Dave’s QML Binding Loop Backtrace Printer

Binding loops suck, and they can be hard to fix. I wrote a tool that prints a backtrace of the bindings being updated when a loop occurs. See link at bottom.


QML bindings are a very fast and easy way to write a declarative UI. However it’s quite easy to accidentally write an infinite loop.
This can happen if we bind propertyA to affect propertyB and also bind propertyB to affect propertyA, they would constantly update each other.

Consider the following example:

1 import QtQuick 2.0
3 Rectangle {
4     width: childrenRect.width
5     Text {
6        text: parent.width > 10 ? "Hello World" : "Hi"
7     }
8 }

The Rectangle width changes on startup, that changes the text’s size, which in turn changes the Rectangle’s width. If this was undetected the application would loop forever and eventually crash.
QML prints a warning, and ceases processing, but it’s an indication that something is wrong with the logic of your code, and it needs fixing.

However, whilst the loop here is obvious to spot, it can be considerably more complicated when looping through tens of bindings over many many components.

Creating a Tool

The problem with this warning is that on its own is rather unhelpful – trying to find the loop then becomes a manual task of tracing all possible combinations through every bindings that could lead to a short circuit. GDB on its own doesn’t help as the C++ backtrace tells us absolutely nothing we can use.

I’ve created a small script that, using gdb, unwinds the backtrace detecting where the properties changed and then showing the QML code which is responsible.

Simply download here into $PATH and run with

“binding-loop-tracker.py myAppName”

In the case of the loop above we will see output like:

=====Binding loop detected - printing backtrace =====
#0 - file:///home/david/temp/binding_loop.qml:4:12
#1 - file:///home/david/temp/binding_loop.qml:6:15
#2 - file:///home/david/temp/binding_loop.qml:4:12

Which shows which line of QML was being updated when we hit the loop.

It still requires some manual work to follow the trace through, but it’s a useful aid and has already helped me in two real world cases that I couldn’t resolve manually.

Building stable branches with kdesrc-build

When coming up to a release it’s important to track the stable branch of a project so that we’re actually running what we’re going to release and can put our focus on fixing any remaining tiny bugs.

If you build all of Plasma with kdesrc-build you can easily switching to building the stable release with the following command:

kdesrc-build –Branch Plasma/5.7 kf5-workspace-modules

To change back, simply emit the –Branch parameter.

My Randa Plans

The Randa meeting starts this week, and I’ll be working with the KDE multi-platform group, being led by Aleix Pol, and will be spending my time working on both flatpak (formerly xdg-app) and Snappy.

During this past week I have been brought into some technical discussions about deployment on both platforms; so I intend to spend my time working closely with other interested developers solving problems that affect either platform as there is a lot of overlap. Tackling these independently doesn’t make sense.

These two emerging technologies both have a lot of potential to revolutionise Linux packaging and distribution with either being a huge boost over the current state.
Both are going to become relevant in the Linux world over the next few years, and the important thing is making sure our software works best for our users whatever the platform.

So far over this week I’ve spent some time fixing KDE flatpak applications, in particular fixing multiple problems we’ve encountered with Dolphin; namely being able to load plugins and making kio slaves work.

In the meantime I’ve been testing out packaging some Snappy apps, packaging and running a few applications.

Over the week I’ll write some more in depth blog posts, exploring the state of each, where we have problems deploying our apps, and hopefully some concrete solutions 🙂

Be sure to sponsor the sprint to help pay for developers from around the world to come together to work on important projects and be sure to follow PlanetKDE for blog posts about software developments from the people here.

PlasmaShell Sans GL

Since Plasma 5, the main shell is powered by QtQuick, which till now brings a requirement on a working OpenGL setup. This causes problems for Plasma in situations where we can’t run OpenGL; either extremely cheap hardware, xrdc or when a user upgrades and breaks their nvidia setup (a seemingly common occurence).

Qt 5.6 brings a new module which opens some interesting possibilities; the QtQuick 2D renderer, which avoids that.
This has existed for a while, but it has only recently been open sourced as GPL3.

QtQuick Internals

QtQuick is powered by a scenegraph.

Each graphical item updates a tree containing nodes containing either transformations or content. That content being either a rectangle, a picture or a custom openGL shader from the application. The important part is that it stores a tree of items in an way optimised for rendering, but also acts somewhat as an layer between any QtQuick component and the underlying OpenGL renderer.

Using the QtQuick 2D renderer

The QtQuick 2D renderer still uses the same scenegraph, so all custom QQuickItem’s which use the standard QtQuick SceneGraph nodes still work, but instead of calling OpenGL functions, calls are mapped to a raster backend instead.

The Result

My personal desktop, running Plasma using the QtQuick 2D renderer. The screenshot shows some of the parts working, but also highlights some of the bugs

Performance is surprisingly fast, not faster than the OpenGL backend, but plasmashell still remains perfectly usable on my desktop.

Most of the basic scene graph nodes have a 2d renderer implementation, however any node that does custom openGL, such as QtGraphicalEffects or certain parts of Plasma, will simply fail.

What’s broken

There are plenty of known limitations with using the 2D renderer, some cosmetic, some more fundametnal.
Qt provides their own list.

In terms of Plasma, the list of broken items are:

  • We have our own GL check in the shell that needs adapting
  • Icons were broken. We implemented our own shader whilst animating between states rather than uploading a new pixmap per frame. I fixed this by simply turning that off.
  • Our load monitor plasmoids are pure GL. I’ve made a patch that makes it not crash. Making it actually work would mean having two implementations…which isn’t a route I really want to go down.
  • Widget explorer is broken, again we have our own shader for some effects
  • We are missing a lot of minor graphical effects. Fewer shadows and alike, but that’s a hit I think we will just have to accept


In general it seems that with a relatively small amount of work this might be a valid option for users without working openGL.

It will always be a second class citizen, but it should be do-able to support without hindering progress for the vast majority of users.

What’s interesting is to see how easy it is to support a different scene graph backend, as it is a clear indication of what we will encounter when it comes to Vulcan in a few years time.

Neon and Plasma Relationship

As we saw neon, a new and fresh Linux distribution was launched last week. This project is incubated by the KDE Community, sharing KDE’s hosting and community. Hopefully we’ll see neon flourish into an awesome distribution over time.

However, I have seen some potential confusion in an article reaching a conclusion that this might be in some way problematic for other distributions to deploy KDE software. To make sure we’re all on the same page I wanted to give a clarifying statement from the Plasma mantainer.

Plasma is and remains distro-agnostic. It’s in our interest to help all of our distribution channels. As long as distributions continue to keep up with the dependencies we need and work well with us, we support everyone as best as we can.

My new widget in Frameworks

One of the new features coming to Frameworks 5.15 is my new widget KCollapsibleGroupBox.

Acting like QGroupBox it allows you to hide some of the more advanced options out the way till the user expands the header revealing the rest. A common web pattern, but lacking in Qt or KF5.

Good for making a long list of pages navigable or hiding features that only a small fraction of users will find useful.

simple by default, powerful when needed

Handy, but mundane. Why the blog post?

There’s no point in writing a new class in a library if no-one knows about it.

Also I wanted to highlight two more interesting topics:

Working with the VDG

The main reason I wrote it was because every mockup given to me to by the VDG, especially from the usability expert Heiko Tietze, invariably featured this widget somewhere.

Our resources need to match what’s being requested from our design team as described by Thomas in his recent blog post.

Bringing our QWidget library forwards

Another important aspect that’s worth broadcasting is how we’re still moving our QWidget libraries forwards. Whilst there’s talk about them being outdated or replaced, that’s really not the case; and still definitely the best candidate for building any serious desktop application.

This isn’t the only new change; there’s still a lot of areas that can and should be improved to modernise our platforms for the current set of applications.

Legacy system tray icons back in Plasma 5

Didn’t we drop support?

Legacy system tray icons are problematic; they don’t scale, they don’t fit in with the theme, they can’t multiplex (be in two trays) and they’re just generally very dated.

We came up with a new scheme Status Notifier Items (SNIs) back in 2009, which was also adopted by our friends at Ubuntu in Unity, which provides logical information over DBus about what to show, rather than just an arbitrary window.

Our existing xembed code needed an entire rewrite, given wayland would also break xembed it seemed a more useful investment of time to make sure most the toolkits supported the SNI specification than work on the rewrite.

In retrospect, I think we underestimated the fallout this would cause. There are propreitory apps, more random toolkits that weren’t covered, and some distros didn’t apply all the patches needed

There’s nothing wrong with changing our minds in response to user feedback.

Restored Support

Having decided that we can’t drop support just yet, it seems unlikely we can drop it in a few months when we switch to Wayland. So we need to consider how to make that work there. Embedding an X window inside a wayland window isn’t going to work.

My solution was a hidden process, xembedsniproxy, renders the embedded windows offscreen then uses the existing SNI specification to inform Plasma. Plasma gets support, including multiplexing without really knowing about X.

It should be as seemless as anything else.

Current State

It’s at a state where it’s usable, but there are undoubtedly still some bugs. Every toolkit has their own quirks regarding system trays.

Please leave a comment if you find anything.

The code is currently in a test repository here

It is available already in yaourt for Arch users:

I hope to have it merged into Plasma for 5.5, depending on feedback.

Just another manic monday

I love project stats, and I completely obsess over bug reports.
Whilst collecting data for another post, a mystery libreoffice autocomplete popup me to plot something I hadn’t thought of plotting.

What days of the week are bugs opened and closed?

Resolved can mean one of a few things; fixing the bug with a change in the code, marking it as a duplicate of another bug or in some cases closing it as not a direction we want to go with Plasma.


People report more on weekdays than weekends

The difference surprised me. I think it gives a strong indication that Plasma 5 is being used more for work than as a hobby, with people more likely to encounter an area needing to improvement during the normal office week.

Obviously we don’t know for sure and we can’t pull useful timestamp information without knowing timezone of the user, but indications seem to be there.

Bugs are resolved on a weeked at 50% of the rate of the weekday

This isn’t too surprising given that some Plasma maintainers like myself work for Blue Systems or Red Hat and we tend to work office hours.

What’s encouraging is that, whilst slower, this shows that a significant amount of work does happen outside office hours; both employees taking the extra time to care for Plasma and our community contributors stepping up to work here. There’s some shift between the people doing weekday and weekend bug closing.

We get more bugs open than we close

Naturally our graph will show that we have some open bugs, but if we assume this rate continues our number of open bugs will continue to rise.

It’s not as bad as this graph makes it look as there also a lot of bugs in a zombie “needs info” state, where we’re waiting to hear back from a user either with more information or to confirm their issue is fixed.

We definitely need more help here, I want Plasma to be bug free.

It’s all about Tuesdays

Tuesday is apparently the day for the most reports getting handled. It’s also the day Plasma 5.4 gets released.

I always knew Tuesday was an underrated day of the week.

Powered By Swiss Cheese – My Plans For Randa

At the start of September I’m going to be taking a vacation from Blue Systems work; ignoring all things Plasma and spend an entire week to spend some time on some other projects.

This will be the second time I’ll be attending Randa. Last year was simulatenously one of the most hard-working enduring sprints possible yet also one of the most enjoyable weeks in close proximity to some of my best friends.

This year I intend to devote all my time working with the KDE Connect team.

As there is so much cross-project work, it’s a lot easier when I can talk to the right people, making Randa the right opportunity.

Some of my project plans during that week – KDE Connect

Smaller Connections?

Android Wear is becoming more than a gimmick. What should KDE Connect integrate? Should we make the media player remote work? Desktop notifications? Blood Pressure monitor as you read certain email threads… certainly some potential.

Desktop to Desktop

The main thing KDE Connect works is where you are in close proximity to your phone and desktop, but don’t want to keep physically switching between everything. For most normal people, a phone and a desktop is enough…but some of us have two machines that we have to switch between and most the same principles of mouse sharing, notifications(?) and files applies.

Most of the relevant code exists; it just needs that final bit of UI on top.

KDE Connect for SMS Messaging in KTp

I ported all the wonderful work by Alexandr Akulich to Qt5 with the KDE connect refactoring, and it’s all merged, bitrotting slowly, it just needs that final push of work to get it out the door.


I’m one of many many who will be at Randa working on a huge range of interesting projects.

We need money in order for sprints like Randa to go ahead; it costs money to get everyone under the same roof, and the organisation at Randa is done in a way that keeps everyone productive.

Whilst I’m lucky enough to fund my own flights, me from yesteryear would have struggled, and we are all giving up our time to work on KDE. We also need support to help get everyone doing great stuff there.

Please donate to the fundraiser using the giant link below

High DPI in Plasma 5.4

As retrofitting high DPI support into such a large range of both KDE and third party applications is risky to do without breakage, progress is deliberately slow and gradual in order to do this right.

The good news is I get to write lots of blog posts as we make the continual steps forwards.

Iterative Progress

Plasma 5.3 (last release)

* High DPI Scaling via a hidden config option for early testing

Plasma 5.4 (this release)

* User facing dialog with the screen management settings for scaling

Plasma 5.5+ (future releases)

* Automtatically enabled. Providing we see positive feedback in this release.

The New Setting

Plasma 5.4 brings a new setting to adjust the screen scaling to match your device.

In the new setting we’ve tried to simplify things to a simple slider which adjusts both font size which we have full control over, and then above a certain threshold adjusting the entire UI which we can only do at integer values (i.e doubling the size). This is to support the semi-high screens and the current generation all in the same place.

It’s still far from what I’d like to achieve overall, but it does the job of making some devices usable.

What Can We Expect From This Release?

With High DPI support enabled you will see one of three things amongst your applications:

  • The app sized sensibly, looking absolutely gorgeous on your new screen.
  • The app sized sensibly, but some aspects may not necessarily make the full use of the higher resolutions available.
  • The app not scaled at all remaining really small. We should see the font size increase to fit, but icons, dialogs, checkboxes may remain annoyingly small.

We’ve tried to make sure the most common, important KDE applications (Dolphin, Konsole, Kate and so on) fit into that top category. An app like xfig or dia, will remain in the bottom category until Wayland.

There’s also a slim chance there may also be some breakage.
If so please do file a bug report as soon as possible and tag me to the CC list.

Props especially go to Christoph Cullman and Alex Fiestas for their help on some of the applications.