LUA SCRIPTING
=============

mpv can load Lua scripts. Scripts passed to the ``--script`` option, or found in
the ``scripts`` subdirectory of the mpv configuration directory (usually
``~/.config/mpv/scripts/``) will be loaded on program start. mpv also appends the
``scripts`` subdirectory to the end of Lua's path so you can import scripts from
there too. Since it's added to the end, don't name scripts you want to import
the same as Lua libraries because they will be overshadowed by them.

mpv provides the built-in module ``mp``, which contains functions to send
commands to the mpv core and to retrieve information about playback state, user
settings, file information, and so on.

These scripts can be used to control mpv in a similar way to slave mode.
Technically, the Lua code uses the client API internally.

Example
-------

A script which leaves fullscreen mode when the player is paused:

::

    function on_pause_change(name, value)
        if value == true then
            mp.set_property("fullscreen", "no")
        end
    end
    mp.observe_property("pause", "bool", on_pause_change)


Details on the script initialization and lifecycle
--------------------------------------------------

Your script will be loaded by the player at program start from the ``scripts``
configuration subdirectory, or from a path specified with the ``--script``
option. Some scripts are loaded internally (like ``--osc``). Each script runs in
its own thread. Your script is first run "as is", and once that is done, the event loop
is entered. This event loop will dispatch events received by mpv and call your
own event handlers which you have registered with ``mp.register_event``, or
timers added with ``mp.add_timeout`` or similar. Note that since the
script starts execution concurrently with player initialization, some properties
may not be populated with meaningful values until the relevant subsystems have
initialized.

When the player quits, all scripts will be asked to terminate. This happens via
a ``shutdown`` event, which by default will make the event loop return. If your
script got into an endless loop, mpv will probably behave fine during playback,
but it won't terminate when quitting, because it's waiting on your script.

Internally, the C code will call the Lua function ``mp_event_loop`` after
loading a Lua script. This function is normally defined by the default prelude
loaded before your script (see ``player/lua/defaults.lua`` in the mpv sources).
The event loop will wait for events and dispatch events registered with
``mp.register_event``. It will also handle timers added with ``mp.add_timeout``
and similar (by waiting with a timeout).

Since mpv 0.6.0, the player will wait until the script is fully loaded before
continuing normal operation. The player considers a script as fully loaded as
soon as it starts waiting for mpv events (or it exits). In practice this means
the player will more or less hang until the script returns from the main chunk
(and ``mp_event_loop`` is called), or the script calls ``mp_event_loop`` or
``mp.dispatch_events`` directly. This is done to make it possible for a script
to fully setup event handlers etc. before playback actually starts. In older
mpv versions, this happened asynchronously. With mpv 0.29.0, this changes
slightly, and it merely waits for scripts to be loaded in this manner before
starting playback as part of the player initialization phase. Scripts run though
initialization in parallel. This might change again.

mp functions
------------

The ``mp`` module is preloaded, although it can be loaded manually with
``require 'mp'``. It provides the core client API.

``mp.command(string)``
    Run the given command. This is similar to the commands used in input.conf.
    See `List of Input Commands`_.

    By default, this will show something on the OSD (depending on the command),
    as if it was used in ``input.conf``. See `Input Command Prefixes`_ how
    to influence OSD usage per command.

    Returns ``true`` on success, or ``nil, error`` on error.

``mp.commandv(arg1, arg2, ...)``
    Similar to ``mp.command``, but pass each command argument as separate
    parameter. This has the advantage that you don't have to care about
    quoting and escaping in some cases.

    Example:

    ::

        mp.command("loadfile " .. filename .. " append")
        mp.commandv("loadfile", filename, "append")

    These two commands are equivalent, except that the first version breaks
    if the filename contains spaces or certain special characters.

    Note that properties are *not* expanded.  You can use either ``mp.command``,
    the ``expand-properties`` prefix, or the ``mp.get_property`` family of
    functions.

    Unlike ``mp.command``, this will not use OSD by default either (except
    for some OSD-specific commands).

``mp.command_native(table [,def])``
    Similar to ``mp.commandv``, but pass the argument list as table. This has
    the advantage that in at least some cases, arguments can be passed as
    native types. It also allows you to use named argument.

    If the table is an array, each array item is like an argument in
    ``mp.commandv()`` (but can be a native type instead of a string).

    If the table contains string keys, it's interpreted as command with named
    arguments. This requires at least an entry with the key ``name`` to be
    present, which must be a string, and contains the command name. The special
    entry ``_flags`` is optional, and if present, must be an array of
    `Input Command Prefixes`_ to apply. All other entries are interpreted as
    arguments.

    Returns a result table on success (usually empty), or ``def, error`` on
    error. ``def`` is the second parameter provided to the function, and is
    nil if it's missing.

``mp.command_native_async(table [,fn])``
    Like ``mp.command_native()``, but the command is ran asynchronously (as far
    as possible), and upon completion, fn is called. fn has two arguments:
    ``fn(success, result, error)``. ``success`` is always a Boolean and is true
    if the command was successful, otherwise false. The second parameter is
    the result value (can be nil) in case of success, nil otherwise (as returned
    by ``mp.command_native()``). The third parameter is the error string in case
    of an error, nil otherwise.

    Returns a table with undefined contents, which can be used as argument for
    ``mp.abort_async_command``.

    If starting the command failed for some reason, ``nil, error`` is returned,
    and ``fn`` is called indicating failure, using the same error value.

``mp.abort_async_command(t)``
    Abort a ``mp.command_native_async`` call. The argument is the return value
    of that command (which starts asynchronous execution of the command).
    Whether this works and how long it takes depends on the command and the
    situation. The abort call itself is asynchronous. Does not return anything.

``mp.get_property(name [,def])``
    Return the value of the given property as string. These are the same
    properties as used in input.conf. See `Properties`_ for a list of
    properties. The returned string is formatted similar to ``${=name}``
    (see `Property Expansion`_).

    Returns the string on success, or ``def, error`` on error. ``def`` is the
    second parameter provided to the function, and is nil if it's missing.

``mp.get_property_osd(name [,def])``
    Similar to ``mp.get_property``, but return the property value formatted for
    OSD. This is the same string as printed with ``${name}`` when used in
    input.conf.

    Returns the string on success, or ``def, error`` on error. ``def`` is the
    second parameter provided to the function, and is an empty string if it's
    missing. Unlike ``get_property()``, assigning the return value to a variable
    will always result in a string.

``mp.get_property_bool(name [,def])``
    Similar to ``mp.get_property``, but return the property value as Boolean.

    Returns a Boolean on success, or ``def, error`` on error.

``mp.get_property_number(name [,def])``
    Similar to ``mp.get_property``, but return the property value as number.

    Note that while Lua does not distinguish between integers and floats,
    mpv internals do. This function simply request a double float from mpv,
    and mpv will usually convert integer property values to float.

    Returns a number on success, or ``def, error`` on error.

``mp.get_property_native(name [,def])``
    Similar to ``mp.get_property``, but return the property value using the best
    Lua type for the property. Most time, this will return a string, Boolean,
    or number. Some properties (for example ``chapter-list``) are returned as
    tables.

    Returns a value on success, or ``def, error`` on error. Note that ``nil``
    might be a possible, valid value too in some corner cases.

``mp.set_property(name, value)``
    Set the given property to the given string value. See ``mp.get_property``
    and `Properties`_ for more information about properties.

    Returns true on success, or ``nil, error`` on error.

``mp.set_property_bool(name, value)``
    Similar to ``mp.set_property``, but set the given property to the given
    Boolean value.

``mp.set_property_number(name, value)``
    Similar to ``mp.set_property``, but set the given property to the given
    numeric value.

    Note that while Lua does not distinguish between integers and floats,
    mpv internals do. This function will test whether the number can be
    represented as integer, and if so, it will pass an integer value to mpv,
    otherwise a double float.

``mp.set_property_native(name, value)``
    Similar to ``mp.set_property``, but set the given property using its native
    type.

    Since there are several data types which cannot represented natively in
    Lua, this might not always work as expected. For example, while the Lua
    wrapper can do some guesswork to decide whether a Lua table is an array
    or a map, this would fail with empty tables. Also, there are not many
    properties for which it makes sense to use this, instead of
    ``set_property``, ``set_property_bool``, ``set_property_number``.
    For these reasons, this function should probably be avoided for now, except
    for properties that use tables natively.

``mp.get_time()``
    Return the current mpv internal time in seconds as a number. This is
    basically the system time, with an arbitrary offset.

``mp.add_key_binding(key, name|fn [,fn [,flags]])``
    Register callback to be run on a key binding. The binding will be mapped to
    the given ``key``, which is a string describing the physical key. This uses
    the same key names as in input.conf, and also allows combinations
    (e.g. ``ctrl+a``). If the key is empty or ``nil``, no physical key is
    registered, but the user still can create own bindings (see below).

    After calling this function, key presses will cause the function ``fn`` to
    be called (unless the user remapped the key with another binding).

    The ``name`` argument should be a short symbolic string. It allows the user
    to remap the key binding via input.conf using the ``script-message``
    command, and the name of the key binding (see below for
    an example). The name should be unique across other bindings in the same
    script - if not, the previous binding with the same name will be
    overwritten. You can omit the name, in which case a random name is generated
    internally.

    The last argument is used for optional flags. This is a table, which can
    have the following entries:

        ``repeatable``
            If set to ``true``, enables key repeat for this specific binding.

        ``complex``
            If set to ``true``, then ``fn`` is called on both key up and down
            events (as well as key repeat, if enabled), with the first
            argument being a table. This table has an ``event`` entry, which
            is set to one of the strings ``down``, ``repeat``, ``up`` or
            ``press`` (the latter if key up/down can't be tracked). It further
            has an ``is_mouse`` entry, which tells whether the event was caused
            by a mouse button.

    Internally, key bindings are dispatched via the ``script-message-to`` or
    ``script-binding`` input commands and ``mp.register_script_message``.

    Trying to map multiple commands to a key will essentially prefer a random
    binding, while the other bindings are not called. It is guaranteed that
    user defined bindings in the central input.conf are preferred over bindings
    added with this function (but see ``mp.add_forced_key_binding``).

    Example:

    ::

        function something_handler()
            print("the key was pressed")
        end
        mp.add_key_binding("x", "something", something_handler)

    This will print the message ``the key was pressed`` when ``x`` was pressed.

    The user can remap these key bindings. Then the user has to put the
    following into their input.conf to remap the command to the ``y`` key:

    ::

        y script-binding something


    This will print the message when the key ``y`` is pressed. (``x`` will
    still work, unless the user remaps it.)

    You can also explicitly send a message to a named script only. Assume the
    above script was using the filename ``fooscript.lua``:

    ::

        y script-binding fooscript/something

``mp.add_forced_key_binding(...)``
    This works almost the same as ``mp.add_key_binding``, but registers the
    key binding in a way that will overwrite the user's custom bindings in their
    input.conf. (``mp.add_key_binding`` overwrites default key bindings only,
    but not those by the user's input.conf.)

``mp.remove_key_binding(name)``
    Remove a key binding added with ``mp.add_key_binding`` or
    ``mp.add_forced_key_binding``. Use the same name as you used when adding
    the bindings. It's not possible to remove bindings for which you omitted
    the name.

``mp.register_event(name, fn)``
    Call a specific function when an event happens. The event name is a string,
    and the function fn is a Lua function value.

    Some events have associated data. This is put into a Lua table and passed
    as argument to fn. The Lua table by default contains a ``name`` field,
    which is a string containing the event name. If the event has an error
    associated, the ``error`` field is set to a string describing the error,
    on success it's not set.

    If multiple functions are registered for the same event, they are run in
    registration order, which the first registered function running before all
    the other ones.

    Returns true if such an event exists, false otherwise.

    See `Events`_ and `List of events`_ for details.

``mp.unregister_event(fn)``
    Undo ``mp.register_event(..., fn)``. This removes all event handlers that
    are equal to the ``fn`` parameter. This uses normal Lua ``==`` comparison,
    so be careful when dealing with closures.

``mp.observe_property(name, type, fn)``
    Watch a property for changes. If the property ``name`` is changed, then
    the function ``fn(name)`` will be called. ``type`` can be ``nil``, or be
    set to one of ``none``, ``native``, ``bool``, ``string``, or ``number``.
    ``none`` is the same as ``nil``. For all other values, the new value of
    the property will be passed as second argument to ``fn``, using
    ``mp.get_property_<type>`` to retrieve it. This means if ``type`` is for
    example ``string``, ``fn`` is roughly called as in
    ``fn(name, mp.get_property_string(name))``.

    If possible, change events are coalesced. If a property is changed a bunch
    of times in a row, only the last change triggers the change function. (The
    exact behavior depends on timing and other things.)

    In some cases the function is not called even if the property changes.
    Whether this can happen depends on the property.

    If the ``type`` is ``none`` or ``nil``, sporadic property change events are
    possible. This means the change function ``fn`` can be called even if the
    property doesn't actually change.

``mp.unobserve_property(fn)``
    Undo ``mp.observe_property(..., fn)``. This removes all property handlers
    that are equal to the ``fn`` parameter. This uses normal Lua ``==``
    comparison, so be careful when dealing with closures.

``mp.add_timeout(seconds, fn)``
    Call the given function fn when the given number of seconds has elapsed.
    Note that the number of seconds can be fractional. For now, the timer's
    resolution may be as low as 50 ms, although this will be improved in the
    future.

    This is a one-shot timer: it will be removed when it's fired.

    Returns a timer object. See ``mp.add_periodic_timer`` for details.

``mp.add_periodic_timer(seconds, fn)``
    Call the given function periodically. This is like ``mp.add_timeout``, but
    the timer is re-added after the function fn is run.

    Returns a timer object. The timer object provides the following methods:
        ``stop()``
            Disable the timer. Does nothing if the timer is already disabled.
            This will remember the current elapsed time when stopping, so that
            ``resume()`` essentially unpauses the timer.

        ``kill()``
            Disable the timer. Resets the elapsed time. ``resume()`` will
            restart the timer.

        ``resume()``
            Restart the timer. If the timer was disabled with ``stop()``, this
            will resume at the time it was stopped. If the timer was disabled
            with ``kill()``, or if it's a previously fired one-shot timer (added
            with ``add_timeout()``), this starts the timer from the beginning,
            using the initially configured timeout.

        ``is_enabled()``
            Whether the timer is currently enabled or was previously disabled
            (e.g. by ``stop()`` or ``kill()``).

        ``timeout`` (RW)
            This field contains the current timeout period. This value is not
            updated as time progresses. It's only used to calculate when the
            timer should fire next when the timer expires.

            If you write this, you can call ``t:kill() ; t:resume()`` to reset
            the current timeout to the new one. (``t:stop()`` won't use the
            new timeout.)

        ``oneshot`` (RW)
            Whether the timer is periodic (``false``) or fires just once
            (``true``). This value is used when the timer expires (but before
            the timer callback function fn is run).

    Note that these are methods, and you have to call them using ``:`` instead
    of ``.`` (Refer to http://www.lua.org/manual/5.2/manual.html#3.4.9 .)

    Example:

    ::

        seconds = 0
        timer = mp.add_periodic_timer(1, function()
            print("called every second")
            # stop it after 10 seconds
            seconds = seconds + 1
            if seconds >= 10 then
                timer:kill()
            end
        end)


``mp.get_opt(key)``
    Return a setting from the ``--script-opts`` option. It's up to the user and
    the script how this mechanism is used. Currently, all scripts can access
    this equally, so you should be careful about collisions.

``mp.get_script_name()``
    Return the name of the current script. The name is usually made of the
    filename of the script, with directory and file extension removed. If
    there are several scripts which would have the same name, it's made unique
    by appending a number.

    .. admonition:: Example

        The script ``/path/to/fooscript.lua`` becomes ``fooscript``.

``mp.osd_message(text [,duration])``
    Show an OSD message on the screen. ``duration`` is in seconds, and is
    optional (uses ``--osd-duration`` by default).

Advanced mp functions
---------------------

These also live in the ``mp`` module, but are documented separately as they
are useful only in special situations.

``mp.suspend()``
    This function has been deprecated in mpv 0.21.0 and does nothing starting
    with mpv 0.23.0 (no replacement).

``mp.resume()``
    This function has been deprecated in mpv 0.21.0 and does nothing starting
    with mpv 0.23.0 (no replacement).

``mp.resume_all()``
    This function has been deprecated in mpv 0.21.0 and does nothing starting
    with mpv 0.23.0 (no replacement).

``mp.get_wakeup_pipe()``
    Calls ``mpv_get_wakeup_pipe()`` and returns the read end of the wakeup
    pipe. This is deprecated, but still works. (See ``client.h`` for details.)

``mp.get_next_timeout()``
    Return the relative time in seconds when the next timer (``mp.add_timeout``
    and similar) expires. If there is no timer, return ``nil``.

``mp.dispatch_events([allow_wait])``
    This can be used to run custom event loops. If you want to have direct
    control what the Lua script does (instead of being called by the default
    event loop), you can set the global variable ``mp_event_loop`` to your
    own function running the event loop. From your event loop, you should call
    ``mp.dispatch_events()`` to dequeue and dispatch mpv events.

    If the ``allow_wait`` parameter is set to ``true``, the function will block
    until the next event is received or the next timer expires. Otherwise (and
    this is the default behavior), it returns as soon as the event loop is
    emptied. It's strongly recommended to use ``mp.get_next_timeout()`` and
    ``mp.get_wakeup_pipe()`` if you're interested in properly working
    notification of new events and working timers.

``mp.register_idle(fn)``
    Register an event loop idle handler. Idle handlers are called before the
    script goes to sleep after handling all new events. This can be used for
    example to delay processing of property change events: if you're observing
    multiple properties at once, you might not want to act on each property
    change, but only when all change notifications have been received.

``mp.unregister_idle(fn)``
    Undo ``mp.register_idle(fn)``. This removes all idle handlers that
    are equal to the ``fn`` parameter. This uses normal Lua ``==`` comparison,
    so be careful when dealing with closures.

``mp.enable_messages(level)``
    Set the minimum log level of which mpv message output to receive. These
    messages are normally printed to the terminal. By calling this function,
    you can set the minimum log level of messages which should be received with
    the ``log-message`` event. See the description of this event for details.
    The level is a string, see ``msg.log`` for allowed log levels.

``mp.register_script_message(name, fn)``
    This is a helper to dispatch ``script-message`` or ``script-message-to``
    invocations to Lua functions. ``fn`` is called if ``script-message`` or
    ``script-message-to`` (with this script as destination) is run
    with ``name`` as first parameter. The other parameters are passed to ``fn``.
    If a message with the given name is already registered, it's overwritten.

    Used by ``mp.add_key_binding``, so be careful about name collisions.

``mp.unregister_script_message(name)``
    Undo a previous registration with ``mp.register_script_message``. Does
    nothing if the ``name`` wasn't registered.

mp.msg functions
----------------

This module allows outputting messages to the terminal, and can be loaded
with ``require 'mp.msg'``.

``msg.log(level, ...)``
    The level parameter is the message priority. It's a string and one of
    ``fatal``, ``error``, ``warn``, ``info``, ``v``, ``debug``, ``trace``. The
    user's settings will determine which of these messages will be
    visible. Normally, all messages are visible, except ``v``, ``debug`` and
    ``trace``.

    The parameters after that are all converted to strings. Spaces are inserted
    to separate multiple parameters.

    You don't need to add newlines.

``msg.fatal(...)``, ``msg.error(...)``, ``msg.warn(...)``, ``msg.info(...)``, ``msg.verbose(...)``, ``msg.debug(...)``, ``msg.trace(...)``
    All of these are shortcuts and equivalent to the corresponding
    ``msg.log(level, ...)`` call.

mp.options functions
--------------------

mpv comes with a built-in module to manage options from config-files and the
command-line. All you have to do is to supply a table with default options to
the read_options function. The function will overwrite the default values
with values found in the config-file and the command-line (in that order).

``options.read_options(table [, identifier])``
    A ``table`` with key-value pairs. The type of the default values is
    important for converting the values read from the config file or
    command-line back. Do not use ``nil`` as a default value!

    The ``identifier`` is used to identify the config-file and the command-line
    options. These needs to unique to avoid collisions with other scripts.
    Defaults to ``mp.get_script_name()``.


Example implementation::

    require 'mp.options'
    local options = {
        optionA = "defaultvalueA",
        optionB = -0.5,
        optionC = true,
    }
    read_options(options, "myscript")
    print(options.optionA)


The config file will be stored in ``script-opts/identifier.conf`` in mpv's user
folder. Comment lines can be started with # and stray spaces are not removed.
Boolean values will be represented with yes/no.

Example config::

    # comment
    optionA=Hello World
    optionB=9999
    optionC=no


Command-line options are read from the ``--script-opts`` parameter. To avoid
collisions, all keys have to be prefixed with ``identifier-``.

Example command-line::

     --script-opts=myscript-optionA=TEST,myscript-optionB=0,myscript-optionC=yes


mp.utils functions
------------------

This built-in module provides generic helper functions for Lua, and have
strictly speaking nothing to do with mpv or video/audio playback. They are
provided for convenience. Most compensate for Lua's scarce standard library.

Be warned that any of these functions might disappear any time. They are not
strictly part of the guaranteed API.

``utils.getcwd()``
    Returns the directory that mpv was launched from. On error, ``nil, error``
    is returned.

``utils.readdir(path [, filter])``
    Enumerate all entries at the given path on the filesystem, and return them
    as array. Each entry is a directory entry (without the path).
    The list is unsorted (in whatever order the operating system returns it).

    If the ``filter`` argument is given, it must be one of the following
    strings:

        ``files``
            List regular files only. This excludes directories, special files
            (like UNIX device files or FIFOs), and dead symlinks. It includes
            UNIX symlinks to regular files.

        ``dirs``
            List directories only, or symlinks to directories. ``.`` and ``..``
            are not included.

        ``normal``
            Include the results of both ``files`` and ``dirs``. (This is the
            default.)

        ``all``
            List all entries, even device files, dead symlinks, FIFOs, and the
            ``.`` and ``..`` entries.

    On error, ``nil, error`` is returned.

``utils.file_info(path)``
    Stats the given path for information and returns a table with the
    following entries:

        ``mode``
            protection bits (on Windows, always 755 (octal) for directories
            and 644 (octal) for files)
        ``size``
            size in bytes
        ``atime``
            time of last access
        ``mtime``
            time of last modification
        ``ctime``
            time of last metadata change (Linux) / time of creation (Windows)
        ``is_file``
            Whether ``path`` is a regular file (boolean)
        ``is_dir``
            Whether ``path`` is a directory (boolean)

    ``mode`` and ``size`` are integers.
    Timestamps (``atime``, ``mtime`` and ``ctime``) are integer seconds since
    the Unix epoch (Unix time).
    The booleans ``is_file`` and ``is_dir`` are provided as a convenience;
    they can be and are derived from ``mode``.

    On error (eg. path does not exist), ``nil, error`` is returned.

``utils.split_path(path)``
    Split a path into directory component and filename component, and return
    them. The first return value is always the directory. The second return
    value is the trailing part of the path, the directory entry.

``utils.join_path(p1, p2)``
    Return the concatenation of the 2 paths. Tries to be clever. For example,
    if ```p2`` is an absolute path, p2 is returned without change.

``utils.subprocess(t)``
    Runs an external process and waits until it exits. Returns process status
    and the captured output. This is a legacy wrapper around calling the
    ``subprocess`` command with ``mp.command_native``. It does the following
    things:

    - copy the table ``t``
    - rename ``cancellable`` field to ``playback_only``
    - rename ``max_size`` to ``capture_size``
    - set ``capture_stdout`` field to ``true`` if unset
    - set ``name`` field to ``subprocess``
    - call ``mp.command_native(copied_t)``
    - if the command failed, create a dummy result table
    - copy ``error_string`` to ``error`` field if the string is non-empty
    - return the result table

    It is recommended to use ``mp.command_native`` or ``mp.command_native_async``
    directly, instead of calling this legacy wrapper. It is for compatibility
    only.

``utils.subprocess_detached(t)``
    Runs an external process and detaches it from mpv's control.

    The parameter ``t`` is a table. The function reads the following entries:

        ``args``
            Array of strings of the same semantics as the ``args`` used in the
            ``subprocess`` function.

    The function returns ``nil``.

    This is a legacy wrapper around calling the ``run`` command with
    ``mp.commandv`` and other functions.

``utils.getpid()``
    Returns the process ID of the running mpv process. This can be used to identify
    the calling mpv when launching (detached) subprocesses.

``utils.parse_json(str [, trail])``
    Parses the given string argument as JSON, and returns it as a Lua table. On
    error, returns ``nil, error``. (Currently, ``error`` is just a string
    reading ``error``, because there is no fine-grained error reporting of any
    kind.)

    The returned value uses similar conventions as ``mp.get_property_native()``
    to distinguish empty objects and arrays.

    If the ``trail`` parameter is ``true`` (or any value equal to ``true``),
    then trailing non-whitespace text is tolerated by the function, and the
    trailing text is returned as 3rd return value. (The 3rd return value is
    always there, but with ``trail`` set, no error is raised.)

``utils.format_json(v)``
    Format the given Lua table (or value) as a JSON string and return it. On
    error, returns ``nil, error``. (Errors usually only happen on value types
    incompatible with JSON.)

    The argument value uses similar conventions as ``mp.set_property_native()``
    to distinguish empty objects and arrays.

``utils.to_string(v)``
    Turn the given value into a string. Formats tables and their contents. This
    doesn't do anything special; it is only needed because Lua is terrible.

Events
------

Events are notifications from player core to scripts. You can register an
event handler with ``mp.register_event``.

Note that all scripts (and other parts of the player) receive events equally,
and there's no such thing as blocking other scripts from receiving events.

Example:

::

    function my_fn(event)
        print("start of playback!")
    end

    mp.register_event("file-loaded", my_fn)



List of events
--------------

``start-file``
    Happens right before a new file is loaded. When you receive this, the
    player is loading the file (or possibly already done with it).

``end-file``
    Happens after a file was unloaded. Typically, the player will load the
    next file right away, or quit if this was the last file.

    The event has the ``reason`` field, which takes one of these values:

    ``eof``
        The file has ended. This can (but doesn't have to) include
        incomplete files or broken network connections under
        circumstances.

    ``stop``
        Playback was ended by a command.

    ``quit``
        Playback was ended by sending the quit command.

    ``error``
        An error happened. In this case, an ``error`` field is present with
        the error string.

    ``redirect``
        Happens with playlists and similar. Details see
        ``MPV_END_FILE_REASON_REDIRECT`` in the C API.

    ``unknown``
        Unknown. Normally doesn't happen, unless the Lua API is out of sync
        with the C API. (Likewise, it could happen that your script gets
        reason strings that did not exist yet at the time your script was
        written.)

``file-loaded``
    Happens after a file was loaded and begins playback.

``seek``
    Happens on seeking. (This might include cases when the player seeks
    internally, even without user interaction. This includes e.g. segment
    changes when playing ordered chapters Matroska files.)

``playback-restart``
    Start of playback after seek or after file was loaded.

``idle``
    Idle mode is entered. This happens when playback ended, and the player was
    started with ``--idle`` or ``--force-window``. This mode is implicitly ended
    when the ``start-file`` or ``shutdown`` events happen.

``tick``
    Called after a video frame was displayed. This is a hack, and you should
    avoid using it. Use timers instead and maybe watch pausing/unpausing events
    to avoid wasting CPU when the player is paused.

``shutdown``
    Sent when the player quits, and the script should terminate. Normally
    handled automatically. See `Details on the script initialization and lifecycle`_.

``log-message``
    Receives messages enabled with ``mp.enable_messages``. The message data
    is contained in the table passed as first parameter to the event handler.
    The table contains, in addition to the default event fields, the following
    fields:

    ``prefix``
        The module prefix, identifies the sender of the message. This is what
        the terminal player puts in front of the message text when using the
        ``--v`` option, and is also what is used for ``--msg-level``.

    ``level``
        The log level as string. See ``msg.log`` for possible log level names.
        Note that later versions of mpv might add new levels or remove
        (undocumented) existing ones.

    ``text``
        The log message. The text will end with a newline character. Sometimes
        it can contain multiple lines.

    Keep in mind that these messages are meant to be hints for humans. You
    should not parse them, and prefix/level/text of messages might change
    any time.

``get-property-reply``
    Undocumented (not useful for Lua scripts).

``set-property-reply``
    Undocumented (not useful for Lua scripts).

``command-reply``
    Undocumented (not useful for Lua scripts).

``client-message``
    Undocumented (used internally).

``video-reconfig``
    Happens on video output or filter reconfig.

``audio-reconfig``
    Happens on audio output or filter reconfig.

The following events also happen, but are deprecated: ``tracks-changed``,
``track-switched``, ``pause``, ``unpause``, ``metadata-update``,
``chapter-change``. Use ``mp.observe_property()`` instead.

Extras
------

This documents experimental features, or features that are "too special" to
guarantee a stable interface.

``mp.add_hook(type, priority, fn)``
    Add a hook callback for ``type`` (a string identifying a certain kind of
    hook). These hooks allow the player to call script functions and wait for
    their result (normally, the Lua scripting interface is asynchronous from
    the point of view of the player core). ``priority`` is an arbitrary integer
    that allows ordering among hooks of the same kind. Using the value 50 is
    recommended as neutral default value. ``fn`` is the function that will be
    called during execution of the hook.

    See `Hooks`_ for currently existing hooks and what they do - only the hook
    list is interesting; handling hook execution is done by the Lua script
    function automatically.
