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

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

+// 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.
+
+#import "ios/clean/chrome/browser/ui/commands/command_dispatcher.h"
+
+#include <unordered_map>
+#include <vector>
+
+#include "base/logging.h"
+#include "base/strings/sys_string_conversions.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation CommandDispatcher {
+  // Stores which target to forward to for a given selector.
+  std::unordered_map<SEL, __weak id> _forwardingTargets;
+}
+
+- (void)registerTarget:(id)target forSelector:(SEL)selector {
+  DCHECK(_forwardingTargets.find(selector) == _forwardingTargets.end());
+
+  _forwardingTargets[selector] = target;
+}
+
+// |-stopDispatchingForTarget| should be called much less often than
+// |-forwardingTargetForSelector|, so removal is intentionally O(n) in order
+// to prioritize the speed of lookups.
+- (void)stopDispatchingForTarget:(id)target {
+  std::vector<SEL> selectorsToErase;
+  for (auto& kv : _forwardingTargets) {
+    if (kv.second == target) {
+      selectorsToErase.push_back(kv.first);
+    }
+  }
+
+  for (auto* selector : selectorsToErase) {
+    _forwardingTargets.erase(selector);
+  }
+}
+
+#pragma mark - NSObject
+
+// Overridden to forward messages to registered handlers.
+- (id)forwardingTargetForSelector:(SEL)selector {
+  auto target = _forwardingTargets.find(selector);
+  if (target != _forwardingTargets.end()) {
+    return target->second;
+  }
+  return [super forwardingTargetForSelector:selector];
+}
+
+@end