Handling time

Time in SFML

Unlike many other libraries where time is a number of milliseconds, or a decimal number of seconds, SFML doesn't impose any specific unit or type for time values. Instead it leaves this choice to the user through a flexible class: SF::Time. All SFML classes and functions that manipulate time values use this class.

SF::Time represents a time period (in other words, the time that elapses between two events). It is not a date-time class which would represent the current year/month/day/hour/minute/second as a timestamp, it's just a value that represents a certain amount of time, and how to interpret it depends on the context where it is used.

Converting time

A SF::Time value can be constructed from different source units: seconds, milliseconds and microseconds. There is a (non-member) function to turn each of them into a SF::Time:

t1 = SF.microseconds(10000)
t2 = SF.milliseconds(10)
t3 = SF.seconds(0.01)

Note that these three times are all equal.

Similarly, a SF::Time can be converted back to either seconds, milliseconds or microseconds:

time = (...)

microseconds = time.as_microseconds
milliseconds = time.as_milliseconds
seconds = time.as_seconds

Playing with time values

SF::Time is just an amount of time, so it supports arithmetic operations such as addition, subtraction, comparison, etc. Times can also be negative.

t1 = (...)
t2 = t1 * 2
t3 = t1 + t2
t4 = -t3

b1 = (t1 == t2)
b2 = (t3 > t4)

Measuring time

Relevant example: transformable

Now that we've seen how to manipulate time values with CrSFML, let's see how to do something that almost every program needs: measuring the time elapsed.

CrSFML has a very simple class for measuring time: SF::Clock. It only has two methods: elapsed_time, to retrieve the time elapsed since the clock started, and restart, to restart the clock.

clock = SF::Clock.new # Starts the clock

sleep(2)

elapsed_1 = clock.elapsed_time
p elapsed_1.as_seconds
clock.restart

sleep(3)

elapsed_2 = clock.elapsed_time
p elapsed_2.as_seconds

Note that restart also returns the elapsed time, so that you can avoid the slight gap that would exist if you had to call elapsed_time explicitly before restart.
Here is an example that uses the time elapsed at each iteration of the game loop to update the game logic:

clock = SF::Clock.new

while window.open?
  elapsed = clock.restart
  update_game(elapsed)
  # [...]
end