Add isIntialized to Messages.

- Add isInitialized Message protocol
- Provide default isInitialized so the generator can emit a custom one as needed.
- Add isInitialized to ExtensionFields so Message can call to it.
- Add Internal.swift as a place to hold public code (because the generated code
  has to be able to call it), but that is not intended to be part of the
  "developer public" api.
  - Helpers for checking a map and repeated field for being initialized.
- Generate isInitialized - Walk the message fields to decided if
  directly/indirectly can have messages with required fields.

Re apple#98

Sources/SwiftProtobuf/ExtensionFieldValueSet.swift

@@ -71,4 +71,13 @@ public struct ExtensionFieldValueSet: Equatable, Sequence {
     }
     return nil
   }
+
+  public var isInitialized: Bool {
+    for (_, v) in values {
+      if !v.isInitialized {
+        return false
+      }
+    }
+    return true
+  }
 }

Sources/SwiftProtobuf/ExtensionFields.swift

@@ -36,6 +36,15 @@ public protocol AnyExtensionField: CustomDebugStringConvertible {
 
   /// Fields know their own type, so can dispatch to a visitor
   func traverse(visitor: Visitor) throws
+
+  /// Check if the field is initialized.
+  var isInitialized: Bool { get }
+}
+
+public extension AnyExtensionField {
+  // Default implementation for extensions fields.  The message types below provide
+  // custom versions.
+  var isInitialized: Bool { return true }
 }
 
 ///
@@ -247,6 +256,13 @@ public struct OptionalMessageExtensionField<M: Message & Equatable>:
         value: v, fieldNumber: protobufExtension.protoFieldNumber)
     }
   }
+
+  public var isInitialized: Bool {
+    if let v = value {
+      return v.isInitialized
+    }
+    return true
+  }
 }
 
 public struct RepeatedMessageExtensionField<M: Message & Equatable>:
@@ -294,6 +310,10 @@ public struct RepeatedMessageExtensionField<M: Message & Equatable>:
         value: value, fieldNumber: protobufExtension.protoFieldNumber)
     }
   }
+
+  public var isInitialized: Bool {
+    return Internal.areAllInitialized(value)
+  }
 }
 
 //
@@ -337,6 +357,13 @@ public struct OptionalGroupExtensionField<G: Message & Hashable>:
         value: v, fieldNumber: protobufExtension.protoFieldNumber)
     }
   }
+
+  public var isInitialized: Bool {
+    if let v = value {
+      return v.isInitialized
+    }
+    return true
+  }
 }
 
 public struct RepeatedGroupExtensionField<G: Message & Hashable>:
@@ -384,4 +411,8 @@ public struct RepeatedGroupExtensionField<G: Message & Hashable>:
         value: value, fieldNumber: protobufExtension.protoFieldNumber)
     }
   }
+
+  public var isInitialized: Bool {
+    return Internal.areAllInitialized(value)
+  }
 }

Sources/SwiftProtobuf/Internal.swift

@@ -0,0 +1,42 @@
+// Sources/SwiftProtobuf/Message.swift - Message support
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See LICENSE.txt for license information:
+// https://github.com/apple/swift-protobuf/blob/master/LICENSE.txt
+//
+// -----------------------------------------------------------------------------
+///
+/// Internal helpers on Messages for the library. These are public
+/// just so the generated code can call them, but shouldn't be called
+/// by developers directly.
+///
+// -----------------------------------------------------------------------------
+
+
+import Foundation
+import Swift
+
+public struct Internal {
+  private init() {}
+
+  public static func areAllInitialized(_ listOfMessages: [Message]) -> Bool {
+    for msg in listOfMessages {
+      if !msg.isInitialized {
+        return false
+      }
+    }
+    return true
+  }
+
+  public static func areAllInitialized<K: Hashable>(_ mapToMessages: [K: Message]) -> Bool {
+    for (_, msg) in mapToMessages {
+      if !msg.isInitialized {
+        return false
+      }
+    }
+    return true
+  }
+
+}

Sources/SwiftProtobuf/Message.swift

@@ -41,6 +41,10 @@ public protocol Message: CustomDebugStringConvertible {
   var anyTypePrefix: String { get }
   var anyTypeURL: String { get }
 
+  /// Check if all required fields (if any) have values set on this message
+  /// on any messages withing this message.
+  var isInitialized: Bool { get }
+
   //
   // General serialization/deserialization machinery
   //
@@ -116,6 +120,12 @@ public protocol Message: CustomDebugStringConvertible {
 }
 
 public extension Message {
+
+  var isInitialized: Bool {
+    // The generated code will include a specialization as needed.
+    return true;
+  }
+
   var hashValue: Int {
     let visitor = HashVisitor()
     try? traverse(visitor: visitor)

Sources/protoc-gen-swift/MessageFieldGenerator.swift

@@ -287,6 +287,30 @@ struct MessageFieldGenerator {
         (descriptor.options.hasPacked ? descriptor.options.packed : isProto3)}
     var isRepeated: Bool {return descriptor.isRepeated}
 
+    // True/False if the field will be hold a Message. If the field is a map,
+    // the value type for the map is checked.
+    var fieldHoldsMessage: Bool {
+        switch descriptor.label {
+        case .required, .optional:
+            let type = descriptor.type
+            return type == .message || type == .group
+        case .repeated:
+            let type = descriptor.type
+            if type == .group { return true }
+            if type == .message {
+                let m = context.getMessageForPath(path: descriptor.typeName)!
+                if m.options.mapEntry {
+                    let valueField = m.field[1]
+                    return valueField.type == .message
+                } else {
+                    return true
+                }
+            } else {
+                return false
+            }
+        }
+    }
+
     var name: String {return descriptor.name}
 
     var swiftBaseType: String {return descriptor.getSwiftBaseType(context: context)}