The Policy SDK contains one observer class. Observer members are virtual and should be overridden to handle callbacks for asynchronous operations.
異步操作完成時， OnXxx() 會呼叫對應至結果的成員函式。 範例包括OnLoadSuccess()、OnLoadFailure()和OnAddEngineSuccess()用於mip::Profile::Observer。
The examples below demonstrate the promise/future pattern, which is also used by the SDK samples, and can be extended to implement the desired callback behavior.
Profile Observer Implementation
In the following example, we've created a class, ProfileObserver that is derived from mip::Profile::Observer. The member functions have been overridden to use the future/promise pattern used throughout the samples.
Note: The below samples are only partially implemented and do not include overrides for the mip::ProfileEngine related observers.
profile_observer.h
In the header, we define ProfileObserver, deriving from mip::Profile::Observer, then override each of the member functions.
class ProfileObserver final : public mip::Profile::Observer {
public:
ProfileObserver() { }
void OnLoadSuccess(const std::shared_ptr<mip::Profile>& profile, const std::shared_ptr<void>& context) override;
void OnLoadFailure(const std::exception_ptr& error, const std::shared_ptr<void>& context) override;
//TODO: Implement remaining members
};
profile_observer.cpp
In the implementation itself, we define an action to take for each observer member function.
每個成員都接受兩個參數。 The first is a shared pointer to the class handled by the function.
ProfileObserver::OnLoadSuccess would expect to receive a mip::Profile.
ProfileObserver::OnAddEngineSuccess would expect mip::ProfileEngine.
The second is a shared pointer to the context. In our implementation the context is a reference to a std::promise, passed in as shared_ptr<void>. 函式的第一行會將這個 std::promise轉換成 ，然後儲存在名為 promise的物件中。
最後，藉由設定 promise->set_value() 和 傳入 mip::Profile 物件，即可備妥未來。
#include "profile_observer.h"
#include <future>
//Called when Profile is successfully loaded
void ProfileObserver::OnLoadSuccess(const std::shared_ptr<mip::Profile>& profile, const std::shared_ptr<void>& context) {
//cast context to promise
auto promise = std::static_pointer_cast<std::promise<std::shared_ptr<mip::Profile>>>(context);
//set promise value to profile
promise->set_value(profile);
}
//Called when Profile fails to load
void ProfileObserver::OnLoadFailure(const std::exception_ptr& error, const std::shared_ptr<void>& context) {
auto promise = std::static_pointer_cast<std::promise<std::shared_ptr<mip::Profile>>>(context);
promise->set_exception(error);
}
//TODO: Implement remaining observer members
When performing any asynchronous operation, the observer implementation is passed to the settings constructor or async function itself.