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/*!

    \example network/blockingfortuneclient

    \title Blocking Fortune Client Example

    The Blocking Fortune Client example shows how to create a client for a

    network service using QTcpSocket's synchronous API in a non-GUI thread.

    \image blockingfortuneclient-example.png

    QTcpSocket supports two general approaches to network programming:

    \list

    \o \e{The asynchronous (non-blocking) approach.} Operations are scheduled

    and performed when control returns to Qt's event loop. When the operation

    is finished, QTcpSocket emits a signal. For example,

    QTcpSocket::connectToHost() returns immediately, and when the connection

    has been established, QTcpSocket emits

    \l{QTcpSocket::connected()}{connected()}.

    \o \e{The synchronous (blocking) approach.} In non-GUI and multithreaded

    applications, you can call the \c waitFor...() functions (e.g.,

    QTcpSocket::waitForConnected()) to suspend the calling thread until the

    operation has completed, instead of connecting to signals.

    \endlist

    The implementation is very similar to the

    \l{network/fortuneclient}{Fortune Client} example, but instead of having

    QTcpSocket as a member of the main class, doing asynchronous networking in

    the main thread, we will do all network operations in a separate thread

    and use QTcpSocket's blocking API.

    The purpose of this example is to demonstrate a pattern that you can use

    to simplify your networking code, without losing responsiveness in your

    user interface.  Use of Qt's blocking network API often leads to

    simpler code, but because of its blocking behavior, it should only be used

    in non-GUI threads to prevent the user interface from freezing. But

    contrary to what many think, using threads with QThread does not

    necessarily add unmanagable complexity to your application.

    We will start with the FortuneThread class, which handles the network

    code.

    \snippet examples/network/blockingfortuneclient/fortunethread.h 0

    FortuneThread is a QThread subclass that provides an API for scheduling

    requests for fortunes, and it has signals for delivering fortunes and

    reporting errors. You can call requestNewFortune() to request a new

    fortune, and the result is delivered by the newFortune() signal. If any

    error occurs, the error() signal is emitted.

    It's important to notice that requestNewFortune() is called from the main,

    GUI thread, but the host name and port values it stores will be accessed

    from FortuneThread's thread. Because we will be reading and writing

    FortuneThread's data members from different threads concurrently, we use

    QMutex to synchronize access.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 2

    The requestNewFortune() function stores the host name and port of the

    fortune server as member data, and we lock the mutex with QMutexLocker to

    protect this data. We then start the thread, unless it is already

    running. We will come back to the QWaitCondition::wakeOne() call later.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 4

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 5

    In the run() function, we start by acquiring the mutex lock, fetching the

    host name and port from the member data, and then releasing the lock

    again. The case that we are protecting ourselves against is that \c

    requestNewFortune() could be called at the same time as we are fetching

    this data. QString is \l reentrant but \e not \l{thread-safe}, and we must

    also avoid the unlikely risk of reading the host name from one request,

    and port of another. And as you might have guessed, FortuneThread can only

    handle one request at a time.

    The run() function now enters a loop:

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 6

    The loop will continue requesting fortunes for as long as \e quit is

    false. We start our first request by creating a QTcpSocket on the stack,

    and then we call \l{QTcpSocket::connectToHost()}{connectToHost()}. This

    starts an asynchronous operation which, after control returns to Qt's

    event loop, will cause QTcpSocket to emit

    \l{QTcpSocket::connected()}{connected()} or

    \l{QTcpSocket::error()}{error()}.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 8

    But since we are running in a non-GUI thread, we do not have to worry

    about blocking the user interface. So instead of entering an event loop,

    we simply call QTcpSocket::waitForConnected(). This function will wait,

    blocking the calling thread, until QTcpSocket emits connected() or an

    error occurs. If connected() is emitted, the function returns true; if the

    connection failed or timed out (which in this example happens after 5

    seconds), false is returned. QTcpSocket::waitForConnected(), like the

    other \c waitFor...() functions, is part of QTcpSocket's \e{blocking

    API}.

    After this statement, we have a connected socket to work with. Now it's

    time to see what the fortune server has sent us.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 9

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 10

    This step is to read the size of the packet. Although we are only reading

    two bytes here, and the \c while loop may seem to overdo it, we present this

    code to demonstrate a good pattern for waiting for data using

    QTcpSocket::waitForReadyRead(). It goes like this: For as long as we still

    need more data, we call waitForReadyRead(). If it returns false,

    we abort the operation. After this statement, we know that we have received

    enough data.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 11

    Now we can create a QDataStream object, passing the socket to

    QDataStream's constructor, and as in the other client examples we set

    the stream protocol version to QDataStream::Qt_4_0, and read the size

    of the packet.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 12

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 13

    Again, we'll use a loop that waits for more data by calling

    QTcpSocket::waitForReadyRead(). In this loop, we're waiting until

    QTcpSocket::bytesAvailable() returns the full packet size.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 14

    Now that we have all the data that we need, we can use QDataStream to

    read the fortune string from the packet. The resulting fortune is

    delivered by emitting newFortune().

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 15

    The final part of our loop is that we acquire the mutex so that we can

    safely read from our member data. We then let the thread go to sleep by

    calling QWaitCondition::wait(). At this point, we can go back to

    requestNewFortune() and look closed at the call to wakeOne():

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 1

    \dots

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 3

    What happened here was that because the thread falls asleep waiting for a

    new request, we needed to wake it up again when a new request

    arrives. QWaitCondition is often used in threads to signal a wakeup call

    like this.

    \snippet examples/network/blockingfortuneclient/fortunethread.cpp 0

    Finishing off the FortuneThread walkthrough, this is the destructor that

    sets \e quit to true, wakes up the thread and waits for the thread to exit

    before returning. This lets the \c while loop in run() will finish its current

    iteration. When run() returns, the thread will terminate and be destroyed.

    Now for the BlockingClient class:

    \snippet examples/network/blockingfortuneclient/blockingclient.h 0

    BlockingClient is very similar to the Client class in the

    \l{network/fortuneclient}{Fortune Client} example, but in this class

    we store a FortuneThread member instead of a pointer to a QTcpSocket.

    When the user clicks the "Get Fortune" button, the same slot is called,

    but its implementation is slightly different:

    \snippet examples/network/blockingfortuneclient/blockingclient.cpp 0

    \snippet examples/network/blockingfortuneclient/blockingclient.cpp 1

    We connect our FortuneThread's two signals newFortune() and error() (which

    are somewhat similar to QTcpSocket::readyRead() and QTcpSocket::error() in

    the previous example) to requestNewFortune() and displayError().

    \snippet examples/network/blockingfortuneclient/blockingclient.cpp 2

    The requestNewFortune() slot calls FortuneThread::requestNewFortune(),

    which \e shedules the request. When the thread has received a new fortune

    and emits newFortune(), our showFortune() slot is called:

    \snippet examples/network/blockingfortuneclient/blockingclient.cpp 3

    \codeline

    \snippet examples/network/blockingfortuneclient/blockingclient.cpp 4

    Here, we simply display the fortune we received as the argument.

    \sa {Fortune Client Example}, {Fortune Server Example}

*/