# Tutorial

## How to Read this Tutorial

This tutorial is not meant to be read linearly. Its top-level structure roughly separates different concepts in the library (e.g., handling calling multiple slots, passing values to and from slots) and in each of these concepts the basic ideas are presented first and then more complex uses of the library are described later. Each of the sections is marked Beginner, Intermediate, or Advanced to help guide the reader. The Beginner sections include information that all library users should know; one can make good use of the Signals2 library after having read only the Beginner sections. The Intermediate sections build on the Beginner sections with slightly more complex uses of the library. Finally, the Advanced sections detail very advanced uses of the Signals2 library, that often require a solid working knowledge of the Beginner and Intermediate topics; most users will not need to read the Advanced sections.

## Hello, World! (Beginner)

The following example writes "Hello, World!" using signals and slots. First, we create a signal `sig`, a signal that takes no arguments and has a void return value. Next, we connect the `hello` function object to the signal using the `connect` method. Finally, use the signal `sig` like a function to call the slots, which in turns invokes `HelloWorld::operator()` to print "Hello, World!".

## Calling Multiple Slots

### Connecting Multiple Slots (Beginner)

Calling a single slot from a signal isn't very interesting, so we can make the Hello, World program more interesting by splitting the work of printing "Hello, World!" into two completely separate slots. The first slot will print "Hello" and may look like this:

The second slot will print ", World!" and a newline, to complete the program. The second slot may look like this:

Like in our previous example, we can create a signal `sig` that takes no arguments and has a `void` return value. This time, we connect both a `hello` and a `world` slot to the same signal, and when we call the signal both slots will be called.

By default, slots are pushed onto the back of the slot list, so the output of this program will be as expected:

    Hello, World!

### Ordering Slot Call Groups (Intermediate)

Slots are free to have side effects, and that can mean that some slots will have to be called before others even if they are not connected in that order. The Boost.Signals2 library allows slots to be placed into groups that are ordered in some way. For our Hello, World program, we want "Hello" to be printed before ", World!", so we put "Hello" into a group that must be executed before the group that ", World!" is in. To do this, we can supply an extra parameter at the beginning of the `connect` call that specifies the group. Group values are, by default, `int`s, and are ordered by the integer < relation. Here's how we construct Hello, World:

Invoking the signal will correctly print "Hello, World!", because the `Hello` object is in group 0, which precedes group 1 where the `World` object resides. The group parameter is, in fact, optional. We omitted it in the first Hello, World example because it was unnecessary when all of the slots are independent. So what happens if we mix calls to connect that use the group parameter and those that don't? The "unnamed" slots (i.e., those that have been connected without specifying a group name) can be placed at the front or back of the slot list (by passing `boost::signals2::at_front` or `boost::signals2::at_back` as the last parameter to ``connect``, respectively), and default to the end of the list. When a group is specified, the final `at_front` or `at_back` parameter describes where the slot will be placed within the group ordering. Ungrouped slots connected with `at_front` will always precede all grouped slots. Ungrouped slots connected with `at_back` will always succeed all grouped slots.

If we add a new slot to our example like this:

... we will get the result we wanted:

    Hello, World!
    ... and good morning!

## Passing Values to and from Slots

### Slot Arguments (Beginner)

Signals can propagate arguments to each of the slots they call. For instance, a signal that propagates mouse motion events might want to pass along the new mouse coordinates and whether the mouse buttons are pressed.

As an example, we'll create a signal that passes two `float` arguments to its slots. Then we'll create a few slots that print the results of various arithmetic operations on these values.

This program will print out the following:

    The arguments are 5 and 3
    The sum is 8
    The product is 15
    The difference is 2
    The quotient is 1.66667

So any values that are given to `sig` when it is called like a function are passed to each of the slots. We have to declare the types of these values up front when we create the signal. The type ``boost::signals2::signal`<void (float, float)>` means that the signal has a `void` return value and takes two `float` values. Any slot connected to `sig` must therefore be able to take two `float` values.

### Signal Return Values (Advanced)

Just as slots can receive arguments, they can also return values. These values can then be returned back to the caller of the signal through a combiner. The combiner is a mechanism that can take the results of calling slots (there may be no results or a hundred; we don't know until the program runs) and coalesces them into a single result to be returned to the caller. The single result is often a simple function of the results of the slot calls: the result of the last slot call, the maximum value returned by any slot, or a container of all of the results are some possibilities.

We can modify our previous arithmetic operations example slightly so that the slots all return the results of computing the product, quotient, sum, or difference. Then the signal itself can return a value based on these results to be printed:

    boost::signals2::signal<float (float, float)> sig;

This example program will output `2`. This is because the default behavior of a signal that has a return type (`float`, the first template argument given to the ``boost::signals2::signal`` class template) is to call all slots and then return a `boost::optional` containing the result returned by the last slot called. This behavior is admittedly silly for this example, because slots have no side effects and the result is the last slot connected.

A more interesting signal result would be the maximum of the values returned by any slot. To do this, we create a custom combiner that looks like this:

The `maximum` class template acts as a function object. Its result type is given by its template parameter, and this is the type it expects to be computing the maximum based on (e.g., `maximum<float>` would find the maximum `float` in a sequence of `float`s). When a `maximum` object is invoked, it is given an input iterator sequence `[first, last)` that includes the results of calling all of the slots. `maximum` uses this input iterator sequence to calculate the maximum element, and returns that maximum value.

We actually use this new function object type by installing it as a combiner for our signal. The combiner template argument follows the signal's calling signature:

    boost::signals2::signal<float (float x, float y),
                  maximum<float> > sig;

Now we can connect slots that perform arithmetic functions and use the signal:

The output of this program will be `15`, because regardless of the order in which the slots are connected, the product of 5 and 3 will be larger than the quotient, sum, or difference.

In other cases we might want to return all of the values computed by the slots together, in one large data structure. This is easily done with a different combiner:

Again, we can create a signal with this new combiner:

    boost::signals2::signal<float (float, float),
        aggregate_values<std::vector<float> > > sig;

The output of this program will contain 15, 8, 1.6667, and 2. It is interesting here that the first template argument for the `signal` class, `float`, is not actually the return type of the signal. Instead, it is the return type used by the connected slots and will also be the `value_type` of the input iterators passed to the combiner. The combiner itself is a function object and its `result_type` member type becomes the return type of the signal.

The input iterators passed to the combiner transform dereference operations into slot calls. Combiners therefore have the option to invoke only some slots until some particular criterion is met. For instance, in a distributed computing system, the combiner may ask each remote system whether it will handle the request. Only one remote system needs to handle a particular request, so after a remote system accepts the work we do not want to ask any other remote systems to perform the same task. Such a combiner need only check the value returned when dereferencing the iterator, and return when the value is acceptable. The following combiner returns the first non-NULL pointer to a `FulfilledRequest` data structure, without asking any later slots to fulfill the request:

    struct DistributeRequest {
      typedef FulfilledRequest* result_type;

      template<typename InputIterator>
      result_type operator()(InputIterator first, InputIterator last) const
      {
        while (first != last) {
          if (result_type fulfilled = *first)
            return fulfilled;
          ++first;
        }
        return 0;
      }
    };

## Connection Management

### Disconnecting Slots (Beginner)

Slots aren't expected to exist indefinitely after they are connected. Often slots are only used to receive a few events and are then disconnected, and the programmer needs control to decide when a slot should no longer be connected.

The entry point for managing connections explicitly is the ``boost::signals2::connection`` class. The `connection` class uniquely represents the connection between a particular signal and a particular slot. The ``connected`()` method checks if the signal and slot are still connected, and the ``disconnect()`` method disconnects the signal and slot if they are connected before it is called. Each call to the signal's `connect()` method returns a connection object, which can be used to determine if the connection still exists or to disconnect the signal and slot.

### Blocking Slots (Beginner)

Slots can be temporarily "blocked", meaning that they will be ignored when the signal is invoked but have not been permanently disconnected. This is typically used to prevent infinite recursion in cases where otherwise running a slot would cause the signal it is connected to to be invoked again. A `boost::signals2::shared_connection_block` object will temporarily block a slot. The connection is unblocked by either destroying or calling `unblock` on all the `shared_connection_block` objects that reference the connection. Here is an example of blocking/unblocking slots:

### Scoped Connections (Intermediate)

The `boost::signals2::scoped_connection` class references a signal/slot connection that will be disconnected when the `scoped_connection` class goes out of scope. This ability is useful when a connection need only be temporary, e.g.,

Note, attempts to initialize a scoped_connection with the assignment syntax will fail due to it being noncopyable. Either the explicit initialization syntax or default construction followed by assignment from a `signals2::connection` will work:

    // doesn't compile due to compiler attempting to copy a temporary scoped_connection object
    // boost::signals2::scoped_connection c0 = sig.connect(ShortLived());

    // okay
    boost::signals2::scoped_connection c1(sig.connect(ShortLived()));

    // also okay
    boost::signals2::scoped_connection c2;
    c2 = sig.connect(ShortLived());

### Disconnecting Equivalent Slots (Intermediate)

One can disconnect slots that are equivalent to a given function object using a form of the ``signal::disconnect`` method, so long as the type of the function object has an accessible `==` operator. For instance:

    boost::signals2::signal<void ()> sig;

### Automatic Connection Management (Intermediate)

Boost.Signals2 can automatically track the lifetime of objects involved in signal/slot connections, including automatic disconnection of slots when objects involved in the slot call are destroyed. For instance, consider a simple news delivery service, where clients connect to a news provider that then sends news to all connected clients as information arrives. The news delivery service may be constructed like this:

    class NewsItem { /* ... */ };

    typedef boost::signals2::signal<void (const NewsItem&)> signal_type;
    signal_type deliverNews;

Clients that wish to receive news updates need only connect a function object that can receive news items to the `deliverNews` signal. For instance, we may have a special message area in our application specifically for news, e.g.,:

    struct NewsMessageArea : public MessageArea
    {
    public:
      // ...

      void displayNews(const NewsItem& news) const
      {
        messageText = news.text();
        update();
      }
    };

    // ...
    NewsMessageArea *newsMessageArea = new NewsMessageArea(/* ... */);
    // ...
    deliverNews.connect(boost::bind(&NewsMessageArea::displayNews,
      newsMessageArea, _1));

However, what if the user closes the news message area, destroying the `newsMessageArea` object that `deliverNews` knows about? Most likely, a segmentation fault will occur. However, with Boost.Signals2 one may track any object which is managed by a shared_ptr, by using `slot::track`. A slot will automatically disconnect when any of its tracked objects expire. In addition, Boost.Signals2 will ensure that no tracked object expires while the slot it is associated with is in mid-execution. It does so by creating temporary shared_ptr copies of the slot's tracked objects before executing it. To track `NewsMessageArea`, we use a shared_ptr to manage its lifetime, and pass the shared_ptr to the slot via its `slot::track` method before connecting it, e.g.:

    // ...
    boost::shared_ptr<NewsMessageArea> newsMessageArea(new NewsMessageArea(/* ... */));
    // ...
    deliverNews.connect(signal_type::slot_type(&NewsMessageArea::displayNews,
      newsMessageArea.get(), _1).track(newsMessageArea));

Note there is no explicit call to bind() needed in the above example. If the `signals2::slot` constructor is passed more than one argument, it will automatically pass all the arguments to `bind` and use the returned function object.

Also note, we pass an ordinary pointer as the second argument to the slot constructor, using `newsMessageArea.get()` instead of passing the `shared_ptr` itself. If we had passed the `newsMessageArea` itself, a copy of the `shared_ptr` would have been bound into the slot function, preventing the `shared_ptr` from expiring. However, the use of `slot::track` implies we wish to allow the tracked object to expire, and automatically disconnect the connection when this occurs.

`shared_ptr` classes other than `boost::shared_ptr` (such as `std::shared_ptr`) may also be tracked for connection management purposes. They are supported by the `slot::track_foreign` method.

### Postconstructors and Predestructors (Advanced)

One limitation of using `shared_ptr` for tracking is that an object cannot setup tracking of itself in its constructor. However, it is possible to set up tracking in a post-constructor which is called after the object has been created and passed to a `shared_ptr`. The Boost.Signals2 library provides support for post-constructors and pre-destructors via the deconstruct() factory function.

For most cases, the simplest and most robust way to setup postconstructors for a class is to define an associated `adl_postconstruct` function which can be found by deconstruct(), make the class' constructors private, and give deconstruct access to the private constructors by declaring `deconstruct_access` a friend. This will ensure that objects of the class may only be created through the deconstruct() function, and their associated `adl_postconstruct()` function will always be called.

The examples section contains several examples of defining classes with postconstructors and predestructors, and creating objects of these classes using deconstruct()

Be aware that the postconstructor/predestructor support in Boost.Signals2 is in no way essential to the use of the library. The use of deconstruct is purely optional. One alternative is to define static factory functions for your classes. The factory function can create an object, pass ownership of the object to a `shared_ptr`, setup tracking for the object, then return the `shared_ptr`.

### When Can Disconnections Occur? (Intermediate)

Signal/slot disconnections occur when any of these conditions occur:

- The connection is explicitly disconnected via the connection's `disconnect` method directly, or indirectly via the signal's `disconnect` method, or `scoped_connection`'s destructor.

- An object tracked by the slot is destroyed.

- The signal is destroyed.

These events can occur at any time without disrupting a signal's calling sequence. If a signal/slot connection is disconnected at any time during a signal's calling sequence, the calling sequence will still continue but will not invoke the disconnected slot. Additionally, a signal may be destroyed while it is in a calling sequence, in which case it will complete its slot call sequence but may not be accessed directly.

Signals may be invoked recursively (e.g., a signal A calls a slot B that invokes signal A...). The disconnection behavior does not change in the recursive case, except that the slot calling sequence includes slot calls for all nested invocations of the signal.

Note, even after a connection is disconnected, its's associated slot may still be in the process of executing. In other words, disconnection does not block waiting for the connection's associated slot to complete execution. This situation may occur in a multi-threaded environment if the disconnection occurs concurrently with signal invocation, or in a single-threaded environment if a slot disconnects itself.

### Passing Slots (Intermediate)

Slots in the Boost.Signals2 library are created from arbitrary function objects, and therefore have no fixed type. However, it is commonplace to require that slots be passed through interfaces that cannot be templates. Slots can be passed via the `slot_type` for each particular signal type and any function object compatible with the signature of the signal can be passed to a `slot_type` parameter. For instance:

The `doOnClick` method is now functionally equivalent to the `connect` method of the `onClick` signal, but the details of the `doOnClick` method can be hidden in an implementation detail file.

## Example: Document-View

Signals can be used to implement flexible Document-View architectures. The document will contain a signal to which each of the views can connect. The following `Document` class defines a simple text document that supports mulitple views. Note that it stores a single signal to which all of the views will be connected.

Next, we can begin to define views. The following `TextView` class provides a simple view of the document text.

Alternatively, we can provide a view of the document translated into hex values using the `HexView` view:

To tie the example together, here is a simple `main` function that sets up two views and then modifies the document:

The complete example source, contributed by Keith MacDonald, is available in the examples section. We also provide variations on the program which employ automatic connection management to disconnect views on their destruction.

## Giving a Slot Access to its Connection (Advanced)

You may encounter situations where you wish to disconnect or block a slot's connection from within the slot itself. For example, suppose you have a group of asynchronous tasks, each of which emits a signal when it completes. You wish to connect a slot to all the tasks to retrieve their results as each completes. Once a given task completes and the slot is run, the slot no longer needs to be connected to the completed task. Therefore, you may wish to clean up old connections by having the slot disconnect its invoking connection when it runs.

For a slot to disconnect (or block) its invoking connection, it must have access to a `signals2::connection` object which references the invoking signal-slot connection. The difficulty is, the `connection` object is returned by the `signal::connect` method, and therefore is not available until after the slot is already connected to the signal. This can be particularly troublesome in a multi-threaded environment where the signal may be invoked concurrently by a different thread while the slot is being connected.

Therefore, the signal classes provide `signal::connect_extended` methods, which allow slots which take an extra argument to be connected to a signal. The extra argument is a `signals2::connection` object which refers to the signal-slot connection currently invoking the slot. `signal::connect_extended` uses slots of the type given by the `signal::extended_slot_type` typedef.

The examples section includes an extended_slot program which demonstrates the syntax for using `signal::connect_extended`.

## Changing the `Mutex` Type of a Signal (Advanced).

For most cases the default type of `boost::signals2::mutex` for a `signals2::signal`'s `Mutex` template type parameter should be fine. If you wish to use an alternate mutex type, it must be default-constructible and fulfill the `Lockable` concept defined by the Boost.Thread library. That is, it must have `lock()` and `unlock()` methods (the `Lockable` concept also includes a `try_lock()` method but this library does not require try locking).

The Boost.Signals2 library provides one alternate mutex class for use with `signal`: `boost::signals2::dummy_mutex`. This is a fake mutex for use in single-threaded programs, where locking a real mutex would be useless overhead. Other mutex types you could use with `signal` include `boost::mutex`, or the `std::mutex` from C++11.

Changing a signal's `Mutex` template type parameter can be tedious, due to the large number of template parameters which precede it. The `signal_type` metafunction is particularly useful in this case, since it enables named template type parameters for the `signals2::signal` class. For example, to declare a signal which takes an `int` as an argument and uses a `boost::signals2::dummy_mutex` for its `Mutex` types, you could write:

    namespace bs2 = boost::signals2;
    using namespace bs2::keywords;
    bs2::signal_type<void (int), mutex_type<bs2::dummy_mutex> >::type sig;

## Linking against the Signals2 library

Unlike the original Boost.Signals library, Boost.Signals2 is currently header-only.