eventpp/doc/callbacklist.md

# Class CallbackList reference

## Tutorials

### CallbackList tutorial 1, basic

**Code**  
```c++
// The namespace is eventpp
// the first parameter is the prototype of the listener.
eventpp::CallbackList<void ()> callbackList;

// Add a callback.
// []() {} is the callback.
// Lambda is not required, any function or std::function
// or whatever function object with the required prototype is fine.
callbackList.append([]() {
	std::cout << "Got callback 1." << std::endl;
});
callbackList.append([]() {
	std::cout << "Got callback 2." << std::endl;
});

// Invoke the callback list
callbackList();
```

**Output**  
> Got callback 1.  
> Got callback 2.  

**Remarks**  
First let's define a callback list.
```c++
eventpp::CallbackList<void ()> callbackList;
```
class CallbackList takes at least one template arguments. It is the *prototype* of the callback.  
The *prototype* is C++ function type, such as `void (int)`, `void (const std::string &, const MyClass &, int, bool)`.  

Now let's add a callback.  
```c++
callbackList.append([]() {
	std::cout << "Got callback 1." << std::endl;
});
```
Function `append` takes one arguments, the *callback*.  
The *callback* can be any callback target -- functions, pointers to functions, , pointers to member functions, lambda expressions, and function objects. It must be able to be called with the *prototype* declared in `callbackList`.  
In the tutorial, we also add another callback.  

Now let's invoke the callbackList.
```c++
callbackList();
```
During the invoking, all callbacks will be invoked one by one in the order of they were added.

### CallbackList tutorial 2, callback with parameters

**Code**  
```c++
// The callback list has two parameters.
eventpp::CallbackList<void (const std::string &, const bool)> callbackList;

callbackList.append([](const std::string & s, const bool b) {
	std::cout << std::boolalpha << "Got callback 1, s is " << s << " b is " << b << std::endl;
});
// The callback prototype doesn't need to be exactly same as the callback list.
// It would be find as long as the arguments is compatible with the dispatcher.
callbackList.append([](std::string s, int b) {
	std::cout << std::boolalpha << "Got callback 2, s is " << s << " b is " << b << std::endl;
});

// Invoke the callback list
callbackList("Hello world", true);
```

**Output**  
> Got callback 1, s is Hello world b is true  
> Got callback 2, s is Hello world b is 1  

**Remarks**  
Now the callback list prototype takes two parameters, `const std::string &` and `const bool`.  
The callback's prototype is not required to be same as the callback list, it's fine as long as the prototype is compatible with the callback list. See the second callback, `[](std::string s, int b)`, its prototype is not same as the callback list.

### CallbackList tutorial 3, remove

**Code**  
```c++
using CL = eventpp::CallbackList<void ()>;
CL callbackList;

CL::Handle handle2;

// Add some callbacks.
callbackList.append([]() {
	std::cout << "Got callback 1." << std::endl;
});
handle2 = callbackList.append([]() {
	std::cout << "Got callback 2." << std::endl;
});
callbackList.append([]() {
	std::cout << "Got callback 3." << std::endl;
});

callbackList.remove(handle2);

// Invoke the callback list
// The "Got callback 2" callback should not be triggered.
callbackList();
```

**Output**  
> Got callback 1.  
> Got callback 3.  

**Remarks**  

### CallbackList tutorial 4, for each

**Code**  
```c++
using CL = eventpp::CallbackList<void ()>;
CL callbackList;

// Add some callbacks.
callbackList.append([]() {
	std::cout << "Got callback 1." << std::endl;
});
callbackList.append([]() {
	std::cout << "Got callback 2." << std::endl;
});
callbackList.append([]() {
	std::cout << "Got callback 3." << std::endl;
});

// Now call forEach to remove the second callback
// The forEach callback prototype is void(const CallbackList::Handle & handle, const CallbackList::Callback & callback)
int index = 0;
callbackList.forEach([&callbackList, &index](const CL::Handle & handle, const CL::Callback & callback) {
	std::cout << "forEach(Handle, Callback), invoked " << index << std::endl;
	if(index == 1) {
		callbackList.remove(handle);
		std::cout << "forEach(Handle, Callback), removed second callback" << std::endl;
	}
	++index;
});

// The forEach callback prototype can also be void(const CallbackList::Handle & handle)
callbackList.forEach([&callbackList, &index](const CL::Handle & handle) {
	std::cout << "forEach(Handle), invoked" << std::endl;
});

// The forEach callback prototype can also be void(const CallbackList::Callback & callback)
callbackList.forEach([&callbackList, &index](const CL::Callback & callback) {
	std::cout << "forEach(Callback), invoked" << std::endl;
});

// Invoke the callback list
// The "Got callback 2" callback should not be triggered.
callbackList();
```

**Output**  
> Got callback 1.  
> Got callback 3.  

**Remarks**  

## API reference

**Template parameters**

```c++
template <
	typename Prototype,
	typename Callback = void,
	typename Threading = MultipleThreading
>
class CallbackList;
```
`Prototype`:  the callback prototype. It's C++ function type such as `void(int, std::string, const MyClass *)`.  
`Callback`: the underlying type to hold the callback. Default is `void`, which will be expanded to `std::function`.  
`Threading`: threading model. Default is 'MultipleThreading'. Possible values:  
  * `MultipleThreading`: the core data is protected with mutex. It's the default value.  
  * `SingleThreading`: the core data is not protected and can't be accessed from multiple threads.  

**Public types**

`Handle`: the handle type returned by appendListener, prependListener and insertListener. A handle can be used to insert a callback or remove a callback. To check if a `Handle` is empty, convert it to boolean, *false* is empty. `Handle` is copyable.  
`Callback`: the callback storage type.

**Functions**

```c++
CallbackList() = default;
CallbackList(CallbackList &&) = delete;
CallbackList(const CallbackList &) = delete;
CallbackList & operator = (const CallbackList &) = delete;
```

CallbackList can not be copied, moved, or assigned.

```c++
Handle append(const Callback & callback)
```  
Add the *callback* to the callback list.  
The callback is added to the end of the callback list.  
Return a handle which represents the callback. The handle can be used to remove this callback or insert other callback before this callback.  
If `append` is called in another callback during the invoking of the callback list, the new callback is guaranteed not triggered during the same callback list invoking.  
The time complexity is O(1).

```c++
Handle prepend(const Callback & callback)
```  
Add the *callback* to the callback list.  
The callback is added to the beginning of the callback list.  
Return a handle which represents the callback. The handle can be used to remove this callback or insert other callback before this callback.  
If `prepend` is called in another callback during the invoking of the callback list, the new callback is guaranteed not triggered during the same callback list invoking.  
The time complexity is O(1).

```c++
Handle insert(const Callback & callback, const Handle before)
```  
Insert the *callback* to the callback list before the callback handle *before*. If *before* is not found, *callback* is added at the end of the callback list.  
Return a handle which represents the callback. The handle can be used to remove this callback or insert other callback before this callback.  
If `insert` is called in another callback during the invoking of the callback list, the new callback is guaranteed not triggered during the same callback list invoking.  
The time complexity is O(1).  

```c++
bool remove(const Handle handle)
```  
Remove the callback *handle* from the callback list.  
Return true if the callback is removed successfully, false if the callback is not found.  
The time complexity is O(1).  

```c++
template <typename Func>  
void forEach(Func && func)
```  
Apply `func` to all callbacks.  
The `func` can be one of the three prototypes:  
```c++
AnyReturnType func(const EventDispatcher::Handle &, const EventDispatcher::Callback &);
AnyReturnType func(const EventDispatcher::Handle &);
AnyReturnType func(const EventDispatcher::Callback &);
```
**Note**: the `func` can remove any callbacks, or add other callbacks, safely.

```c++
void operator() (Args ...args)
```  
Invoke each callbacks in the callback list.  
The callbacks are called with arguments `args`.  
The callbacks are called in the thread same as the callee of `operator()`.

## Nested callback safety
1. If a callback adds another callback to the callback list during a invoking, the new callback is guaranteed not to be triggered within the same invoking. This is guaranteed by an unsigned 64 bits integer counter. This rule will be broken is the counter is overflowed to zero in a invoking, but this rule will continue working on the subsequence invoking.  
2. Any callbacks that are removed during a invoking are guaranteed not triggered.  
3. All above points are not true in multiple threading. That's to say, if one thread is dispatching an event, the other thread add or remove a listener, the added or removed listener may be triggered during the dispatch.


## Thread safety
`CallbackList` is thread safe. All public functions can be invoked from multiple threads simultaneously. If it failed, please report a bug.  
`CallbackList` guarantees the integration of each append/prepend/insert/remove/invoking operations, but it doesn't guarantee the order of the operations in multiple threads. For example, if a thread is invoking a callback list, the other thread removes a callback in the mean time, the removed callback may be still triggered after it's removed.  
The operations on the listeners, such as copying, moving, comparing, or invoking, may be not thread safe. It depends on listeners.  


## Exception safety

CallbackList doesn't throw any exceptions.  
Exceptions may be thrown by underlying code when,  
1. Out of memory, new memory can't be allocated.  
2. The callbacks throw exceptions during copying, moving, comparing, or invoking.

## Time complexities
* `append`: O(1)
* `prepend`: O(1)
* `insert`: O(1)
* `remove`: O(1)

## Internal data structure

CallbackList uses doubly linked list to manage the callbacks.  
Each node is linked by shared pointer. Using shared pointer allows the node be removed while iterating.