hump/docs/timer.rst

hump.timer
==========

::

    Timer = require "hump.timer"

hump.timer offers a simple interface to schedule the execution of functions. It
is possible to run functions *after* and *for* some amount of time. For
example, a timer could be set to move critters every 5 seconds or to make the
player invincible for a short amount of time.

In addition to that, ``hump.timer`` offers various `tweening
<http://en.wikipedia.org/wiki/Inbetweening>`_ functions that make it
easier to produce `juicy games <http://www.youtube.com/watch?v=Fy0aCDmgnxg>`_.

**Example**::

    function love.keypressed(key)
        if key == ' ' then
            Timer.after(1, function() print("Hello, world!") end)
        end
    end
    
    function love.update(dt)
        Timer.update(dt)
    end

Function Reference
------------------

.. function:: Timer.new()

   :returns: A timer instance.


Creates a new timer instance that is independent of the global timer: It will
manage it's own list of scheduled functions and does not in any way affect the
the global timer. Likewise, the global timer does not affect timer instances.

.. note::
    If you don't need multiple independent schedulers, you can use the
    global/default timer (see examples).

**Example**::

    menuTimer = Timer.new()


.. function:: Timer.after(delay, func)

   :param number delay: Number of seconds the function will be delayed.
   :param function func: The function to be delayed.
   :returns: The timer handle. See also :func:`Timer.cancel`.


Schedule a function. The function will be executed after ``delay`` seconds have
elapsed, given that ``update(dt)`` is called every frame.

.. note::
    There is no guarantee that the delay will not be exceeded, it is only
    guaranteed that the function will *not* be executed *before* the delay has
    passed.

``func`` will receive itself as only parameter. This is useful to implement
periodic behavior (see the example).

**Examples**::

    -- grant the player 5 seconds of immortality
    player.isInvincible = true
    Timer.after(5, function() player.isInvincible = false end)

::

    -- print "foo" every second. See also every()
    Timer.after(1, function(func) print("foo") Timer.after(1, func) end)

::

    --Using a timer instance:
    menuTimer:after(1, finishAnimation)


.. function:: Timer.every(delay, func[, count])

   :param number delay: Number of seconds between two consecutive function calls.
   :param function func: The function to be called periodically.
   :param number count:  Number of times the function is to be called (optional).
   :returns: The timer handle. See also :func:`Timer.cancel`.


Add a function that will be called ``count`` times every ``delay`` seconds.

If ``count`` is omitted, the function will be called until it returns ``false``
or :func:`Timer.cancel` or :func:`Timer.clear` is called on the timer instance.

**Example**::

    -- toggle light on and off every second
    Timer.every(1, function() lamp:toggleLight() end)

::

    -- launch 5 fighters in quick succession (using a timer instance)
    mothership_timer.every(0.3, function() self:launchFighter() end, 5)

::

    -- flicker player's image as long as he is invincible
    Timer.every(0.1, function()
        player:flipImage()
        return player.isInvincible
    end)


.. function:: Timer.during(delay, func[, after])

   :param number delay: Number of seconds the func will be called.
   :param function func: The function to be called on ``update(dt)``.
   :param function after: A function to be called after delay seconds (optional).
   :returns: The timer handle. See also :func:`Timer.cancel`.


Run ``func(dt)`` for the next ``delay`` seconds. The function is called every
time ``update(dt)`` is called. Optionally run ``after()`` once ``delay``
seconds have passed.

``after()`` will receive itself as only parameter.

.. note::
    You should not add new timers in ``func(dt)``, as this can lead to random
    crashes.

**Examples**::

    -- play an animation for 5 seconds
    Timer.during(5, function(dt) animation:update(dt) end)

::

    -- shake the camera for one second
    local orig_x, orig_y = camera:pos()
    Timer.during(1, function()
        camera:lookAt(orig_x + math.random(-2,2), orig_y + math.random(-2,2))
    end, function()
        -- reset camera position
        camera:lookAt(orig_x, orig_y)
    end)

::

    player.isInvincible = true
    -- flash player for 3 seconds
    local t = 0
    player.timer.during(3, function(dt)
        t = t + dt
        player.visible = (t % .2) < .1
    end, function()
        -- make sure the player is visible after three seconds
        player.visible = true
        player.isInvincible = false
    end)


.. function:: Timer.cancel(handle)

   :param table handle:  The function to be canceled.

Prevent a timer from being executed in the future.

**Examples**::

    function tick()
        print('tick... tock...')
    end
    handle = Timer.every(1, tick)
    -- later
    Timer.cancel(handle) -- NOT: Timer.cancel(tick)

::

    -- using a timer instance
    function tick()
        print('tick... tock...')
    end
    handle = menuTimer:every(1, tick)
    -- later
    menuTimer:cancel(handle)


.. function:: Timer.clear()

Remove all timed and periodic functions. Functions that have not yet been
executed will discarded.

**Examples**::

    Timer.clear()

::

    menu_timer:clear()


.. function:: Timer.update(dt)

   :param number dt:  Time that has passed since the last ``update()``.

Update timers and execute functions if the deadline is reached. Call in
``love.update(dt)``.

**Examples**::

    function love.update(dt)
        do_stuff()
        Timer.update(dt)
    end

::

    -- using hump.gamestate and a timer instance
    function menuState:update(dt)
        self.timer.update(dt)
    end


.. function:: Timer.tween(duration, subject, target, method, after, ...)

   :param number duration: Duration of the tween.
   :param table subject: Object to be tweened.
   :param table target: Target values.
   :param string method: Tweening method, defaults to 'linear' (:ref:`see here
                         <tweening-methods>`, optional).
   :param function after: Function to execute after the tween has finished
                          (optiona).
   :param mixed ...:  Additional arguments to the *tweening* function.
   :returns: A timer handle.


`Tweening <http://en.wikipedia.org/wiki/Inbetweening>`_ (short for
in-betweening) is the process that happens between two defined states. For
example, a tween can be used to gradually fade out a graphic or move a text
message to the center of the screen. For more information why tweening should
be important to you, check out this great talk on `juicy games
<http://www.youtube.com/watch?v=Fy0aCDmgnxg>`_.

``hump.timer`` offers two interfaces for tweening: the low-level
:func:`Timer.during` and the higher level interface :func:`Timer.tween`.

To see which tweening methods hump offers, :ref:`see below <tweening-methods>`.

**Examples**::

    function love.load()
        color = {0, 0, 0}
        Timer.tween(10, color, {255, 255, 255}, 'in-out-quad')
    end

    function love.update(dt)
        Timer.update(dt)
    end

    function love.draw()
        love.graphics.setBackgroundColor(color)
    end

::

    function love.load()
        circle = {rad = 10, pos = {x = 400, y = 300}}
        -- multiple tweens can work on the same subject
        -- and nested values can be tweened, too
        Timer.tween(5, circle, {rad = 50}, 'in-out-quad')
        Timer.tween(2, circle, {pos = {y = 550}}, 'out-bounce')
    end

    function love.update(dt)
        Timer.update(dt)
    end

    function love.draw()
        love.graphics.circle('fill', circle.pos.x, circle.pos.y, circle.rad)
    end

::

    function love.load()
        -- repeated tweening

        circle = {rad = 10, x = 100, y = 100}
        local grow, shrink, move_down, move_up
        grow = function()
            Timer.tween(1, circle, {rad = 50}, 'in-out-quad', shrink)
        end
        shrink = function()
            Timer.tween(2, circle, {rad = 10}, 'in-out-quad', grow)
        end

        move_down = function()
            Timer.tween(3, circle, {x = 700, y = 500}, 'bounce', move_up)
        end
        move_up = function()
            Timer.tween(5, circle, {x = 200, y = 200}, 'out-elastic', move_down)
        end

        grow()
        move_down()
    end

    function love.update(dt)
        Timer.update(dt)
    end

    function love.draw()
        love.graphics.circle('fill', circle.x, circle.y, circle.rad)
    end



.. _tweening-methods:

Tweening methods
----------------

At the core of tweening lie interpolation methods. These methods define how the
output should look depending on how much time has passed. For example, consider
the following tween::

    -- now: player.x = 0, player.y = 0
    Timer.tween(2, player, {x = 2})
    Timer.tween(4, player, {y = 8})

At the beginning of the tweens (no time passed), the interpolation method would
place the player at ``x = 0, y = 0``. After one second, the player should be at
``x = 1, y = 2``, and after two seconds the output is ``x = 2, y = 4``.

The actual duration of and time since starting the tween is not important, only
the fraction of the two. Similarly, the starting value and output are not
important to the interpolation method, since it can be calculated from the
start and end point. Thus an interpolation method can be fully characterized by
a function that takes a number between 0 and 1 and returns a number that
defines the output (usually also between 0 and 1). The interpolation function
must hold that the output is 0 for input 0 and 1 for input 1.

**hump** predefines several commonly used interpolation methods, which are
generalized versions of `Robert Penner's easing
functions <http://www.robertpenner.com/easing/>`_. Those are:

``'linear'``,
``'quad'``,
``'cubic'``,
``'quart'``,
``'quint'``,
``'sine'``,
``'expo'``,
``'circ'``,
``'back'``,
``'bounce'``, and
``'elastic'``.

It's hard to understand how these functions behave by staring at a graph, so
below are some animation examples. You can change the type of the tween by
changing the selections.

.. raw:: html

    <div id="tween-graph"></div>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js" charset="utf-8"></script>
    <script src="_static/graph-tweens.js"></script>

Note that while the animations above show tweening of shapes, other attributes
(color, opacity, volume of a sound, ...) can be changed as well.


Custom interpolators
^^^^^^^^^^^^^^^^^^^^

.. warning:
    This is a stub

You can add custom interpolation methods by adding them to the `tween` table::

    Timer.tween.sqrt = function(t) return math.sqrt(t) end
    -- or just Timer.tween.sqrt = math.sqrt

Access the your method like you would the predefined ones. You can even use the
modyfing prefixes::

    Timer.tween(5, 'in-out-sqrt', circle, {radius = 50})

You can also invert and chain functions::

    outsqrt = Timer.tween.out(math.sqrt)
    inoutsqrt = Timer.tween.chain(math.sqrt, outsqrt)