Doc: Discuss the concept of thread affinity in more detail

- Create a section dedicated to this concept, which is fundamental to
  signal/event handling
- Move relevant content from the very broad "Thread Basics" page to the
  QObject class ref
- Flesh out existing content, including distinguishing signals from
  events
- Address the common misconception that "member" = "child"; this has
  been encountered several times in the Qt Project forums

Change-Id: I5e7e6c848596e72c0bb623f8442c8389b81ad9ef
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/doc/src/threads-basics.qdoc

@@ -185,19 +185,9 @@
 
     \section2 QObject and Threads
 
-    A QObject is said to have a \e{thread affinity} or, in other words, that it
-    lives in a certain thread. This means that, at creation time, QObject saves
-    a pointer to the current thread. This information becomes relevant when an
-    event is posted with \l{QCoreApplication::}{postEvent()}. The event will be
-    put in the corresponding thread's event loop. If the thread where the
-    QObject lives doesn't have an event loop, the event will never be delivered.
-
-    To start an event loop, \l{QThread::}{exec()} must be called inside
-    \l{QThread::}{run()}. Thread affinity can be changed using
-    \l{QObject::}{moveToThread()}.
-
     As mentioned above, developers must always be careful when calling objects'
-    methods from other threads. Thread affinity does not change this situation.
+    methods from other threads. \l{QObject#Thread Affinity}{Thread affinity}
+    does not change this situation.
     Qt documentation marks several methods as thread-safe.
     \l{QCoreApplication::}{postEvent()} is a noteworthy example. A thread-safe
     method may be called from different threads simultaneously.
@@ -234,18 +224,6 @@
        has terminated.
     \endlist
 
-    A QObject's parent must always be in the same thread. This has a surprising
-    consequence for objects generated within the \l{QThread::}{run()} method:
-
-    \code
-    void HelloThread::run()
-    {
-         QObject *object1 = new QObject(this);  //error, parent must be in the same thread
-         QObject object2;  // OK
-         QSharedPointer <QObject> object3(new QObject); // OK
-    }
-    \endcode
-
     \section2 Using a Mutex to Protect the Integrity of Data
 
     A mutex is an object that has \l{QMutex::}{lock()} and \l{QMutex::}{unlock()}

src/corelib/kernel/qobject.cpp

@@ -522,13 +522,6 @@ void QMetaCallEvent::placeMetaCall(QObject *object)
     details. A convenience handler, childEvent(), can be reimplemented
     to catch child events.
 
-    Events are delivered in the thread in which the object was
-    created; see \l{Thread Support in Qt} and thread() for details.
-    Note that event processing is not done at all for QObjects with no
-    thread affinity (thread() returns zero). Use the moveToThread()
-    function to change the thread affinity for an object and its
-    children (the object cannot be moved if it has a parent).
-
     Last but not least, QObject provides the basic timer support in
     Qt; see QTimer for high-level support for timers.
 
@@ -549,6 +542,41 @@ void QMetaCallEvent::placeMetaCall(QObject *object)
     Some QObject functions, e.g. children(), return a QObjectList.
     QObjectList is a typedef for QList<QObject *>.
 
+    \section1 Thread Affinity
+
+    A QObject instance is said to have a \e{thread affinity}, or that
+    it \e{lives} in a certain thread. When a QObject receives a
+    \l{Qt::QueuedConnection}{queued signal} or a \l{The Event
+    System#Sending Events}{posted event}, the slot or event handler
+    will run in the thread that the object lives in.
+
+    \note If a QObject has no thread affinity (that is, if thread()
+    returns zero), or if it lives in a thread that has no running event
+    loop, then it cannot receive queued signals or posted events.
+
+    By default, a QObject lives in the thread in which it is created.
+    An object's thread affinity can be queried using thread() and
+    changed using moveToThread().
+
+    All QObjects must live in the same thread as their parent. Consequently:
+
+    \list
+    \li setParent() will fail if the two QObjects involved live in
+        different threads.
+    \li When a QObject is moved to another thread, all its children
+        will be automatically moved too.
+    \li moveToThread() will fail if the QObject has a parent.
+    \li If \l{QObject}s are created within QThread::run(), they cannot
+        become children of the QThread object because the QThread does
+        not live in the thread that calls QThread::run().
+    \endlist
+
+    \note A QObject's member variables \e{do not} automatically become
+    its children. The parent-child relationship must be set by either
+    passing a pointer to the child's \l{QObject()}{constructor}, or by
+    calling setParent(). Without this step, the object's member variables
+    will remain in the old thread when moveToThread() is called.
+
     \target No copy constructor
     \section1 No copy constructor or assignment operator