Integrating NSUserDefaults plugin to iOS (#291)

Summary:
I have a few details left, but its almost done. This PR addresses #145

- The NSUserDefaults plugin uses the SharedPreferences Desktop Part since we can reuse all of it.
- The NSUserDefaults plugin uses swizzling in order to be notified of what specific event changed at runtime.
- Added Test harness in both Sample Swift and Sample apps for iOS in order to test the plugin.
- Updated the documentation in `docs/shared-preferences-plugin.md` and` README.md`

I am open to suggestions since the desktop sharedPreferences version doesn't support deletion of preferences. Most likely I would have to modify the UI, and for that matter, I might as well build a user defaults desktop version

I wanted to add xiphirx in this MR since he developed the shared preferences plugin for Android and Desktop. I don't see a way to remove preferences from the flipper desktop app so I was wondering if you would be OK with me adding that.
Pull Request resolved: https://github.com/facebook/flipper/pull/291

Reviewed By: passy

Differential Revision: D10334685

Pulled By: priteshrnandgaonkar

fbshipit-source-id: d798c01a46df7ddecf713924799f046b560ea922

iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.m

@@ -0,0 +1,80 @@
+//
+//  FKUserDefaultsPlugin.m
+//  Sample
+//
+//  Created by Marc Terns on 9/30/18.
+//  Copyright © 2018 Facebook. All rights reserved.
+//
+
+#import "FKUserDefaultsPlugin.h"
+#import <FlipperKit/FlipperConnection.h>
+#import <FlipperKit/FlipperResponder.h>
+#import "FKUserDefaultsSwizzleUtility.h"
+
+@interface FKUserDefaultsPlugin ()
+@property (nonatomic, strong) id<FlipperConnection> flipperConnection;
+@property (nonatomic, strong) NSUserDefaults *userDefaults;
+@property (nonatomic, copy) NSString *key;
+@property (nonatomic, copy) NSString *suiteName;
+@end
+
+@implementation FKUserDefaultsPlugin
+
+- (instancetype)initWithSuiteName:(NSString *)suiteName {
+    if (self = [super init]) {
+        _userDefaults = [NSUserDefaults standardUserDefaults];
+        _suiteName = suiteName;
+        __weak typeof(self) weakSelf = self;
+        [FKUserDefaultsSwizzleUtility swizzleSelector:@selector(setObject:forKey:) class:[NSUserDefaults class] block:^(NSInvocation * _Nonnull invocation) {
+            __unsafe_unretained id firstArg = nil;
+            __unsafe_unretained id secondArg = nil;
+            [invocation getArgument:&firstArg atIndex:2];
+            [invocation getArgument:&secondArg atIndex:3];
+            [invocation invoke];
+            [weakSelf userDefaultsChangedWithValue:firstArg key:secondArg];
+        }];
+    }
+    return self;
+        
+}
+
+- (void)didConnect:(id<FlipperConnection>)connection {
+    self.flipperConnection = connection;
+    [connection receive:@"getSharedPreferences" withBlock:^(NSDictionary *params, id<FlipperResponder> responder) {
+        [responder success:[self.userDefaults dictionaryRepresentation]];
+    }];
+    
+    [connection receive:@"setSharedPreference" withBlock:^(NSDictionary *params , id<FlipperResponder> responder) {
+        NSString *preferenceName = params[@"preferenceName"];
+        [self.userDefaults setObject:params[@"preferenceValue"] forKey:preferenceName];
+        [responder success:[self.userDefaults dictionaryRepresentation]];
+    }];
+}
+
+- (void)didDisconnect {
+    self.flipperConnection = nil;
+}
+
+- (NSString *)identifier {
+    return @"Preferences";
+}
+
+#pragma mark - Private methods
+
+- (void)userDefaultsChangedWithValue:(id)value key:(NSString *)key {
+    NSTimeInterval interval = [[NSDate date] timeIntervalSince1970] * 1000;
+    NSString *intervalStr = [NSString stringWithFormat:@"%f", interval];
+    NSMutableDictionary *params = [@{@"name":key,
+                                    @"time":intervalStr
+                                    } mutableCopy];
+    
+    if (!value) {
+        [params setObject:@"YES" forKey:@"deleted"];
+    } else {
+        [params setObject:value forKey:@"value"];
+    }
+    
+    [self.flipperConnection send:@"sharedPreferencesChange" withParams:[params copy]];
+}
+
+@end