Call Slot In Qt
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- Call Slot In Qts
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To establish a signal and slot connection between two widgets in a dialog, you first need to switch to Qt Designer's Edit Signals/Slots mode. To do that, you can press the F4 key, select the EditEdit Signals/Slots option in the main menu, or click on the Edit Signals/Slots button on the toolbar. You can use Qt's meta-object data to ask about its type (as a string), or ask which calls it supports. That means you can then dynamically find, and get call signatures and through Qt's signals/slots, invoke via text an object. Call(T &function, QObject.receiver, void.args): A static function that will call the function, applying the given parameters. Qt still supports C98 compiler which means we unfortunately cannot require support for variadic templates. Therefore we had to specialize our trait function for each number of arguments. Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. The minimal example requires a class with one signal, one slot and one connection: counter.h. QTTRFUNCTIONS is a macro that defines all the 'tr' functions used for multi-language support. Qtmetacast performs a dynamic cast, given the class name or the name of one of its base classes (Qt doesn't rely on RTTI, obviously). Qtmetacall calls an internal signal or slot by its index.
As Qt WebKit is replaced by Qt WebEngine(you can refer to this post about porting issues), accessing html elements from C++ directly becomes impossible. Many works originally done by QWebKit classes are now transferred to javascript. Javascript is used to manipulate web content. And you need to call runJavaScript from C++ code to run javascript on the web page loaded by QWebEngineView.To get web elements, a communication mechanism is invented to bridge the C++ world and the javascript world. The bridging mechanism is more than obtaining the values of web page elements. It provides the ways in which C++ code can call javascript functions, and javascript can invoke C++ functions as well.The values of variables can also be passed from C++ to javascript, and vice versa. Let’s consider the following application scenarios:
javascript code calls C++ function
C++ code should provide a class which contains the to-be-called function(as a slot), then register an object of this class to a QWebChannel object, and set the web channel object to the web page object in the QWebEngineView
To invoke the C++ function jscallme, javascript should new an instance of QWebChannel object.
QWebChannel is defined in qwebchannel.js(you can find this file in the example folder of Qt installation directory) so the script should be loaded first. In the function passed as the second parameter of function QWebChannel, the exposed object from C++ world(webobj in C++) channel.objects.webobj is assigned to the javascript variable webobj, and then assigned to window.foo so you can use foo elsewhere to refer to the object. After the function is executed, javascript can call the C++ slot function jscallme as follows:
Pass data from javascript to C++
We’ve known how to call C++ function from javascript. You should be able to figure out a way to pass data from javascript to C++, i.e., as parameter(s) of function. We re-implement jscallme as follows:
, and invoking of the function from js would be:
Note that the “const” before the parameter can not be omitted, otherwise, you will get the following error:
Could not convert argument QJsonValue(string, “sd”) to target type .
Although data can be passed as parameters of function, it would be more convenient if we can pass data by setting an attribute of an object like:
We expect after the code is executed, “somedata” will be stored in a member variable of the exposed object (webobj) in C++. This is done by delaring a qt property in C++ class:
Now if you execute foo.someattribute=”somedata” in javascript, m_someattribute in C++ will be “somedata”.
Pass data from C++ to javascript
We can send data from C++ to javascript using signals. We emit a signal with the data to send as the parameter of the signal. Javascript must connect the signal to a function to receive the data.
The line “webobj.someattributeChanged.connect(updateattribute)” connects the C++ signal someattributeChanged to the javascript function updateattribute. Note that although updateattribute takes one parameter “text”, we did not provide the parameter value in connect. In fact, we do not know the parameter value passed to updateattribute until the signal is received. The signal is accompanied by a parameter “attr” which is passed as the “text” parameter of updateattribute. Now, if you call webobj->setsomeattribute(“hello”), you will see the value of the html element with id “#attrid” is set to “hello”. Note that although we connect the member m_someattribute to the qt property someattribute in the above example, it is not a required step. The signal mechanism alone can realize the delivery of data.
We can make things even simpler by adding the NOTIFY parameter when declaring the someattribute property.
Now, if you call webobj->setProperty(“someattribute”,”hello”) somewhere in C++, the signal “someattributeChanged” is automatically emitted and our web page gets updated.
C++ invokes javascript function
This is much straightforward compared with invoking C++ function from js. Just use runJavaScript passing the function as the parameter as follows:
It assumes the jsfun is already defined on your web page, otherwise, you have to define it in the string parameter. The return value is asynchronously passed to the lambda expression as the parameter v.
Now, back to the question raised at the beginning of the post: How to get the value of an html element in C++? It can be done as follows:
It uses jQuery functions so make sure jQuery lib is running on your web page.
Slots in D-Bus adaptors are declared just like normal, public slots, but their parameters must follow certain rules (see The Qt D-Bus Type System for more information). Slots whose parameters do not follow those rules or that are not public will not be accessible via D-Bus.
Slots can have one parameter of type const QDBusMessage &, which must appear at the end of the input parameter list, before any output parameters. This parameter, if present, will be initialized with a copy of the current message being processed, which allows the callee to obtain information about the caller, such as its connection name.
Slots can be of three kinds:
- Asynchronous
- Input-only
- Input-and-output
Asynchronous Slots
Call Slot In Qt Download
Asynchronous slots are those that do not normally return any reply to the caller. For that reason, they cannot take any output parameters. In most cases, by the time the first line of the slot is run, the caller function has already resumed working.
However, slots must not rely on that behavior. Scheduling and message-dispatching issues could change the order in which the slot is run. Code intending to synchronize with the caller should provide its own method of synchronization.
Asynchronous slots are marked by the keyword Q_NOREPLY in the method signature, before the void return type and the slot name. The quit() slot in the D-Bus Complex Ping Pong Example is an example of this.
Input-Only Slots
Input-only slots are normal slots that take parameters passed by value or by constant reference. However, unlike asynchronous slots, the caller is usually waiting for completion of the callee before resuming operation. Therefore, non-asynchronous slots should not block or should state it its documentation that they may do so.
Input-only slots have no special marking in their signature, except that they take only parameters passed by value or by constant reference. Optionally, slots can take a QDBusMessage parameter as a last parameter, which can be used to perform additional analysis of the method call message.
Input and Output Slots
Like input-only slots, input-and-output slots are those that the caller is waiting for a reply. Unlike input-only ones, though, this reply will contain data. Slots that output data may contain non-constant references and may return a value as well. However, the output parameters must all appear at the end of the argument list and may not have input arguments interleaved. Optionally, a QDBusMessage argument may appear between the input and the output arguments.
Automatic Replies
Method replies are generated automatically with the contents of the output parameters (if there were any) by the Qt D-Bus implementation. Slots need not worry about constructing proper QDBusMessage objects and sending them over the connection.
However, the possibility of doing so remains there. Should the slot find out it needs to send a special reply or even an error, it can do so by using QDBusMessage::createReply() or QDBusMessage::createErrorReply() on the QDBusMessage parameter and send it with QDBusConnection::send(). The Qt D-Bus implementation will not generate any reply if the slot did so.
Warning: When a caller places a method call and waits for a reply, it will only wait for a limited amount of time. Slots intending to take a long time to complete should make that fact clear in documentation so that callers properly set higher timeouts.
Delayed Replies
In some circumstances, the called slot may not be able to process the request immediately. This is frequently the case when the request involves an I/O or networking operation which may block.
If this is the case, the slot should return control to the application's main loop to avoid freezing the user interface, and resume the process later. To accomplish this, it should make use of the extra QDBusMessage parameter at the end of the input parameter list and request a delayed reply.
We do this by writing a slot that stores the request data in a persistent structure, indicating to the caller using QDBusMessage::setDelayedReply(true) that the response will be sent later.
In this case, the return value is unimportant; we return an arbitrary value to satisfy the compiler.
When the request is processed and a reply is available, it should be sent using the QDBusMessage object that was obtained. In our example, the reply code could be something as follows:
As can be seen in the example, when a delayed reply is in place, the return value(s) from the slot will be ignored by Qt D-Bus. They are used only to determine the slot's signature when communicating the adaptor's description to remote applications, or in case the code in the slot decides not to use a delayed reply.
Qt Call Slot In Different Thread
The delayed reply itself is requested from Qt D-Bus by calling QDBusMessage::reply() on the original message. It then becomes the resposibility of the called code to eventually send a reply to the caller.
Warning: When a caller places a method call and waits for a reply, it will only wait for a limited amount of time. Slots intending to take a long time to complete should make that fact clear in documentation so that callers properly set higher timeouts.
Call Slot In Qts
See also Using Qt D-Bus Adaptors, Declaring Signals in D-Bus Adaptors, The Qt D-Bus Type System, QDBusConnection, and QDBusMessage.
Qt Call Slot In Another Thread
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