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Jubler performance load3/5/2023 ![]() Id: rect anchors.fill: parent color: "blue" In the following example, we have a block of code which is run often (in this case, it is the contents of an explicit loop but it could be a commonly-evaluated binding expression, for example) and in it, we resolve the object with the "rect" id and its "color" property multiple times: While in some cases the result of a lookup can be cached and reused, it is always best to avoid doing unnecessary work altogether, if possible. In general, "property var" should be considered to be superior to "property variant" for every use-case from QtQuick 2.0 and newer (note that "property variant" is marked as obsolete), as it allows a true JavaScript reference to be stored (which can reduce the number of conversions required in certain expressions). If you must expose a QVariantMap to QML, use a "var" property rather than a "variant" property. Using the closest matching property type will avoid unnecessary conversion. Lists can also be expensive, although sequences of specific types ( QList of int, qreal, bool, QString, and QUrl) should be inexpensive other list types involve an expensive conversion cost (creating a new JavaScript Array, and adding new types one by one, with per-type conversion from C++ type instance to JavaScript value).Ĭonverting between some basic property types (such as "string" and "url" properties) can also be expensive. One example of where it is expensive is assigning a C++ QVariantMap Q_PROPERTY to a QML "variant" property. In most cases, this is fairly inexpensive, but in others it can be quite expensive. One major cost of using JavaScript is that in most cases when a property from a QML type is accessed, a JavaScript object with an external resource containing the underlying C++ data (or a reference to it) is created. However, care must be taken to ensure that unnecessary processing isn't triggered accidentally. Thanks to some optimizations in the QML engine, such as those done to the bindings compiler, it can (in some use-cases) be faster than calling a C++ function. ![]() Most QML applications will have a large amount of JavaScript code in them, in the form of dynamic functions, signal handlers, and property binding expressions. Attempting to optimize code without profiling is likely to result in very minor rather than significant performance improvements. See the Qt Creator manual for more information on how to use the QML profiling tool.ĭetermining which bindings are being run the most often, or which functions your application is spending the most time in, will allow you to decide whether you need to optimize the problem areas, or redesign some implementation details of your application so that the performance is improved. Knowing where time is spent in an application will allow you to focus on problem areas which actually exist, rather than problem areas which potentially exist. The most important tip is: use the QML profiler included with Qt Creator. Those bindings can then cause side effects which, for example, destroy the hierarchy containing your event loop. This is dangerous, because when an event loop is entered in a signal handler or binding, the QML engine continues to run other bindings, animations, transitions, etc. Note: A pattern which is tempting, but should never be used, is creating your own QEventLoop or calling QCoreApplication::processEvents() in order to avoid blocking within a C++ code block invoked from QML.
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