Implement Clipboard hasStrings method on iOS

Author: justinmc

shell/platform/darwin/ios/framework/Source/FlutterPlatformPlugin.mm

@@ -88,6 +88,8 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
   } else if ([method isEqualToString:@"Clipboard.setData"]) {
     [self setClipboardData:args];
     result(nil);
+  } else if ([method isEqualToString:@"Clipboard.hasStrings"]) {
+    result([self clipboardHasStrings]);
   } else {
     result(FlutterMethodNotImplemented);
   }
@@ -248,4 +250,16 @@ - (void)setClipboardData:(NSDictionary*)data {
   }
 }
 
+- (NSDictionary*)clipboardHasStrings {
+  bool hasStrings = false;
+  UIPasteboard* pasteboard = [UIPasteboard generalPasteboard];
+  if (@available(iOS 10, *)) {
+    hasStrings = pasteboard.hasStrings;
+  } else {
+    NSString* stringInPasteboard = pasteboard.string;
+    hasStrings = stringInPasteboard != nil;
+  }
+  return @{@"value" : @(hasStrings)};
+}
+
 @end