Reviewed By: JoshuaGross

Differential Revision: D17764967

fbshipit-source-id: 16e54ebe1f0c58b80a6491db970a60c01fec8a15

React/Views/RCTPickerManager.m

@@ -10,6 +10,7 @@
 #import "RCTBridge.h"
 #import "RCTPicker.h"
 #import "RCTFont.h"
+#import <React/RCTUIManager.h>
 
 @implementation RCTPickerManager
 
@@ -42,4 +43,22 @@ - (UIView *)view
   view.font = [RCTFont updateFont:view.font withFamily:json ?: defaultView.font.familyName];
 }
 
+RCT_EXPORT_METHOD(setNativeSelectedIndex : (nonnull NSNumber *)viewTag toIndex : (NSNumber *)index)
+{
+  [self.bridge.uiManager addUIBlock:^(RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
+    UIView *view = viewRegistry[viewTag];
+
+    if ([view isKindOfClass:[RCTPicker class]]) {
+      [(RCTPicker *)view setSelectedIndex:index.integerValue];
+    } else {
+      UIView *subview = view.subviews.firstObject;
+      if ([subview isKindOfClass:[RCTPicker class]]) {
+        [(RCTPicker *)subview setSelectedIndex:index.integerValue];
+      } else {
+        RCTLogError(@"view type must be RCTPicker");
+      }
+    }
+  }];
+}
+
 @end