… | |
… | |
1860 | |
1860 | |
1861 | In this case, it would be more efficient to leave the C<ev_timer> alone, |
1861 | In this case, it would be more efficient to leave the C<ev_timer> alone, |
1862 | but remember the time of last activity, and check for a real timeout only |
1862 | but remember the time of last activity, and check for a real timeout only |
1863 | within the callback: |
1863 | within the callback: |
1864 | |
1864 | |
|
|
1865 | ev_tstamp timeout = 60.; |
1865 | ev_tstamp last_activity; // time of last activity |
1866 | ev_tstamp last_activity; // time of last activity |
|
|
1867 | ev_timer timer; |
1866 | |
1868 | |
1867 | static void |
1869 | static void |
1868 | callback (EV_P_ ev_timer *w, int revents) |
1870 | callback (EV_P_ ev_timer *w, int revents) |
1869 | { |
1871 | { |
1870 | ev_tstamp now = ev_now (EV_A); |
1872 | // calculate when the timeout would happen |
1871 | ev_tstamp timeout = last_activity + 60.; |
1873 | ev_tstamp after = last_activity - ev_now (EV_A) + timeout; |
1872 | |
1874 | |
1873 | // if last_activity + 60. is older than now, we did time out |
1875 | // if negative, it means we the timeout already occured |
1874 | if (timeout < now) |
1876 | if (after < 0.) |
1875 | { |
1877 | { |
1876 | // timeout occurred, take action |
1878 | // timeout occurred, take action |
1877 | } |
1879 | } |
1878 | else |
1880 | else |
1879 | { |
1881 | { |
1880 | // callback was invoked, but there was some activity, re-arm |
1882 | // callback was invoked, but there was some recent |
1881 | // the watcher to fire in last_activity + 60, which is |
1883 | // activity. simply restart the timer to time out |
1882 | // guaranteed to be in the future, so "again" is positive: |
1884 | // after "after" seconds, which is the earliest time |
1883 | w->repeat = timeout - now; |
1885 | // the timeout can occur. |
|
|
1886 | ev_timer_set (w, after, 0.); |
1884 | ev_timer_again (EV_A_ w); |
1887 | ev_timer_start (EV_A_ w); |
1885 | } |
1888 | } |
1886 | } |
1889 | } |
1887 | |
1890 | |
1888 | To summarise the callback: first calculate the real timeout (defined |
1891 | To summarise the callback: first calculate in how many seconds the |
1889 | as "60 seconds after the last activity"), then check if that time has |
1892 | timeout will occur (by calculating the absolute time when it would occur, |
1890 | been reached, which means something I<did>, in fact, time out. Otherwise |
1893 | C<last_activity + timeout>, and subtracting the current time, C<ev_now |
1891 | the callback was invoked too early (C<timeout> is in the future), so |
1894 | (EV_A)> from that). |
1892 | re-schedule the timer to fire at that future time, to see if maybe we have |
|
|
1893 | a timeout then. |
|
|
1894 | |
1895 | |
1895 | Note how C<ev_timer_again> is used, taking advantage of the |
1896 | If this value is negative, then we are already past the timeout, i.e. we |
1896 | C<ev_timer_again> optimisation when the timer is already running. |
1897 | timed out, and need to do whatever is needed in this case. |
|
|
1898 | |
|
|
1899 | Otherwise, we now the earliest time at which the timeout would trigger, |
|
|
1900 | and simply start the timer with this timeout value. |
|
|
1901 | |
|
|
1902 | In other words, each time the callback is invoked it will check whether |
|
|
1903 | the timeout cocured. If not, it will simply reschedule itself to check |
|
|
1904 | again at the earliest time it could time out. Rinse. Repeat. |
1897 | |
1905 | |
1898 | This scheme causes more callback invocations (about one every 60 seconds |
1906 | This scheme causes more callback invocations (about one every 60 seconds |
1899 | minus half the average time between activity), but virtually no calls to |
1907 | minus half the average time between activity), but virtually no calls to |
1900 | libev to change the timeout. |
1908 | libev to change the timeout. |
1901 | |
1909 | |
1902 | To start the timer, simply initialise the watcher and set C<last_activity> |
1910 | To start the machinery, simply initialise the watcher and set |
1903 | to the current time (meaning we just have some activity :), then call the |
1911 | C<last_activity> to the current time (meaning there was some activity just |
1904 | callback, which will "do the right thing" and start the timer: |
1912 | now), then call the callback, which will "do the right thing" and start |
|
|
1913 | the timer: |
1905 | |
1914 | |
|
|
1915 | last_activity = ev_now (EV_A); |
1906 | ev_init (timer, callback); |
1916 | ev_init (&timer, callback); |
1907 | last_activity = ev_now (loop); |
1917 | callback (EV_A_ &timer, 0); |
1908 | callback (loop, timer, EV_TIMER); |
|
|
1909 | |
1918 | |
1910 | And when there is some activity, simply store the current time in |
1919 | When there is some activity, simply store the current time in |
1911 | C<last_activity>, no libev calls at all: |
1920 | C<last_activity>, no libev calls at all: |
1912 | |
1921 | |
|
|
1922 | if (activity detected) |
1913 | last_activity = ev_now (loop); |
1923 | last_activity = ev_now (EV_A); |
|
|
1924 | |
|
|
1925 | When your timeout value changes, then the timeout can be changed by simply |
|
|
1926 | providing a new value, stopping the timer and calling the callback, which |
|
|
1927 | will agaion do the right thing (for example, time out immediately :). |
|
|
1928 | |
|
|
1929 | timeout = new_value; |
|
|
1930 | ev_timer_stop (EV_A_ &timer); |
|
|
1931 | callback (EV_A_ &timer, 0); |
1914 | |
1932 | |
1915 | This technique is slightly more complex, but in most cases where the |
1933 | This technique is slightly more complex, but in most cases where the |
1916 | time-out is unlikely to be triggered, much more efficient. |
1934 | time-out is unlikely to be triggered, much more efficient. |
1917 | |
|
|
1918 | Changing the timeout is trivial as well (if it isn't hard-coded in the |
|
|
1919 | callback :) - just change the timeout and invoke the callback, which will |
|
|
1920 | fix things for you. |
|
|
1921 | |
1935 | |
1922 | =item 4. Wee, just use a double-linked list for your timeouts. |
1936 | =item 4. Wee, just use a double-linked list for your timeouts. |
1923 | |
1937 | |
1924 | If there is not one request, but many thousands (millions...), all |
1938 | If there is not one request, but many thousands (millions...), all |
1925 | employing some kind of timeout with the same timeout value, then one can |
1939 | employing some kind of timeout with the same timeout value, then one can |
… | |
… | |
3557 | { |
3571 | { |
3558 | struct my_biggy big = (struct my_biggy *) |
3572 | struct my_biggy big = (struct my_biggy *) |
3559 | (((char *)w) - offsetof (struct my_biggy, t2)); |
3573 | (((char *)w) - offsetof (struct my_biggy, t2)); |
3560 | } |
3574 | } |
3561 | |
3575 | |
|
|
3576 | =head2 AVOIDING FINISHING BEFORE RETURNING |
|
|
3577 | |
|
|
3578 | Often you have structures like this in event-based programs: |
|
|
3579 | |
|
|
3580 | callback () |
|
|
3581 | { |
|
|
3582 | free (request); |
|
|
3583 | } |
|
|
3584 | |
|
|
3585 | request = start_new_request (..., callback); |
|
|
3586 | |
|
|
3587 | The intent is to start some "lengthy" operation. The C<request> could be |
|
|
3588 | used to cancel the operation, or do other things with it. |
|
|
3589 | |
|
|
3590 | It's not uncommon to have code paths in C<start_new_request> that |
|
|
3591 | immediately invoke the callback, for example, to report errors. Or you add |
|
|
3592 | some caching layer that finds that it can skip the lengthy aspects of the |
|
|
3593 | operation and simply invoke the callback with the result. |
|
|
3594 | |
|
|
3595 | The problem here is that this will happen I<before> C<start_new_request> |
|
|
3596 | has returned, so C<request> is not set. |
|
|
3597 | |
|
|
3598 | Even if you pass the request by some safer means to the callback, you |
|
|
3599 | might want to do something to the request after starting it, such as |
|
|
3600 | canceling it, which probably isn't working so well when the callback has |
|
|
3601 | already been invoked. |
|
|
3602 | |
|
|
3603 | A common way around all these issues is to make sure that |
|
|
3604 | C<start_new_request> I<always> returns before the callback is invoked. If |
|
|
3605 | C<start_new_request> immediately knows the result, it can artificially |
|
|
3606 | delay invoking the callback by e.g. using a C<prepare> or C<idle> watcher |
|
|
3607 | for example, or more sneakily, by reusing an existing (stopped) watcher |
|
|
3608 | and pushing it into the pending queue: |
|
|
3609 | |
|
|
3610 | ev_set_cb (watcher, callback); |
|
|
3611 | ev_feed_event (EV_A_ watcher, 0); |
|
|
3612 | |
|
|
3613 | This way, C<start_new_request> can safely return before the callback is |
|
|
3614 | invoked, while not delaying callback invocation too much. |
|
|
3615 | |
3562 | =head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS |
3616 | =head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS |
3563 | |
3617 | |
3564 | Often (especially in GUI toolkits) there are places where you have |
3618 | Often (especially in GUI toolkits) there are places where you have |
3565 | I<modal> interaction, which is most easily implemented by recursively |
3619 | I<modal> interaction, which is most easily implemented by recursively |
3566 | invoking C<ev_run>. |
3620 | invoking C<ev_run>. |