Add class to track app version.

// FREEBIE

src/Util/AppVersion.h

@@ -0,0 +1,15 @@
+//
+//  AppVersion.h
+//
+//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//
+
+@interface AppVersion : NSObject
+
+@property (nonatomic, readonly) NSString *firstAppVersion;
+@property (nonatomic, readonly) NSString *lastAppVersion;
+@property (nonatomic, readonly) NSString *currentAppVersion;
+
++ (instancetype)instance;
+
+@end

src/Util/AppVersion.m

@@ -0,0 +1,64 @@
+//
+//  AppVersion.m
+//
+//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//
+
+#import "AppVersion.h"
+
+@interface AppVersion ()
+
+@property (nonatomic) NSString *firstAppVersion;
+@property (nonatomic) NSString *lastAppVersion;
+@property (nonatomic) NSString *currentAppVersion;
+
+@end
+
+#pragma mark -
+
+@implementation AppVersion
+
++ (instancetype)instance {
+    static AppVersion *instance = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        instance = [AppVersion new];
+        [instance configure];
+    });
+    return instance;
+}
+
+- (void)configure {
+    self.currentAppVersion = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"CFBundleShortVersionString"];
+    
+    NSString *kNSUserDefaults_FirstAppVersion = @"kNSUserDefaults_FirstAppVersion";
+    NSString *kNSUserDefaults_LastAppVersion = @"kNSUserDefaults_LastVersion";
+    
+    // The version of the app when it was first launched.
+    // nil if the app has never been launched before.
+    self.firstAppVersion = [[NSUserDefaults standardUserDefaults] objectForKey:kNSUserDefaults_FirstAppVersion];
+    // The version of the app the last time it was launched.
+    // nil if the app has never been launched before.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-variable"
+    self.lastAppVersion = [[NSUserDefaults standardUserDefaults] objectForKey:kNSUserDefaults_LastAppVersion];
+#pragma clang diagnostic pop
+    
+    // Ensure the value for the "first launched version".
+    if (!self.firstAppVersion) {
+        self.firstAppVersion = self.currentAppVersion;
+        [[NSUserDefaults standardUserDefaults] setObject:self.currentAppVersion
+                                                  forKey:kNSUserDefaults_FirstAppVersion];
+    }
+    
+    // Update the value for the "most recently launched version".
+    [[NSUserDefaults standardUserDefaults] setObject:self.currentAppVersion
+                                              forKey:kNSUserDefaults_LastAppVersion];
+    [[NSUserDefaults standardUserDefaults] synchronize];
+
+    DDLogInfo(@"firstAppVersion: %@", self.firstAppVersion);
+    DDLogInfo(@"lastAppVersion: %@", self.lastAppVersion);
+    DDLogInfo(@"currentAppVersion: %@", self.currentAppVersion);
+}
+
+@end