参考文章：https://coderwall.com/p/u4w9ra/implementing-signals-in-c-11
最近在完成C++大作业时，碰到了监听者模式的需求。
尽管C++下也可以通过声明IObserver这样的接口，做继承，然后实现类似Java中的监听者模式。
但是这种方法并不是最适合C++的。通过利用C++11中的函数对象和RAII，我们可以实现一个更符合C++国情的监听者模式。
代码如下：
/* Signal class for implementing event. */
template <typename... TFuncArgs>
class Signal {
public:
using Callback = std::function<void(TFuncArgs...)>;
/* Connection class.
Disconnect() will be called automatically once it's out of scope.
*/
class SignalConnection {
private:
friend class Signal;
/* We only allow class Signal to create a connection. */
SignalConnection(Signal& signal, int id) noexcept:id(id), signal(signal) {}
public:
/* A copy constructor of "connection" is really confusing. just delete it. */
SignalConnection(const SignalConnection& copy) = delete;
/* without a copy constructor, we can't return SignalConnection, unless we provide a move constructor. */
SignalConnection(SignalConnection&& toMove) noexcept : id(toMove.id), signal(toMove.signal),disconnected(toMove.disconnected) {}
~SignalConnection() {
Disconnect();
}
int id;
Signal& signal;
bool disconnected = false;
void Disconnect() {
if (disconnected)
return;
disconnected = true;
signal.Disconnect(*this);
}
};
/* <b>Register to the Signal.</b>
Returns a connection object.
the connection object will automatically disconnect once it's out of scope.*/
SignalConnection Connect(Callback callback) {
callbacks.push_back(std::pair<int, Callback>(idRoller++, callback));
return SignalConnection(*this, idRoller - 1);
}
void Invoke(TFuncArgs... args) {
for (auto& con : callbacks) {
(con.second)(args...);
}
}
void operator()(TFuncArgs... args) {
Invoke(args...);
}
private:
/* ID Counter.
We look for the connection's corresponding callback using id, since the operator== of std::function doesn't work as imagine.
*/
int idRoller = 0;
std::list<std::pair<int, Callback>> callbacks;
void Disconnect(SignalConnection& t) {
callbacks.erase(std::remove_if(callbacks.begin(), callbacks.end(), [&](auto& pCallback) {
return pCallback.first == t.id;
}), callbacks.end());
}
};
一共两个类，Signal类表示事件，SignalConnection是由Signal返回给监听者的一个句柄，用于取消监听。
SignalConnection在析构函数中会自动调用进行取消监听，这样监听者不用担心内存泄露的问题。
Signal中保存的回调是pair<int,Callback>的结构。因为std::function对象不能用==直接进行比较，因此我们需要对每个监听者分配一个独一无二的id，在取消监听时，通过比较这个id，来确定删除哪个监听者。
为了避免歧义，我们将SignalConnection类的复制构造函数设置为delete，但是允许move构造，不然Signal也无法在函数里返回一个SignalConnection了。
使用方法：
void TestFunc1(int t) {
cout << "Func1" << endl;
}
void TestFunc2(int t,string someArg) {
cout << "Func2" << endl;
}
int main(){
using TestDelegate = Signal<int>;
TestDelegate testEvent;
TestDelegate::SignalConnection testCon1 = testEvent.Connect(&TestFunc1);
TestDelegate::SignalConnection testCon2 = testEvent.Connect(std::bind(TestFunc2 ,std::placeholders::_1, "arg")); //用bind去绑定参数
auto testCon3 = testEvent.Connect(&TestFunc1); //用auto简化声明
testEvent(1);
...
可以看到使用起来非常简洁自然。
上面这个实现有一定的缺陷，就是当Signal被析构后，没办法通知SignalConnection去Disconnect。在这之后此时SignalConnection被析构的话，调用signal.Disconnect()会导致引用错误。
最先想到的改进方法是让SignalConnection在Disconnect之前检查Signal还在不在。
但是既然都已经析构了，我们是没有办法去检查的。除非我们让Signal在最开始构造的时候，必须在堆上构造，然后用shared_ptr保存。让SignalConnection保存一个weak_ptr去检查Signal是否被析构。实际情况下这种实现多少有点不美观。
第二种方法，就是尝试在Signal析构时，将连接到它的SignalConnection全部Disconnect。
这种方法，我们需要在Signal中保存SignalConnection的指针。在Connect时，我们不再返回SignalConnection的对象，而是SignalConnection的shared_ptr，同时保存一个对应的weak_ptr。这种方法相对来说要简洁一点，只是将Connect的返回类型改为了shared_ptr，其他特性都得到了保留。
/* Signal class for implementing event. */
template <typename... TFuncArgs>
class Signal {
public:
using Callback = std::function<void(TFuncArgs...)>;
/* Connection class.
Disconnect() will be called automatically once it's out of scope.
*/
class SignalConnection {
private:
friend class Signal;
/* We only allow class Signal to create a connection. */
SignalConnection(Signal& signal) noexcept: signal(signal) {}
public:
/* A copy constructor of "connection" is really confusing. just delete it. */
SignalConnection(const SignalConnection& copy) = delete;
/* without a copy constructor, we can't return SignalConnection, unless we provide a move constructor. */
SignalConnection(SignalConnection&& toMove) noexcept : id(toMove.id), signal(toMove.signal),disconnected(toMove.disconnected) {}
~SignalConnection() {
Disconnect();
}
Signal& signal;
bool disconnected = false;
void Disconnect() {
if (disconnected)
return;
disconnected = true;
signal.Disconnect(this);
}
};
using SPConnection = std::shared_ptr<SignalConnection>;
/* Register to the Signal.
Returns a connection object.
the connection object will automatically disconnect once it's out of scope.*/
SPConnection Connect(Callback callback) {
auto t = std::shared_ptr<SignalConnection>(new SignalConnection(*this));
callbacks.push_back(std::pair<std::weak_ptr<SignalConnection>, Callback>(t, callback));
return t;
}
void Invoke(TFuncArgs... args) {
for (auto& con : callbacks) {
(con.second)(args...);
}
}
void operator()(TFuncArgs... args) {
Invoke(args...);
}
~Signal() {
for (auto& t : callbacks) {
if (auto sp = t.first.lock()) { //translate to shared_ptr
sp->disconnected = true;
}
}
}
private:
std::list<std::pair<std::weak_ptr<SignalConnection>, Callback>> callbacks;
void Disconnect(SignalConnection* t) {
callbacks.erase(std::remove_if(callbacks.begin(), callbacks.end(), [&](auto& pCallback) {
auto ptr = pCallback.first.lock();
return !ptr || ptr.get() == t;
}), callbacks.end());
}
};
最后一个办法，我们不用修改任何函数签名就可以解决这个问题。
解决这个问题的关键是让SignalConnection得知Signal是否被析构。所以我们要在不受析构影响的堆内存中找一个地方存放这个信息，让Signal被析构的时候在这个地方表示自己已经被析构。然后在构造SignalConnection时，将这个内存地址一同传入SignalConnection。SignalConnection去检查这个内存就能知道Signal是否被析构了。为了防止内存泄露，最后一个检查的SignalConnection还需要把这块内存回收。
这其实就类似于引用计数了，而我们可以用智能指针去模拟这些行为，而不用自己真的去管理内存。代码如下。
/* Signal class for implementing event. */
template <typename... TFuncArgs>
class Signal {
public:
using Callback = std::function<void(TFuncArgs...)>;
std::shared_ptr<int> survivePtr;
Signal() : survivePtr(make_shared<int>(0)) {
}
/* Connection class.
Disconnect() will be called automatically once it's out of scope.
*/
class SignalConnection {
private:
friend class Signal;
/* We only allow class Signal to create a connection. */
SignalConnection(Signal& signal, int id, std::shared_ptr<int> survivePtr) noexcept:id(id), signal(signal), signalSurvivePtr(survivePtr){}
public:
/* A copy constructor of "connection" is really confusing. just delete it. */
SignalConnection(const SignalConnection& copy) = delete;
/* without a copy constructor, we can't return SignalConnection, unless we provide a move constructor. */
SignalConnection(SignalConnection&& toMove) noexcept : id(toMove.id), signal(toMove.signal), disconnected(toMove.disconnected), signalSurvivePtr(toMove.signalSurvivePtr) {}
~SignalConnection() {
Disconnect();
}
int id;
Signal& signal;
bool disconnected = false;
std::weak_ptr<int> signalSurvivePtr;
void Disconnect() {
if (disconnected || signalSurvivePtr.expired())
return;
disconnected = true;
signal.Disconnect(*this);
}
};
/* <b>Register to the Signal.</b>
Returns a connection object.
the connection object will automatically disconnect once it's out of scope.*/
SignalConnection Connect(Callback callback) {
callbacks.push_back(std::pair<int, Callback>(idRoller++, callback));
return SignalConnection(*this, idRoller - 1, survivePtr);
}
void Invoke(TFuncArgs... args) {
for (auto& con : callbacks) {
(con.second)(args...);
}
}
void operator()(TFuncArgs... args) {
Invoke(args...);
}
private:
/* ID Counter.
We look for the connection's corresponding callback using id, since the operator== of std::function doesn't work as imagine.
*/
int idRoller = 0;
std::list<std::pair<int, Callback>> callbacks;
void Disconnect(SignalConnection& t) {
callbacks.erase(std::remove_if(callbacks.begin(), callbacks.end(), [&](auto& pCallback) {
return pCallback.first == t.id;
}), callbacks.end());
}
};
Signal中我们保存一个shared_ptr，在SignalConnection中保存的是weak_ptr。这样当Signal被析构时，该内存空间的引用计数归零，此时SignalConnection可以通过自己的weak_ptr得知Signal是否被析构。