--- title: How Long Does It Take for a QQuickItem to Become... description: A technique is presented for measuring how long it takes a QQuickItem to actually appear on screen after becoming visible, using QQuickItem::visibleChanged,... canonical: https://daily.dev/posts/how-long-does-it-take-for-a-qquickitem-to-become-visible-measuring-frame-delays-in-qt-quick-keub0ngbb twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: How Long Does It Take for a QQuickItem to Become Visible? Measuring Frame Delays in Qt Quick | daily.dev og:description: A technique is presented for measuring how long it takes a QQuickItem to actually appear on screen after becoming visible, using QQuickItem::visibleChanged,... og:url: https://daily.dev/posts/how-long-does-it-take-for-a-qquickitem-to-become-visible-measuring-frame-delays-in-qt-quick-keub0ngbb og:image: https://api.daily.dev/og/posts/KeUB0ngbB.png og:image:alt: How Long Does It Take for a QQuickItem to Become Visible? Measuring Frame Delays in Qt Quick og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # How Long Does It Take for a QQuickItem to Become Visible? Measuring Frame Delays in Qt Quick **[KDAB Blog](https://daily.dev/sources/kdab)** · 4 min read · 0 upvotes · 0 comments ## Summary A technique is presented for measuring how long it takes a QQuickItem to actually appear on screen after becoming visible, using QQuickItem::visibleChanged, ensurePolish/updatePolish, and QQuickWindow::afterFrameEnd to precisely detect when the correct frame is swapped in. The approach converts the measured delay into a frame-count metric using the display's refresh rate, allowing developers to detect and quantify perceived frame drops in Qt Quick applications. It also explains a C++ constructor trick for reliably triggering visibleChanged in QML-instantiated items, and includes reusable subclass code. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How can I measure how long it takes a QQuickItem to actually become visible on screen in Qt Quick? Connect a slot to QQuickItem::visibleChanged that starts a timer, then override updatePolish (triggered via ensurePolished) to set a flag marking that the next swapped frame corresponds to your item. Check that flag inside a QQuickWindow::afterFrameEnd connection to take the measurement only when your item's frame is actually swapped onto screen, avoiding false triggers from other animated elements. _Developers tracking Qt Quick rendering delays can follow this kind of low-level profiling technique via daily.dev._ ### How do I convert a measured rendering delay in milliseconds into a number of dropped frames in Qt Quick? Divide the measured delay by the time per frame, calculated as 1000 ms divided by the display's refresh rate obtained from QScreen::refreshRate, then apply a floor function to the result. For a 60Hz display, one frame takes about 16 ms, so any measurement under 16 ms counts as an instant swap with zero dropped frames, while larger values indicate the number of frames the component was late. _Anyone benchmarking QML frame timing can find similarly concrete rendering techniques through daily.dev._ ### Why does QQuickItem::visibleChanged sometimes fail to fire when setting visible to true in QML? The QML engine can optimize initial property evaluation so that a property set to false then immediately true in the same instantiation is evaluated only to its final value of true, meaning no actual change occurs and visibleChanged never emits. Setting visible to false directly in the C++ constructor of a QQuickItem subclass guarantees the signal fires whenever visible is later set true in QML. _Qt developers debugging missed property-change signals can dig into edge cases like this via daily.dev._ ## Similar posts on daily.dev - [Qt Widgets to Qt Quick, An Application Journey Part 3](https://daily.dev/posts/qt-widgets-to-qt-quick-an-application-journey-part-3-vdfmbgxpz) · Qt · 0 upvotes · 0 comments - [How to Implement an FPS Counter](https://daily.dev/posts/how-to-implement-an-fps-counter-7opok5rjb) · Hacker News · 0 upvotes · 0 comments - [QtWidgets to QtQuick, An Application Journey Part 2](https://daily.dev/posts/qtwidgets-to-qtquick-an-application-journey-part-2-etgs3keds) · Qt · 0 upvotes · 0 comments --- Tags: [#c++](https://daily.dev/tags/c++), [#qt](https://daily.dev/tags/qt), [#qml](https://daily.dev/tags/qml) [View this post on daily.dev](https://daily.dev/posts/how-long-does-it-take-for-a-qquickitem-to-become-visible-measuring-frame-delays-in-qt-quick-keub0ngbb) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"How Long Does It Take for a QQuickItem to Become Visible? 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