etl/docs/callbacks/signal.md

---
title: "signal"
---

{{< callout type="info">}}
  Header: `signal.h`  
  From: `20.44.0`  
{{< /callout >}}

A class that implements simple signal/slot framework.  
Uses `etl::delegate` as the default slot type, though other types may be used.

```cpp
template <typename TFunction, size_t Size, typename TSlot = etl::delegate<TFunction>>
class signal
```
`TFunction`  The callback function signature.
`Size`       The maximum numbr of slots for the signal.
`TSlot`      The callback slot type. Default = `etl::delegate<TFunction>`

## Types
`slot_type` Defined as the slot type.
`size_type` The size type used internally.
`span_type` The span type used in the connect API.

## Constructors
```cpp
template <typename... TSlots>
ETL_CONSTEXPR14 explicit signal(TSlots&&... slots) ETL_NOEXCEPT
```
Construct the signal from a variadic list of slots.  
Can be used as a `constexpr` constructor.  
Static asserts if any of the slots are not `slot_type`.  
Static asserts if the number of slots exceeds capacity.

## Connect
```cpp
bool connect(const slot_type& slot)
```
Connects the slot, if not already connected and returns `true`.  
If the signal is full, asserts `etl::signal_full` and returns `false`.

---

```cpp
bool connect(std::initializer_list<const slot_type> slots)
```
Connects all of the slots and returns `true`.  
If the number of slots exceeds the signal's max size, asserts an `etl::signal_full` and returns `false`.  
Enabled if `ETL_HAS_INITIALIZER_LIST` and `ETL_USING_CPP17` are defined as `1`.

---

```cpp
bool connect(const span_type slots)
```
Connects all of the slots and returns `true`.
If the number of slots exceeds the signal's max size, asserts an `etl::signal_full` and returns `false`.

## Disconnect

```cpp
void disconnect(const slot_type& slot) ETL_NOEXCEPT
```
Disconnects slot from the signal.  
If the signal does not contain the slot there is no error.

---

```cpp
void disconnect(std::initializer_list<const slot_type> slots) ETL_NOEXCEPT
```
Disconnects all of the slots from the signal.  
If the signal does not contain a particular slot, there is no error.  
Enabled if `ETL_HAS_INITIALIZER_LIST` and `ETL_USING_CPP17` are defined as `1`.

---

```cpp
void disconnect(const span_type slots) ETL_NOEXCEPT
```
Disconnects all of the slots from the signal.  
If the signal does not contain a particular slot, there is no error.  

```cpp
void disconnect_all() ETL_NOEXCEPT
```
Disconnects all slots from the signal.

## Status

```cpp 
ETL_CONSTEXPR14 bool connected(const slot_type& slot) const ETL_NOEXCEPT
```
Checks if a slot is connected to the signal.

---

```cpp
ETL_CONSTEXPR14 bool empty() const ETL_NOEXCEPT
```
Return `true` if the signal has no slots connected.

---

```cpp
ETL_CONSTEXPR14 bool full() const ETL_NOEXCEPT
```
Return `true` if the signal has the maximum number of slots connected.

---

```cpp
ETL_CONSTEXPR14 size_type max_size() const ETL_NOEXCEPT
```
Returns the total number of slots that can be connected.

---

```cpp
ETL_CONSTEXPR14 size_type size() const ETL_NOEXCEPT
```
Returns the total slots currently connected.

---

```cpp
ETL_CONSTEXPR14 size_type available() const ETL_NOEXCEPT
```
Returns the total empty slots available.

## Call
```cpp
template <typename... TArgs>
void operator()(TArgs&&... args) const ETL_NOEXCEPT
```
Function operator that calls each slot with the supplied parameters.

## Errors

```cpp
etl::signal_full
```
Indicates that an attempt to add a slot to a full signal occurred.  
Inherits from `etl::signal_exception`.

## Examples

```cpp
constexpr size_t MaxSlots = 3;

using callback_type = void(int a, int b);
using signal_type   = etl::signal<callback_type, MaxSlots>;
using slot_type     = signal_type::slot_type;
using span_type     = signal_type::span_type;

using not_slot_type = etl::delegate<void(int)>;
```

**Defining the slot functions**  

```cpp
void Function1(int a, int b)
{
  std::cout << "Function1: " << a << "," << b << "\n";
}

void Function2(int a, int b)
{
  std::cout << "Function2: " << a << "," << b << "\n";
}

void Function3(int a, int b)
{
  std::cout << "Function3: " << a << "," << b << "\n";
}
0
void Function4(int a, int b)
{
  std::cout << "Function4: " << a << "," << b << "\n";
}

void Function5(int a)
{
  std::cout << "Function5: " << a << "\n";
}
```

**Creating the slots**  

```cpp
constexpr slot_type MakeSlot1() noexcept
{
return slot_type::create<Function1>();
}

constexpr slot_type MakeSlot2() noexcept
{
return slot_type::create<Function2>();
}

constexpr slot_type MakeSlot3() noexcept
{
return slot_type::create<Function3>();
}

constexpr slot_type MakeSlot4() noexcept
{
return slot_type::create<Function4>();
}

constexpr not_slot_type MakeSlot5() noexcept
{
return not_slot_type::create<Function5>();
}
```

**Define the signals as `constexpr`**  

```cpp
// Define the signal and connect as constexpr
constexpr signal_type({ MakeSlot1(), MakeSlot2(), MakeSlot3() });

// Define the signal and connect as constexpr.
// Static assert "Number of slots exceeds capacity"
constexpr signal_type({ MakeSlot1(), MakeSlot2(), MakeSlot3(), MakeSlot4() });

// Define the signal and connect as constexpr.
// Static assert "All slots must be slot_type"
constexpr signal_type({ MakeSlot1(), MakeSlot2(), MakeSlot5() });
```

**Defining the signal at runtime**  

```cpp
// Define the signal.
signal_type signal;

// Connect one at a time.
signal.connect(MakeSlot1());
signal.connect(MakeSlot2());
signal.connect(MakeSlot3());

// Connect using initializer_list.
signal.connect({ MakeSlot1(), MakeSlot2(), MakeSlot3() });

// Connect using span.
const slot_type slot_list[] = { MakeSlot1(), MakeSlot2(), MakeSlot3() };
signal.connect(slot_list);
```

**Checking the status**  

```cpp
// Define the signal.
signal_type signal;

signal.max_size()             // Returns 3
signal.size()                 // Returns 0
signal.available()            // Returns 3
signal.empty();               // Returns true
signal.full();                // Returns false
signal.connected(MakeSlot1()) // Returns false
signal.connected(MakeSlot2()) // Returns false
signal.connected(MakeSlot3()) // Returns false

signal.connect(MakeSlot1());  // Returns true

signal.max_size()             // Returns 3
signal.size()                 // Returns 1
signal.available()            // Returns 2
signal.empty();               // Returns false
signal.full();                // Returns false
signal.connected(MakeSlot1()) // Returns true
signal.connected(MakeSlot2()) // Returns false
signal.connected(MakeSlot3()) // Returns false

signal.connect(MakeSlot1());  // Already connected. Returns true

signal.max_size()             // Returns 3
signal.size()                 // Returns 1
signal.available()            // Returns 2
signal.empty();               // Returns false
signal.full();                // Returns false
signal.connected(MakeSlot1()) // Returns true
signal.connected(MakeSlot2()) // Returns false
signal.connected(MakeSlot3()) // Returns false

signal.connect(MakeSlot2());  // Returns true

signal.max_size()             // Returns 3
signal.size()                 // Returns 2
signal.available()            // Returns 1
signal.empty();               // Returns false
signal.full();                // Returns false
signal.connected(MakeSlot1()) // Returns true
signal.connected(MakeSlot2()) // Returns true
signal.connected(MakeSlot3()) // Returns false

signal.connect(MakeSlot3());  // Returns true

signal.max_size()             // Returns 3
signal.size()                 // Returns 3
signal.available()            // Returns 0
signal.empty();               // Returns false
signal.full();                // Returns true
signal.connected(MakeSlot1()) // Returns true
signal.connected(MakeSlot2()) // Returns true
signal.connected(MakeSlot3()) // Returns true

signal.connect(MakeSlot4());  // ETL_ASSERT etl::signal_full. Returns false
```

**Calling the signal**  

```cpp
signal_type signal({ MakeSlot1(), MakeSlot2(), MakeSlot3() });

signal(1, 2); // Call all of the slots with the parameters 1 & 2
```

**Output**  
```
Function1: 1,2
Function2: 1,2
Function3: 1,2
```