[ios] Adds a CommandDispatcher to proxy method calls to UI handlers.

ios/clean/chrome/browser/ui/commands/command_dispatcher.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IOS_CLEAN_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_
+#define IOS_CLEAN_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_
+
+#import <Foundation/Foundation.h>
+
+// CommandDispatcher allows coordinators to register as command handlers for
+// specific selectors.  Other objects can call these methods on the dispatcher,
+// which in turn will forward the call to the registered handler.
+@interface CommandDispatcher : NSObject

[marq (ping after 24h), 2017/03/16 13:54:46]

Doesn't CommandDispatcher need to publicly conform to all of the command
protocols?

[lpromero, 2017/03/21 15:15:32]

This is discussed upthread.

+
+// Registers the given |target| to receive forwarded messages for the given
+// |selector|.
+- (void)registerTarget:(id)target forSelector:(SEL)selector;

[lpromero, 2017/03/10 16:17:36]

The comment doesn't specify that you can't have several targets for the same
selector. Actually, what happens when that case arises?

+
+// Removes all forwarding registrations for the given |target|.
+- (void)stopDispatchingForTarget:(id)target;

[lpromero, 2017/03/09 16:16:30]

s/…/unregisterTarget: ?

[rohitrao (ping after 24h), 2017/03/10 14:44:31]

I went through a number of iterations:
unregisterTarget
deregisterTarget
stopForwardingForTarget

Eventually I settled on stopDispatchingForTarget because it sounded the least
odd.  I can change the name if no one else likes it =)

I've also been a bit inconsistent about "target" vs "handler".  Do you have a
preference for one vs the other?

[lpromero, 2017/03/10 16:17:36]

Handler might convey more that fact that we can only have one, no?

[marq (ping after 24h), 2017/03/16 13:54:46]

I like -stopDispatching:, so much that I'd like to see the other method become
dispatchToTarget: forSelector:

[lpromero, 2017/03/21 15:15:32]

https://codereview.chromium.org/2766663002/

+
+@end
+
+#endif  // IOS_CLEAN_CHROME_BROWSER_UI_COMMANDS_COMMAND_DISPATCHER_H_