Greatly optimize WebChannel in various ways.

This is a big code drop - sorry for that. The benefits are worth it
though, I'm sure. The optimizations were required to make the
WebChannel useable even on a low-end embedded device with medium
amount of traffic.

The changes in this patch can be grouped into different parts:

a) Do more in C++: Esp. by leveraging e.g. the new classInfoForObjects
in QtMetaObjectPublisher (on the C++ side) one can greatly reduce
the time required for initialization of the webchannel.

b) Property Caching: Instead of requiring a socket roundtrip whenever
a property is read on the HTML side, we now cache the property values
on the HTML side. Note that for this to work properly, one needs to
add proper notify signals to the property declarations, as otherwise
the cache will not get updated.

c) Grouping: Instead of sending separate messages to the clients
for every property update, these signals are grouped by a 50ms timer,
and then send aggregated to the client. This reduces the socket
traffic, as more boiler plate can be shared.

d) Compression: Some data was previously send repeatedly, such as
property name and notify signal. This is now compressed internally
where possible (i.e. for the ${propName}Changed naming scheme).

e) Message Flood Prevention: Previously, one could easily flood an
HTML client by sending data to it. If it could not work off the
incoming stream one would freeze the HTML client. Now, we wait for an
idle signal of the client prior to sending new data to it. Paired
with the message grouping and property cache mentioned above, we
are able to only send the newest data once the HTML client becomes
active again. I.e. we discard now-obsolete property updates etc.

Change-Id: I8f3ae16ed6c1f6a89b644acdce7efbf0f07fc786
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>

3 files changed, 7 insertions, 2 deletions

diff --git a/examples/qtobject/qml/qtobject/main.qml b/examples/qtobject/qml/qtobject/main.qml
index 7a63cd2..d00c9ac 100644
--- a/examples/qtobject/qml/qtobject/main.qml
+++ b/examples/qtobject/qml/qtobject/main.qml
@@ -49,6 +49,7 @@ import QtWebKit.experimental 1.0
 Rectangle {
     MetaObjectPublisher {
         id: publisher
+        webChannel: webChannel
     }
 
     TestObject {
diff --git a/examples/qtobject/testobject.cpp b/examples/qtobject/testobject.cpp
index 4bcd92f..e18dbc0 100644
--- a/examples/qtobject/testobject.cpp
+++ b/examples/qtobject/testobject.cpp
@@ -18,6 +18,7 @@ void TestObject::setProp1(const QString& s)
     p1 = s;
     qWarning() << __func__ << p1;
     emit sig1(1, 0.5, QStringLiteral("asdf"));
+    emit prop1Changed();
 }
 
 void TestObject::setProp2(const QString& s)
@@ -25,6 +26,7 @@ void TestObject::setProp2(const QString& s)
     p2 = s;
     qWarning() << __func__ << p2;
     emit sig2();
+    emit prop2Changed(s);
 }
 
 QString TestObject::manyArgs(int a, float b, const QString& c) const
diff --git a/examples/qtobject/testobject.h b/examples/qtobject/testobject.h
index 39dc608..f3f1c03 100644
--- a/examples/qtobject/testobject.h
+++ b/examples/qtobject/testobject.h
@@ -7,8 +7,8 @@
 class TestObject : public QObject
 {
     Q_OBJECT
-    Q_PROPERTY(QString prop1 READ prop1 WRITE setProp1)
-    Q_PROPERTY(QString prop2 READ prop2 WRITE setProp2)
+    Q_PROPERTY(QString prop1 READ prop1 WRITE setProp1 NOTIFY prop1Changed)
+    Q_PROPERTY(QString prop2 READ prop2 WRITE setProp2 NOTIFY prop2Changed)
 public:
     explicit TestObject(QObject *parent = 0);
     QString prop1() const { return "p1" + p1 + objectName(); }
@@ -21,6 +21,8 @@ signals:
     void timeout();
     void sig1(int a, float b, const QString& c);
     void sig2();
+    void prop1Changed();
+    void prop2Changed(const QString& newValue);
 
 public slots:
     void startTimer(int millis)