Dependency free, header only signals and slot library implemented with C 11.

The following example creates a signal and then connects a lambda as a slot.

// Create a signal which accepts slots with no arguments and void return value.
nod::signal<void()> signal;
// Connect a lambda slot that writes "Hello, World!" to stdout
signal.connect([](){
		std::cout << "Hello, World!" << std::endl;
	});
// Call the slots
signal();

If multiple slots are connected to the same signal, all of the slots will be called when the signal is invoked. The slots will be called in the same order as they where connected.

void endline() {
	std::cout << std::endl;
}

// Create a signal
nod::signal<void()> signal;
// Connect a lambda that prints a message
signal.connect([](){
		std::cout << "Message without endline!";
	});
// Connect a function that prints a endline
signal.connect(endline);

// Call the slots
signal();

The signal types in the library support connection of the same types that is supported by std::function<T>.

When a signal calls it's connected slots, any arguments passed to the signal are propagated to the slots. To make this work, we do need to specify the signature of the signal to accept the arguments.

void print_sum( int x, int y ) {
	std::cout << x << " " << y << "=" << (x y) << std::endl;
}
void print_product( int x, int y ) {
	std::cout << x << "*" << y << "=" << (x*y) << std::endl;
}


// We create a signal with two integer arguments.
nod::signal<void(int,int)> signal;
// Let's connect our slot
signal.connect( print_sum );
signal.connect( print_product );

// Call the slots
signal(10, 15);
signal(-5, 7);	

There are many circumstances where the programmer needs to diconnect a slot that no longer want to recieve events from the signal. This can be really important if the lifetime of the slots are shorter than the lifetime of the signal. That could cause the signal to call slots that have been destroyed but not disconnected, leading to undefined behaviour and probably segmentation faults.

When a slot is connected, the return value from the connect method returns an instance of the class nod::connection, that can be used to disconnect that slot.

// Let's create a signal
nod::signal<void()> signal;
// Connect a slot, and save the connection
nod::connection connection = signal.connect([](){
								 std::cout << "I'm connected!" << std::endl;
							 });
// Triggering the signal will call the slot
signal();
// Now we disconnect the slot
connection.disconnect();
// Triggering the signal will no longer call the slot
signal();

To assist in disconnecting slots, one can use the class nod::scoped_connection to capture a slot connection. A scoped connection will automatically disconnect the slot when the connection object goes out of scope.

// We create a signal
nod::signal<void()> signal;
// Let's use a scope to control lifetime
{ 
	// Let's save the connection in a scoped_connection
	nod::scoped_connection connection =
		signal.connect([](){
			std::cout << "This message should only be emitted once!" << std::endl; 
		});
	// If we trigger the signal, the slot will be called
	signal();
} // Our scoped connection is destructed, and disconnects the slot
// Triggering the signal now will not call the slot
signal();	

It is possible for slots to have a return value. The return values can be returned from the signal using a accumulator, which is a function object that acts as a proxy object that processes the slot return values. When triggering a signal through a accumulator, the accumulator gets called for each slot return value, does the desired accumulation and then return the result to the code triggering the signal. The accumulator is designed to work in a similar way as the STL numerical algorithm std::accumulate.

// We create a singal with slots that return a value
nod::signal<int(int, int)> signal;
// Then we connect some signals
signal.connect( std::plus<int>{} );
signal.connect( std::multiplies<int>{} );
signal.connect( std::minus<int>{} );		
// Let's say we want to calculate the sum of all the slot return values
// when triggering the singal with the parameters 10 and 100.
// We do this by accumulating the return values with the initial value 0
// and a plus function object, like so:
std::cout << "Sum: " << signal.accumulate(0, std::plus<int>{})(10,100) << std::endl;
// Or accumulate by multiplying (this needs 1 as initial value):
std::cout << "Product: " << signal.accumulate(1, std::multiplies<int>{})(10,100) << std::endl;
// If we instead want to build a vector with all the return values
// we can accumulate them this way (start with a empty vector and add each value):			
auto vec = signal.accumulate( std::vector<int>{}, []( std::vector<int> result, int value ) {
		result.push_back( value );
		return result;
	})(10,100);

std::cout << "Vector: ";
for( auto const& element : vec ) {
	std::cout << element << " "; 
}
std::cout << std::endl;

As we can see from the previous example, we can use the accumulate method if we want to aggregate all the return values of the slots. Doing the aggregation that way is not very optimal. It is both a inefficient algorithm for doing aggreagtion to a container, and it obscures the call site as the caller needs to express the aggregation using the verb accumulate. To remedy these shortcomings we can turn to the method aggregate instead. This is a template method, taking the type of container to aggregate to as a template parameter.

// We create a singal
nod::signal<int(int, int)> signal;
// Let's connect some slots
signal.connect( std::plus<int>{} );
signal.connect( std::multiplies<int>{} );
signal.connect( std::minus<int>{} );
// We can now trigger the signal and aggregate the slot return values
auto vec = signal.aggregate<std::vector<int>>(10,100);

std::cout << "Result: ";
for( auto const& element : vec ) {
	std::cout << element << " "; 
}
std::cout << std::endl;

There are two types of signals in the library. The first is nod::signal<T> which is safe to use in a multi threaded environment. Multiple threads can read, write, connect slots and disconnect slots simultaneously, and the signal will provide the nessesary synchronization. When triggering a slignal, all the registered slots will be called and executed by the thread that triggered the signal.

The second type of signal is nod::unsafe_signal<T> which is not safe to use in a multi threaded environment. No syncronization will be performed on the internal state of the signal. Instances of the signal should theoretically be safe to read from multiple thread simultaneously, as long as no thread is writing to the same object at the same time. There can be a performance gain involved in using the unsafe version of a signal, since no syncronization primitives will be used.

nod::connection and nod::scoped_connection are thread safe for reading from multiple threads, as long as no thread is writing to the same object. Writing in this context means calling any non const member function, including destructing the object. If an object is being written by one thread, then all reads and writes to that object from the same or other threads needs to be prevented. This basically means that a connection is only allowed to be disconnected from one thread, and you should not check connection status or reassign the connection while it is being disconnected.

The test project uses premake5 to generate make files or similiar.

To build and run the tests using gcc and gmake on linux, execute the following from the test directory:

premake5 gmake
make -C build/gmake
bin/gmake/debug/nod_tests

To build and run the tests, execute the following from the test directory:

REM Adjust paths to suite your environment
c:\path\to\premake\premake5.exe vs2013
"c:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\Tools\vsvars32.bat"
msbuild /m build\vs2013\nod_tests.sln
bin\vs2013\debug\nod_tests.exe

Copyright (c) 2015 Fredrik Berggren

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