Add applying method and its generator

Author: Pouya Yarandi

Sources/SwiftProtobuf/ProtobufMirror.swift

@@ -132,3 +132,12 @@ public extension ProtobufMirror {
   }
 
 }
+
+public enum MessageModificationError: Error {
+  case typeMismatch
+  case invalidFieldNumber
+}
+
+public protocol MessageModifier: Message {
+  func applying(_ value: Any, for fieldNumber: Int) throws -> Self
+}

Sources/protoc-gen-swift/FieldGenerator.swift

@@ -56,6 +56,8 @@ protocol FieldGenerator {
   /// Generate any support needed to this field's value is initialized.
   /// The generated code should return false if it isn't set.
   func generateIsInitializedCheck(printer: inout CodePrinter)
+
+  func generateApplying(printer: inout CodePrinter)
 }
 
 /// Simple base class for FieldGenerators that also provides fieldMapNames.

Sources/protoc-gen-swift/MessageFieldGenerator.swift

@@ -221,4 +221,8 @@ class MessageFieldGenerator: FieldGeneratorBase, FieldGenerator {
         p.printIndented("try visitor.\(visitMethod)(\(traitsArg)value: \(varName), fieldNumber: \(number))")
         p.print("}\(suffix)")
     }
+
+    func generateApplying(printer p: inout CodePrinter) {
+        p.print("case \(number): copy.\(swiftName) = try valueAs(\(swiftType).self)")
+    }
 }

Sources/protoc-gen-swift/MessageGenerator.swift

@@ -196,7 +196,7 @@ class MessageGenerator {
   func generateRuntimeSupport(printer p: inout CodePrinter, file: FileGenerator, parent: MessageGenerator?) {
     p.print(
         "",
-        "extension \(swiftFullName): \(namer.swiftProtobufModulePrefix)Message, \(namer.swiftProtobufModulePrefix)_MessageImplementationBase, \(namer.swiftProtobufModulePrefix)_ProtoNameProviding {")
+        "extension \(swiftFullName): \(namer.swiftProtobufModulePrefix)Message, \(namer.swiftProtobufModulePrefix)_MessageImplementationBase, \(namer.swiftProtobufModulePrefix)_ProtoNameProviding, \(namer.swiftProtobufModulePrefix)MessageModifier {")
     p.withIndentation { p in
       if let parent = parent {
         p.print("\(visibility)static let protoMessageName: String = \(parent.swiftFullName).protoMessageName + \".\(descriptor.name)\"")
@@ -220,6 +220,8 @@ class MessageGenerator {
       generateTraverse(printer: &p)
       p.print()
       generateMessageEquality(printer: &p)
+      p.print()
+      generateMessageApplying(printer: &p)
     }
     p.print("}")
 
@@ -402,6 +404,29 @@ class MessageGenerator {
     p.print("}")
   }
 
+  private func generateMessageApplying(printer p: inout CodePrinter) {
+    p.print("\(visibility)func applying(_ value: Any, for fieldNumber: Int) throws -> \(swiftFullName) {")
+    p.withIndentation { p in
+      p.print("""
+      func valueAs<T>(_ type: T.Type) throws -> T {
+        guard let _value = value as? T else {
+          throw \(namer.swiftProtobufModulePrefix)MessageModificationError.typeMismatch
+        }
+        return _value
+      }
+      """)
+      p.print("var copy = self")
+      p.print("switch fieldNumber {")
+      for f in fields {
+        f.generateApplying(printer: &p)
+      }
+      p.print("default: throw \(namer.swiftProtobufModulePrefix)MessageModificationError.invalidFieldNumber")
+      p.print("}")
+      p.print("return copy")
+    }
+    p.print("}")
+  }
+
   /// Generates the `isInitialized` property for the message, if needed.
   ///
   /// This may generate nothing, if the `isInitialized` property is not

Sources/protoc-gen-swift/OneofGenerator.swift

@@ -104,6 +104,10 @@ class OneofGenerator {
         func generateTraverse(printer p: inout CodePrinter) {
             oneof.generateTraverse(printer: &p, field: self)
         }
+
+        func generateApplying(printer p: inout CodePrinter) {
+            oneof.generateApplying(printer: &p, field: self)
+        }
     }
 
     private let oneofDescriptor: OneofDescriptor
@@ -419,4 +423,14 @@ class OneofGenerator {
 
         p.print("if let v = \(storedProperty), !v.isInitialized {return false}")
     }
+
+    func generateApplying(printer p: inout CodePrinter, field: MemberFieldGenerator) {
+        // First field causes the output.
+        let group = fieldSortedGrouped[field.group]
+        guard field === fields.first else { return }
+
+        for f in group {
+            p.print("case \(f.number): copy.\(f.swiftName) = try valueAs(\(f.swiftType).self)")
+        }
+    }
 }