# `Membrane.Element.Action`
[🔗](https://github.com/membraneframework/membrane-core/blob/v1.2.7/lib/membrane/element/action.ex#L1)

This module contains type specifications of actions that can be returned
from element callbacks.

Returning actions is a way of element interaction with
other elements and parts of framework. Each action may be returned by any
callback unless explicitly stated otherwise.

# `buffer`

```elixir
@type buffer() ::
  {:buffer, {Membrane.Pad.ref(), Membrane.Buffer.t() | [Membrane.Buffer.t()]}}
```

Sends buffers through a pad.

The pad must have output direction.

Allowed only when playback is playing.

# `common_actions`

```elixir
@type common_actions() ::
  notify_parent()
  | start_timer()
  | timer_interval()
  | stop_timer()
  | terminate()
  | split()
```

Action that can be always returned from each of the callbacks.

# `demand`

```elixir
@type demand() :: {:demand, {Membrane.Pad.ref(), demand_size()} | Membrane.Pad.ref()}
```

Makes a demand on a pad.

The pad must have input direction and work in `:manual` flow control mode. This action does NOT
entail _sending_ demand through the pad, but just _requesting_ some amount
of data from pad's internal queue, which _sends_ demands automatically when it
runs out of data.
If there is any data available at the pad, the data is passed to
`c:Membrane.WithInputPads.handle_buffer/4` callback. Invoked callback is
guaranteed not to receive more data than demanded.

Demand size can be either a non-negative integer, that overrides existing demand,
or a function that is passed current demand, and is to return the new demand. In case only pad
is specified, the demand size defaults to 1.

Allowed only when playback is playing.

# `demand_size`

```elixir
@type demand_size() :: pos_integer() | (pos_integer() -> non_neg_integer())
```

# `end_of_stream`

```elixir
@type end_of_stream() :: {:end_of_stream, Membrane.Pad.ref()}
```

Marks that processing via a pad (output) has been finished and the pad instance
won't be used anymore.

Triggers `end_of_stream/3` callback at the receiver element.
Allowed only when playback is in playing state.

# `event`

```elixir
@type event() :: {:event, {Membrane.Pad.ref(), Membrane.Event.t()}}
```

Sends an event through a pad (input or output).

Allowed only when playback is `playing`.

# `forward`

```elixir
@type forward() ::
  {:forward,
   Membrane.Buffer.t()
   | [Membrane.Buffer.t()]
   | Membrane.StreamFormat.t()
   | Membrane.Event.t()
   | :end_of_stream}
```

Sends buffers/stream format/event/end of stream to all output pads of element (or to input
pads when event occurs on the output pad).

Used by default implementations of
`c:Membrane.Element.WithInputPads.handle_stream_format/4`,
`c:Membrane.Element.Base.handle_event/4` and
`c:Membrane.Element.WithInputPads.handle_end_of_stream/3` callbacks in filter.

Allowed only when _all_ below conditions are met:
- element is filter,
- callback is `c:Membrane.Element.WithInputPads.handle_buffer/4`,
`c:Membrane.Element.WithInputPads.handle_stream_format/4`,
`c:Membrane.Element.Base.handle_event/4` or `c:Membrane.Element.WithInputPads.handle_end_of_stream/3`
- playback is `playing`

Keep in mind that `c:Membrane.WithInputPads.handle_buffer/4` can only
forward buffers, `c:Membrane.Element.WithInputPads.handle_stream_format/4` - stream formats.
`c:Membrane.Element.Base.handle_event/4` - events and
`c:Membrane.Element.WithInputPads.handle_end_of_stream/3` - ends of streams.

# `latency`

```elixir
@type latency() :: {:latency, latency :: Membrane.Time.non_neg()}
```

This action sets the latency for the element.

This action is permitted only in callback `c:Membrane.Element.Base.handle_init/2`.

The example of usage of these actions is [there](../../../guides/timer.md)

# `notify_parent`

```elixir
@type notify_parent() :: {:notify_parent, Membrane.ChildNotification.t()}
```

Sends a message to the parent.

# `pause_auto_demand`

```elixir
@type pause_auto_demand() ::
  {:pause_auto_demand, Membrane.Pad.ref() | [Membrane.Pad.ref()]}
```

Pauses auto-demanding on the specific pad.

The pad must have input direction and work in `:auto` flow control mode.

This action does not guarantee that no more buffers will arrive on the specific pad,
but ensures, that demand on this pad will not increase until returning
`Membrane.Element.Action.resume_auto_demand()` action. Number of buffers, that will
arrive on the pad, depends on the behaviour of the elements earlier in the pipeline.

When auto-demanding is already paused, this action has no effect.

# `redemand`

```elixir
@type redemand() :: {:redemand, Membrane.Pad.ref() | [Membrane.Pad.ref()]}
```

Executes `c:Membrane.Element.WithOutputPads.handle_demand/5` callback
for the given pad (or pads), that have demand greater than 0.

The pad must have output direction and work in pull mode.

## Redemand in Sources and Endpoints

In case of Sources and Endpoints, `:redemand` is just a helper that simplifies element's code.
The element doesn't need to generate the whole demand synchronously at `handle_demand`
or store current demand size in its state, but it can just generate one buffer
and return `:redemand` action.
If there is still one or more buffers to produce, returning `:redemand` triggers
the next invocation of `handle_demand`. In such case, the element is to produce
next buffer and call `:redemand` again.
If there are no more buffers demanded, `handle_demand` is not invoked and
the loop ends.
One more advantage of the approach with `:redemand` action is that produced buffers
are sent one after another in separate messages and this can possibly improve
the latency.

## Redemand in Filters

Redemand in Filters is useful in a situation where not the entire demand of
output pad has been satisfied and there is a need to send a demand for additional
buffers through the input pad.
A typical example of this situation is a parser that has not demanded enough
bytes to parse the whole frame.

## Usage limitations
Allowed only when playback is playing.

# `resume_auto_demand`

```elixir
@type resume_auto_demand() ::
  {:resume_auto_demand, Membrane.Pad.ref() | [Membrane.Pad.ref()]}
```

Resumes auto-demanding on the specific pad.

The pad must have input direction and work in `:auto` flow control mode.

This action reverts the effects of `Membrane.Element.Action.pause_auto_demand()` action.

When auto demanding is not paused, this action has no effect.

# `setup`

```elixir
@type setup() :: {:setup, :incomplete | :complete}
```

Action that manages the end of the component setup.

By default, component setup ends with the end of `c:Membrane.Element.Base.handle_setup/2` callback.
If `{:setup, :incomplete}` is returned there, setup lasts until `{:setup, :complete}`
is returned from antoher callback.

Untils the setup lasts, the component won't enter `:playing` playback.

# `split`

```elixir
@type split() :: {:split, {callback_name :: atom(), args_list :: [[any()]]}}
```

Allows to split callback execution into multiple applications of another callback
(called from now sub-callback).

Executions are synchronous in the element process, and each of them passes
subsequent arguments from the args_list, along with the element state (passed
as the last argument each time).

Return value of each execution of sub-callback can be any valid return value
of the original callback (this also means sub-callback can return any action
valid for the original callback, unless explicitly stated). Returned actions
are executed immediately (they are NOT accumulated and executed after all
sub-callback executions are finished).

Useful when a long action is to be undertaken, and partial results need to
be returned before entire process finishes.

# `start_timer`

```elixir
@type start_timer() ::
  {:start_timer,
   {timer_id :: any(),
    interval :: Ratio.t() | Membrane.Time.non_neg() | :no_interval}
   | {timer_id :: any(),
      interval :: Ratio.t() | Membrane.Time.non_neg() | :no_interval,
      clock :: Membrane.Clock.t()}}
```

Starts a timer that will invoke `c:Membrane.Element.Base.handle_tick/3` callback
every `interval` according to the given `clock`.

The timer's `id` is passed to the `c:Membrane.Element.Base.handle_tick/3`
callback and can be used for changing its interval via `t:timer_interval/0`
or stopping it via `t:stop_timer/0`.

If `interval` is set to `:no_interval`, the timer won't issue any ticks until
the interval is set with `t:timer_interval/0` action.

If no `clock` is passed, parent's clock is chosen.

Timers use `Process.send_after/3` under the hood.

# `stop_timer`

```elixir
@type stop_timer() :: {:stop_timer, timer_id :: any()}
```

Stops a timer started with `t:start_timer/0` action.

This action is atomic: stopping timer guarantees that no ticks will arrive from it.

# `stream_actions`

```elixir
@type stream_actions() ::
  event()
  | stream_format()
  | buffer()
  | demand()
  | pause_auto_demand()
  | resume_auto_demand()
  | end_of_stream()
  | redemand()
```

Actions that can be returned from callbacks when the element is in `playback: :playing` state.

# `stream_format`

```elixir
@type stream_format() ::
  {:stream_format, {Membrane.Pad.ref(), Membrane.StreamFormat.t()}}
```

Sends stream format through a pad.

The pad must have output direction. Sent stream format must fit constraints on the pad.

Allowed only when playback is `playing`.

# `t`

```elixir
@type t() :: common_actions() | stream_actions() | latency() | forward() | setup()
```

Type that defines a union of actions that may be returned from most of the callbacks.

Depending on element type, callback, current playback and other
circumstances there may be different actions available.
Check the typespec and documentation of particular callbacks
for details.

# `terminate`

```elixir
@type terminate() ::
  {:terminate, reason :: :normal | :shutdown | {:shutdown, term()} | term()}
```

Terminates element with given reason.

Termination reason follows the OTP semantics:
- Use `:normal` for graceful termination. Allowed only when the parent already requested termination,
  i.e. after `c:Membrane.Element.Base.handle_terminate_request/2` is called
- If reason is neither `:normal`, `:shutdown` nor `{:shutdown, term}`, an error is logged

# `timer_interval`

```elixir
@type timer_interval() ::
  {:timer_interval,
   {timer_id :: any(),
    interval :: Ratio.t() | Membrane.Time.non_neg() | :no_interval}}
```

Changes interval of a timer started with `t:start_timer/0`.

Permitted only from `c:Membrane.Element.Base.handle_tick/3`, unless the interval
was previously set to `:no_interval`.

If the `interval` is `:no_interval`, the timer won't issue any ticks until
another `t:timer_interval/0` action. Otherwise, the timer will issue ticks every
new `interval`. The next tick after interval change is scheduled at
`new_interval + previous_time`, where previous_time is the time of the latest
tick or the time of returning `t:start_timer/0` action if no tick has been
sent yet. Note that if `current_time - previous_time > new_interval`, a burst
of `div(current_time - previous_time, new_interval)` ticks is issued immediately.

---

*Consult [api-reference.md](api-reference.md) for complete listing*