nullability annotations for TSGroupModel

- add readonly where possible
- group members needn't be a mutable array

// FREEBIE

Signal/src/ViewControllers/DebugUI/DebugUIStress.m

@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
 //
 
 #import "DebugUIStress.h"
@@ -479,7 +479,7 @@ NS_ASSUME_NONNULL_BEGIN
         readWriteWithBlock:^(YapDatabaseReadWriteTransaction *_Nonnull transaction) {
             TSGroupModel *groupModel =
                 [[TSGroupModel alloc] initWithTitle:[groupThread.groupModel.groupName stringByAppendingString:@" Copy"]
-                                          memberIds:[groupThread.groupModel.groupMemberIds mutableCopy]
+                                          memberIds:groupThread.groupModel.groupMemberIds
                                               image:groupThread.groupModel.groupImage
                                             groupId:[SecurityUtils generateRandomBytes:16]];
             thread = [TSGroupThread getOrCreateThreadWithGroupModel:groupModel transaction:transaction];

Signal/src/ViewControllers/UpdateGroupViewController.m

@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
 //
 
 #import "UpdateGroupViewController.h"
@@ -375,7 +375,7 @@ NS_ASSUME_NONNULL_BEGIN
 
     NSString *groupName = [self.groupNameTextField.text ows_stripped];
     TSGroupModel *groupModel = [[TSGroupModel alloc] initWithTitle:groupName
-                                                         memberIds:[self.memberRecipientIds.allObjects mutableCopy]
+                                                         memberIds:self.memberRecipientIds.allObjects
                                                              image:self.groupAvatar
                                                            groupId:self.thread.groupModel.groupId];
     [self.conversationSettingsViewDelegate groupWasUpdated:groupModel];

SignalServiceKit/src/Contacts/Threads/TSGroupThread.m

@@ -44,9 +44,7 @@ NS_ASSUME_NONNULL_BEGIN
     OWSAssert(localNumber.length > 0);
 
     TSGroupModel *groupModel = [[TSGroupModel alloc] initWithTitle:nil
-                                                         memberIds:[@[
-                                                             localNumber,
-                                                         ] mutableCopy]
+                                                         memberIds:@[ localNumber ]
                                                              image:nil
                                                            groupId:groupId];
 

SignalServiceKit/src/Messages/OWSMessageManager.m

@@ -922,7 +922,7 @@ NS_ASSUME_NONNULL_BEGIN
                     [TSGroupThread getOrCreateThreadWithGroupId:groupId transaction:transaction];
 
                 TSGroupModel *newGroupModel = [[TSGroupModel alloc] initWithTitle:dataMessage.group.name
-                                                                        memberIds:[newMemberIds.allObjects mutableCopy]
+                                                                        memberIds:newMemberIds.allObjects
                                                                             image:oldGroupThread.groupModel.groupImage
                                                                           groupId:dataMessage.group.id];
                 NSString *updateGroupInfo = [newGroupThread.groupModel getInfoStringAboutUpdateTo:newGroupModel

SignalServiceKit/src/Messages/TSGroupModel.h

@@ -1,22 +1,24 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
 //
 
 #import "TSYapDatabaseObject.h"
 #import "ContactsManagerProtocol.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface TSGroupModel : TSYapDatabaseObject
 
-@property (nonatomic, strong) NSArray<NSString *> *groupMemberIds;
-@property (nonatomic, strong) NSString *groupName;
-@property (nonatomic, strong) NSData *groupId;
+@property (nonatomic) NSArray<NSString *> *groupMemberIds;
+@property (nullable, readonly, nonatomic) NSString *groupName;
+@property (readonly, nonatomic) NSData *groupId;
 
 #if TARGET_OS_IOS
-@property (nonatomic, strong) UIImage *groupImage;
+@property (nullable, nonatomic, strong) UIImage *groupImage;
 
-- (instancetype)initWithTitle:(NSString *)title
-                    memberIds:(NSMutableArray<NSString *> *)memberIds
-                        image:(UIImage *)image
+- (instancetype)initWithTitle:(nullable NSString *)title
+                    memberIds:(NSArray<NSString *> *)memberIds
+                        image:(nullable UIImage *)image
                       groupId:(NSData *)groupId;
 
 - (BOOL)isEqual:(id)other;
@@ -25,3 +27,5 @@
 #endif
 
 @end
+
+NS_ASSUME_NONNULL_END

SignalServiceKit/src/Messages/TSGroupModel.m

@@ -1,16 +1,18 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
 //
 
 #import "TSGroupModel.h"
 #import "FunctionalUtil.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 @implementation TSGroupModel
 
 #if TARGET_OS_IOS
-- (instancetype)initWithTitle:(NSString *)title
+- (instancetype)initWithTitle:(nullable NSString *)title
                     memberIds:(NSArray<NSString *> *)memberIds
-                        image:(UIImage *)image
+                        image:(nullable UIImage *)image
                       groupId:(NSData *)groupId
 {
     _groupName              = title;
@@ -106,3 +108,5 @@
 #endif
 
 @end
+
+NS_ASSUME_NONNULL_END