Doc: Move multithreading guidelines to the overview page

It's helpful to see how to choose among different solutions, right
after seeing short descriptions of all the solutions.

- Some minor rewording was done during the move
- The example about polling ports in a new thread was removed because
  there are better ways to do that without threads.

Change-Id: I2cb571a4dbf9be93fb0ec88c60fb7406996c345b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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

@@ -172,69 +172,9 @@
 
     \section2 Which Qt Thread Technology Should You Use?
 
-    Sometimes you want to do more than just running a method in the context of
+    See the \l{Multithreading Technologies in Qt} page for an introduction to the
-    another thread. You may want to have an object which lives in another
+    different approaches to multithreading to Qt, and for guidelines on how to
-    thread that provides a service to the GUI thread. Maybe you want another
+    choose among them.
-    thread to stay alive forever to poll hardware ports and send a signal to
-    the GUI thread when something noteworthy has happened. Qt provides
-    different solutions for developing threaded applications. The right
-    solution depends on the purpose of the new thread as well as on the
-    thread's lifetime.
-
-    \table
-    \header
-        \li Lifetime of thread
-        \li Development task
-        \li Solution
-    \row
-        \li One call
-        \li Run one method within another thread and quit the thread when the
-           method is finished.
-        \li Qt provides different solutions:
-           \list
-              \li Write a function and run it with QtConcurrent::run()
-              \li Derive a class from QRunnable and run it in the global thread
-                 pool with QThreadPool::globalInstance()->start()
-              \li Derive a class from QThread, reimplement the QThread::run()
-                 method and use QThread::start() to run it.
-           \endlist
-
-    \row
-        \li One call
-        \li Operations are to be performed on all items of a container.
-           Processing should be performed using all available cores. A common
-           example is to produce thumbnails from a list of images.
-        \li QtConcurrent provides the \l{QtConcurrent::}{map()} function for
-           applying operations on every container element,
-           \l{QtConcurrent::}{filter()} for selecting container elements, and
-           the option of specifying a reduce function for combining the
-           remaining elements.
-    \row
-        \li One call
-        \li A long running operation has to be put in another thread. During the
-           course of processing, status information should be sent to the GUI
-           thread.
-        \li Use QThread, reimplement run and emit signals as needed. Connect the
-           signals to the GUI thread's slots using queued signal/slot
-           connections.
-
-    \row
-        \li Permanent
-        \li Have an object living in another thread and let it perform different
-           tasks upon request.
-           This means communication to and from the worker thread is required.
-        \li Derive a class from QObject and implement the necessary slots and
-           signals, move the object to a thread with a running event loop and
-           communicate with the object over queued signal/slot connections.
-    \row
-        \li Permanent
-        \li Have an object living in another thread, let the object perform
-           repeated tasks such as polling a port and enable communication with
-           the GUI thread.
-        \li Same as above but also use a timer in the worker thread to implement
-           polling. However, the best solution for polling is to avoid it
-           completely. Sometimes using QSocketNotifier is an alternative.
-    \endtable
 
 
     \section1 Qt Thread Basics

src/corelib/doc/src/threads.qdoc

@@ -178,7 +178,14 @@
     See the \l{Qt Concurrent} module documentation for details on the individual functions.
 
 
-    \section1 Comparison of Solutions
+    \section1 Choosing an Appropriate Approach
+
+    As demonstrated above, Qt provides different solutions for developing threaded
+    applications. The right solution for a given application depends on the purpose
+    of the new thread and the thread's lifetime. Below is a comparison of Qt's
+    threading technologies, followed by recommended solutions for some example use cases.
+
+    \section2 Comparison of Solutions
 
     \table
     \header
@@ -228,6 +235,53 @@
         \li Yes
     \endtable
     \sup{\e{*Except QtConcurrent::run(), which is like QRunnable}}
+
+
+    \section2 Example Use Cases
+
+    \table
+    \header
+        \li Lifetime of thread
+        \li Operation
+        \li Solution
+    \row
+        \li One call
+        \li Run a linear function within another thread, optionally with progress
+            updates during the run.
+        \li Qt provides different solutions:
+            \list
+              \li Place the function in a reimplementation of QThread::run() and
+                  start the QThread. Emit signals to update progress. OR
+              \li Place the function in a reimplementation of QRunnable::run() and
+                  add the QRunnable to a QThreadPool. Write to a \l{Synchronizing
+                  Threads}{thread-safe variable} to update progress. OR
+              \li Run the function using QtConcurrent::run(). Write to a \l{Synchronizing
+                  Threads}{thread-safe variable} to update progress.
+            \endlist
+    \row
+        \li One call
+        \li Perform an operation on all items of a container, using all available
+            cores. For example, producing thumbnails from a list of images.
+        \li Use Qt Concurrent's \l{QtConcurrent::}{filter()} function to select
+            container elements, and the \l{QtConcurrent::}{map()} function to apply
+            an operation to each element. To fold the output into a single result,
+            use \l{QtConcurrent::}{filterReduced()} and \l{QtConcurrent::}{mapReduced()}
+            instead.
+    \row
+        \li Permanent
+        \li Have an object living in another thread that can perform different
+            tasks upon request and/or can receive new data to work with.
+        \li Subclass a QObject to create a worker. Instantiate this worker object
+            and a QThread. Move the worker to the new thread. Send commands or
+            data to the worker object over queued signal-slot connections.
+    \row
+        \li Permanent
+        \li Repeatedly perform an expensive operation in another thread, where the
+            thread does not need to receive any signals or events.
+        \li Write the infinite loop directly within a reimplementation of QThread::run().
+            Start the thread without an event loop. Let the thread emit signals to
+            send data back to the GUI thread.
+    \endtable
 */
 
 /*!