Timers: add Qt::TimerId enum class

Which will be used to represent timer IDs. Thanks to Marc for the idea
to use "a strongly typed int".

QTimer got a new id() method that returns Qt::TimerId (can't overload
timerId()). Various classes in qtbase have a member named timerId(), but
a new method is needed anyway in QTimer so id() it is (this is the
reason QChronoTimer only has id() and no timerId()). Besides
timer.timerId() has an extra "timer".

This commit fixes the inconsistency between QObject using `0` timer id
to indicate "failed to start", while QTimer::timerId() returned `-1` to
indicate "timer is inactive". QTimer::id(), being a new method and all,
now returns Qt::TimerId::Invalid, which has value `0`, so that the
values match between the two classes. Extend the unittests to ensure
QTimer::timerId()'s behavior is preserved.

[ChangeLog][Core][QObject] Added Qt::TimerId enum class, that is used to
represent timer IDs.

Change-Id: I0e8564c1461884106d8a797cc980a669035d480a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp b/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
index 3e9f78c0ff..e3bbac7a3c 100644
--- a/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
@@ -469,3 +469,16 @@ const bool wasBlocked = someQObject->blockSignals(true);
 // no signals here
 someQObject->blockSignals(wasBlocked);
 //! [54]
+
+{
+//! [invalid-timer-id]
+    QObject *obj;
+    ...
+    int id = obj->startTimer(100ms);
+    if (id > Qt::TimerId::Invalid)
+        // The timer has been started successfully
+
+    if (id > 0) // Equivalent, albeit less readable
+        // The timer has been started successfully
+//! [invalid-timer-id]
+}