Imports code of the RX library.

src/objective-c/RxLibrary/transformations/GRXMappingWriter.m

+#import "GRXMappingWriter.h"
+
+static id (^kIdentity)(id value) = ^id(id value) {
+  return value;
+};
+
+@interface GRXWriter () <GRXWriteable>
+@end
+
+@implementation GRXMappingWriter {
+  id (^_map)(id value);
+}
+
+- (instancetype)initWithWriter:(id<GRXWriter>)writer {
+  return [self initWithWriter:writer map:nil];
+}
+
+// Designated initializer
+- (instancetype)initWithWriter:(id<GRXWriter>)writer map:(id (^)(id value))map {
+  if ((self = [super initWithWriter:writer])) {
+    _map = map ?: kIdentity;
+  }
+  return self;
+}
+
+// Override
+- (void)didReceiveValue:(id)value {
+  [super didReceiveValue:_map(value)];
+}
+@end