Added PolicyChangeRegistrar, to observe specific policies at the PolicyService.

chrome/browser/policy/policy_service.h

Index: chrome/browser/policy/policy_service.h
diff --git a/chrome/browser/policy/policy_service.h b/chrome/browser/policy/policy_service.h
index 5b5f3f33507a61bb6a964191b3c4571bca07e9bd..f414ba0c82caac57c71a0e03bfd2cb1c6d4348c8 100644
--- a/chrome/browser/policy/policy_service.h
+++ b/chrome/browser/policy/policy_service.h
@@ -6,8 +6,11 @@
 #define CHROME_BROWSER_POLICY_POLICY_SERVICE_H_
 #pragma once
 
+#include <map>
 #include <string>
 
+#include "base/basictypes.h"
+#include "base/callback.h"
 #include "chrome/browser/policy/policy_map.h"
 
 namespace policy {
@@ -75,6 +78,45 @@ class PolicyService {
   virtual bool IsInitializationComplete() const = 0;
 };
 
+// A registrar that only observes changes to particular policies within the
+// PolicyMap for the given policy namespace.
+class PolicyChangeRegistrar : public PolicyService::Observer {
+ public:
+  typedef base::Callback<void(const Value*, const Value*)> UpdateCallback;
+
+  // Observes updates to the given (domain, component_id) namespace in the given
+  // |policy_service|, and notifies |observer| whenever any of the registered
+  // policy keys changes. Both the |policy_service| and the |observer| must
+  // outlive |this|.
+  PolicyChangeRegistrar(PolicyService* policy_service,
+                        PolicyDomain domain,
+                        const std::string component_id);
+
+  virtual ~PolicyChangeRegistrar();
+
+  // Will invoke |callback| whenever |policy_name| changes its value, as long
+  // as this registrar exists.
+  // Only one callback can be registed per policy name; a second call with the
+  // same |policy_name| will overwrite the previous callback.
+  void Observe(const std::string& policy_name, const UpdateCallback& callback);
+
+  // Implementation of PolicyService::Observer:
+  virtual void OnPolicyUpdated(PolicyDomain domain,
+                               const std::string& component_id,
+                               const PolicyMap& previous,
+                               const PolicyMap& current) OVERRIDE;
+
+ private:
+  typedef std::map<std::string, UpdateCallback> CallbackMap;
+
+  PolicyService* policy_service_;
+  PolicyDomain domain_;
+  std::string component_id_;
+  CallbackMap callback_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(PolicyChangeRegistrar);
+};
+
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_POLICY_POLICY_SERVICE_H_