QElapsedTimer¶
The
QElapsedTimer
class provides a fast way to calculate elapsed times. More…
New in version 4.7.
Synopsis¶
Functions¶
def
__eq__
(other)def
__ne__
(other)def
elapsed
()def
hasExpired
(timeout)def
invalidate
()def
isValid
()def
msecsSinceReference
()def
msecsTo
(other)def
nsecsElapsed
()def
restart
()def
secsTo
(other)def
start
()
Static functions¶
def
clockType
()def
isMonotonic
()
Detailed Description¶
The
QElapsedTimer
class is usually used to quickly calculate how much time has elapsed between two events. Its API is similar to that ofQTime
, so code that was using that can be ported quickly to the new class.However, unlike
QTime
,QElapsedTimer
tries to use monotonic clocks if possible. This means it’s not possible to convertQElapsedTimer
objects to a human-readable time.The typical use-case for the class is to determine how much time was spent in a slow operation. The simplest example of such a case is for debugging purposes, as in the following example:
timer = QElapsedTimer() timer.start() slowOperation1() sys.stderr.write("The slow operation took" + timer.elapsed() + "milliseconds")In this example, the timer is started by a call to
start()
and the elapsed timer is calculated by theelapsed()
function.The time elapsed can also be used to recalculate the time available for another operation, after the first one is complete. This is useful when the execution must complete within a certain time period, but several steps are needed. The
waitFor
-type functions inQIODevice
and its subclasses are good examples of such need. In that case, the code could be as follows:def executeSlowOperations(timeout): timer = QElapsedTimer() timer.start() slowOperation1() remainingTime = timeout - timer.elapsed() if remainingTime > 0: slowOperation2(remainingTime)Another use-case is to execute a certain operation for a specific timeslice. For this,
QElapsedTimer
provides thehasExpired()
convenience function, which can be used to determine if a certain number of milliseconds has already elapsed:def executeOperationsForTime(ms): timer = QElapsedTimer() timer.start() while not timer.hasExpired(ms): slowOperation1()It is often more convenient to use
QDeadlineTimer
in this case, which counts towards a timeout in the future instead of tracking elapsed time.
Reference Clocks¶
QElapsedTimer
will use the platform’s monotonic reference clock in all platforms that support it (seeisMonotonic()
). This has the added benefit thatQElapsedTimer
is immune to time adjustments, such as the user correcting the time. Also unlikeQTime
,QElapsedTimer
is immune to changes in the timezone settings, such as daylight-saving periods.On the other hand, this means
QElapsedTimer
values can only be compared with other values that use the same reference. This is especially true if the time since the reference is extracted from theQElapsedTimer
object (msecsSinceReference()
) and serialised. These values should never be exchanged across the network or saved to disk, since there’s no telling whether the computer node receiving the data is the same as the one originating it or if it has rebooted since.It is, however, possible to exchange the value with other processes running on the same machine, provided that they also use the same reference clock.
QElapsedTimer
will always use the same clock, so it’s safe to compare with the value coming from another process in the same machine. If comparing to values produced by other APIs, you should check that the clock used is the same asQElapsedTimer
(seeclockType()
).
32-bit overflows¶
Some of the clocks used by
QElapsedTimer
have a limited range and may overflow after hitting the upper limit (usually 32-bit).QElapsedTimer
deals with this overflow issue and presents a consistent timing. However, when extracting the time since reference fromQElapsedTimer
, two different processes in the same machine may have different understanding of how much time has actually elapsed.The information on which clocks types may overflow and how to remedy that issue is documented along with the clock types.
-
class
QElapsedTimer
¶ QElapsedTimer(QElapsedTimer)
- param QElapsedTimer
Constructs an invalid
QElapsedTimer
. A timer becomes valid once it has been started.
-
PySide2.QtCore.QElapsedTimer.
ClockType
¶ This enum contains the different clock types that
QElapsedTimer
may use.QElapsedTimer
will always use the same clock type in a particular machine, so this value will not change during the lifetime of a program. It is provided so thatQElapsedTimer
can be used with other non-Qt implementations, to guarantee that the same reference clock is being used.Constant
Description
QElapsedTimer.SystemTime
The human-readable system time. This clock is not monotonic.
QElapsedTimer.MonotonicClock
The system’s monotonic clock, usually found in Unix systems. This clock is monotonic and does not overflow.
QElapsedTimer.TickCounter
The system’s tick counter, used on Windows systems. This clock may overflow.
QElapsedTimer.MachAbsoluteTime
The Mach kernel’s absolute time ( macOS and iOS). This clock is monotonic and does not overflow.
QElapsedTimer.PerformanceCounter
The high-resolution performance counter provided by Windows. This clock is monotonic and does not overflow.
SystemTime¶
The system time clock is purely the real time, expressed in milliseconds since Jan 1, 1970 at 0:00 UTC. It’s equivalent to the value returned by the C and POSIX
time
function, with the milliseconds added. This clock type is currently only used on Unix systems that do not support monotonic clocks (see below).This is the only non-monotonic clock that
QElapsedTimer
may use.
MonotonicClock¶
This is the system’s monotonic clock, expressed in milliseconds since an arbitrary point in the past. This clock type is used on Unix systems which support POSIX monotonic clocks (
_POSIX_MONOTONIC_CLOCK
).This clock does not overflow.
TickCounter¶
The tick counter clock type is based on the system’s or the processor’s tick counter, multiplied by the duration of a tick. This clock type is used on Windows platforms. If the high-precision performance counter is available on Windows, the ``
PerformanceCounter
`` clock type is used instead.The
TickCounter
clock type is the only clock type that may overflow. Windows Vista and Windows Server 2008 support the extended 64-bit tick counter, which allows avoiding the overflow.On Windows systems, the clock overflows after 2^32 milliseconds, which corresponds to roughly 49.7 days. This means two processes’ reckoning of the time since the reference may be different by multiples of 2^32 milliseconds. When comparing such values, it’s recommended that the high 32 bits of the millisecond count be masked off.
MachAbsoluteTime¶
This clock type is based on the absolute time presented by Mach kernels, such as that found on macOS . This clock type is presented separately from
MonotonicClock
since macOS and iOS are also Unix systems and may support a POSIX monotonic clock with values differing from the Mach absolute time.This clock is monotonic and does not overflow.
PerformanceCounter¶
This clock uses the Windows functions
QueryPerformanceCounter
andQueryPerformanceFrequency
to access the system’s high-precision performance counter. Since this counter may not be available on all systems,QElapsedTimer
will fall back to the ``TickCounter
`` clock automatically, if this clock cannot be used.This clock is monotonic and does not overflow.
See also
New in version 4.7.
-
static
PySide2.QtCore.QElapsedTimer.
clockType
()¶ - Return type
Returns the clock type that this
QElapsedTimer
implementation uses.See also
-
PySide2.QtCore.QElapsedTimer.
elapsed
()¶ - Return type
qint64
Returns the number of milliseconds since this
QElapsedTimer
was last started.Calling this function on a
QElapsedTimer
that is invalid results in undefined behavior.See also
-
PySide2.QtCore.QElapsedTimer.
hasExpired
(timeout)¶ - Parameters
timeout –
qint64
- Return type
bool
Returns
true
if thisQElapsedTimer
has already expired bytimeout
milliseconds (that is, more thantimeout
milliseconds have elapsed). The value oftimeout
can be -1 to indicate that this timer does not expire, in which case this function will always return false.See also
elapsed()
QDeadlineTimer
-
PySide2.QtCore.QElapsedTimer.
invalidate
()¶ Marks this
QElapsedTimer
object as invalid.An invalid object can be checked with
isValid()
. Calculations of timer elapsed since invalid data are undefined and will likely produce bizarre results.
-
static
PySide2.QtCore.QElapsedTimer.
isMonotonic
()¶ - Return type
bool
Returns
true
if this is a monotonic clock, false otherwise. See the information on the different clock types to understand which ones are monotonic.See also
clockType()
ClockType
-
PySide2.QtCore.QElapsedTimer.
isValid
()¶ - Return type
bool
Returns
false
if the timer has never been started or invalidated by a call toinvalidate()
.See also
-
PySide2.QtCore.QElapsedTimer.
msecsSinceReference
()¶ - Return type
qint64
Returns the number of milliseconds between last time this
QElapsedTimer
object was started and its reference clock’s start.This number is usually arbitrary for all clocks except the
SystemTime
clock. For that clock type, this number is the number of milliseconds since January 1st, 1970 at 0:00 UTC (that is, it is the Unix time expressed in milliseconds).On Linux, Windows and Apple platforms, this value is usually the time since the system boot, though it usually does not include the time the system has spent in sleep states.
See also
-
PySide2.QtCore.QElapsedTimer.
msecsTo
(other)¶ - Parameters
other –
QElapsedTimer
- Return type
qint64
Returns the number of milliseconds between this
QElapsedTimer
andother
. Ifother
was started before this object, the returned value will be negative. If it was started later, the returned value will be positive.The return value is undefined if this object or
other
were invalidated.
-
PySide2.QtCore.QElapsedTimer.
nsecsElapsed
()¶ - Return type
qint64
Returns the number of nanoseconds since this
QElapsedTimer
was last started.Calling this function on a
QElapsedTimer
that is invalid results in undefined behavior.On platforms that do not provide nanosecond resolution, the value returned will be the best estimate available.
See also
-
PySide2.QtCore.QElapsedTimer.
__ne__
(other)¶ - Parameters
other –
QElapsedTimer
- Return type
bool
Returns
true
if this object andother
contain different times.
-
PySide2.QtCore.QElapsedTimer.
__eq__
(other)¶ - Parameters
other –
QElapsedTimer
- Return type
bool
Returns
true
if this object andother
contain the same time.
-
PySide2.QtCore.QElapsedTimer.
restart
()¶ - Return type
qint64
Restarts the timer and returns the time elapsed since the previous start. This function is equivalent to obtaining the elapsed time with
elapsed()
and then starting the timer again withstart()
, but it does so in one single operation, avoiding the need to obtain the clock value twice.Calling this function on a
QElapsedTimer
that is invalid results in undefined behavior.The following example illustrates how to use this function to calibrate a parameter to a slow operation (for example, an iteration count) so that this operation takes at least 250 milliseconds:
timer = QElapsedTimer() count = 1 timer.start() while True: count *= 2 slowOperation2(count) if timer.restart() < 250: break return count
See also
-
PySide2.QtCore.QElapsedTimer.
secsTo
(other)¶ - Parameters
other –
QElapsedTimer
- Return type
qint64
Returns the number of seconds between this
QElapsedTimer
andother
. Ifother
was started before this object, the returned value will be negative. If it was started later, the returned value will be positive.Calling this function on or with a
QElapsedTimer
that is invalid results in undefined behavior.
-
PySide2.QtCore.QElapsedTimer.
start
()¶ Starts this timer. Once started, a timer value can be checked with
elapsed()
ormsecsSinceReference()
.Normally, a timer is started just before a lengthy operation, such as:
timer = QElapsedTimer() timer.start() slowOperation1() sys.stderr.write("The slow operation took" + timer.elapsed() + "milliseconds")
Also, starting a timer makes it valid again.
See also
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