Add documentation to iOS plugin registry (flutter#3665)

shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h

@@ -14,40 +14,169 @@
 NS_ASSUME_NONNULL_BEGIN
 @protocol FlutterPluginRegistrar;
 
+/**
+ Implemented by the iOS part of a Flutter plugin.
+
+ Defines a set of optional callback methods and a method to set up the plugin
+ and register it to be called by other application components.
+ */
 @protocol FlutterPlugin<NSObject>
 @required
+/**
+ Registers this plugin.
+
+ - Parameters:
+   - registrar: A helper providing application context and methods for
+     registering callbacks
+ */
 + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar;
 @optional
+/**
+ Called if this plugin has been registered to receive `FlutterMethodCall`s.
+
+ - Parameters:
+   - call: The method call command object.
+   - result: A callback for submitting the result of the call.
+ */
 - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+
+ - Returns: `NO` if this plugin vetoes application launch.
+ */
 - (BOOL)application:(UIApplication*)application
     didFinishLaunchingWithOptions:(NSDictionary*)launchOptions;
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)applicationDidBecomeActive:(UIApplication*)application;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)applicationWillResignActive:(UIApplication*)application;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)applicationDidEnterBackground:(UIApplication*)application;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)applicationWillEnterForeground:(UIApplication*)application;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)applicationWillTerminate:(UIApplication*)application;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)application:(UIApplication*)application
     didRegisterUserNotificationSettings:(UIUserNotificationSettings*)notificationSettings;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+ */
 - (void)application:(UIApplication*)application
     didRegisterForRemoteNotificationsWithDeviceToken:(NSData*)deviceToken;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+
+ - Returns: `YES` if this plugin handles request.
+ */
 - (BOOL)application:(UIApplication*)application
     didReceiveRemoteNotification:(NSDictionary*)userInfo
           fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler;
+
+/**
+ Called if this plugin has been registered for `UIApplicationDelegate` callbacks.
+
+ - Returns: `YES` if this plugin handles request.
+ */
 - (BOOL)application:(UIApplication*)application
       handleOpenURL:(NSURL*)url;
 @end
 
+/**
+ Registration context for a single `FlutterPlugin`.
+ */
 @protocol FlutterPluginRegistrar<NSObject>
+/**
+ Returns a `FlutterBinaryMessenger` for creating Dart/iOS communication
+ channels to be used by the plugin.
+
+ - Returns: The messenger.
+ */
 - (NSObject<FlutterBinaryMessenger>*)messenger;
+
+/**
+ Publishes a value for external use of the plugin.
+
+ Plugins may publish a single value, such as an instance of the
+ plugin's main class, for situations where external control or
+ interaction is needed.
+
+ The published value will be available from the `FlutterPluginRegistry`.
+ Repeated calls overwrite any previous publication.
+
+ - Parameter value: The value to be published.
+ */
 - (void)publish:(NSObject*)value;
+
+/**
+ Registers the plugin as a receiver of incoming method calls from the Dart side
+ on the specified `FlutterMethodChannel`.
+
+ - Parameters:
+   - delegate: The receiving object, such as the plugin's main class.
+   - channel: The channel
+ */
 - (void)addMethodCallDelegate:(NSObject<FlutterPlugin>*)delegate
                       channel:(FlutterMethodChannel*)channel;
+
+/**
+ Registers the plugin as a receiver of `UIApplicationDelegate` calls.
+
+ - Parameters delegate: The receiving object, such as the plugin's main class.
+ */
 - (void)addApplicationDelegate:(NSObject<FlutterPlugin>*)delegate;
 @end
 
+/**
+ A registry of Flutter iOS plugins.
+
+ Plugins are identified by unique string keys. It is strongly advised to use
+ inverted domain names such as `com.mycompany.myproject.MyPlugin` as keys
+ to avoid clashes.
+ */
 @protocol FlutterPluginRegistry<NSObject>
+/**
+ Returns a registrar for registering a plugin.
+
+ - Parameter pluginKey: The unique key identifying the plugin.
+ */
 - (NSObject<FlutterPluginRegistrar>*)registrarForPlugin:(NSString*)pluginKey;
+/**
+ Returns whether the specified plugin has been registered.
+
+ - Parameter pluginKey: The unique key identifying the plugin.
+ - Returns: `YES` if `registrarForPlugin` has been called with `pluginKey`.
+ */
 - (BOOL)hasPlugin:(NSString*)pluginKey;
+
+/**
+ Returns a value published by the specified plugin.
+
+ - Parameter pluginKey: The unique key identifying the plugin.
+ - Returns: An object published by the plugin, if any. Will be `NSNull` if
+   nothing has been published. Will be `nil` if the plugin has not been
+   registered.
+ */
 - (NSObject*)valuePublishedByPlugin:(NSString*)pluginKey;
 @end