Added binding loop tracker

Author: cajus

Diff for: binding-loops/LICENSE

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+Copyright 2023 GONICUS GmbH <[email protected]>
+Copyright 2016 David Edmundson <[email protected]>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished
+to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

Diff for: binding-loops/README.md

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+# Traces for binding loops
+
+Sometimes you manage to create *binding loops* in QML. That basically means
+that there's a change to a property *x*, which uses QML bindings in its
+calculation, loops back to itself after it has been modified.
+
+While many of them are obvious and easy to fix, it can become tricky to
+understand the cause if there are various nested components involved. You'll
+just get some
+
+```shell
+8:45:32.168 ?foo?|qt_message_output|QDebug::~QDebug \
+  qrc:/de/gonicus/ui/components/controls/Spinner.qml:199:5: \
+  QML TextField: Binding loop detected for property "text"
+```
+
+and you've no idea what it comes from after some digging.
+
+For me, this always ended up in the debugger, looking at the backtrace, and
+trying to understand what's going on manually.
+
+## Automation with gdb
+
+Doing it a couple of times in the past, I stumbled upon a 2016 blog entry of 
+[David Edmundson]([email protected]:gonicus/qml-helper.git), who did a small
+python script to automate the tracing for Qt 5. Based on his work, I've adapted
+it to work with Qt 6.
+
+### Preconditions
+
+To make use of the script, you need Qt 6 and `gdb` packages installed.
+
+### Usage
+
+Just run `loop-tracker.py path/to/your/binary`. This will fire up **gdb** and
+start your app. Slowly. When it traps into a binding loop, it unwinds the stack
+and tries to find places where the update happens. It then prints the QML
+file / line number, and continues.
+
+```
+$ ./loop-tracker.py ~/Qt/6.5.0/gcc_64/bin/qmlscene test.qml 
+Starting gdb - please wait...
+[...]
+Thread 1 "qmlscene" hit Breakpoint 1, QQmlPropertyBinding::createBindingLoopErrorDescription (this=0x43bb20) at /home/qt/work/qt/qtdeclarative/src/qml/qml/qqmlpropertybinding.cpp:265
+265	/home/qt/work/qt/qtdeclarative/src/qml/qml/qqmlpropertybinding.cpp: No such file or directory.
+===== Binding loop detected =====
+
+Backtrace:
+#0 - file:///var/home/prcs1076/Projekte/qml-helper/binding-loops/test.qml:6:9
+
+file:///var/home/prcs1076/Projekte/qml-helper/binding-loops/test.qml:4:5: QML Rectangle: Binding loop detected for property "width"
+[...]
+```
+
+That's not perfect, but helped to find some of the more hidden loops here.

Diff for: binding-loops/loop-tracker.py

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+#!/usr/bin/env python3
+import sys
+import subprocess
+
+try:
+    import gdb
+
+except:
+    from shutil import which
+    from os.path import basename
+
+    args = sys.argv
+    selfName = args.pop(0)
+
+    if not selfName.endswith(".py"):
+        print("'%s' needs to end with '.py'" % basename(selfName))
+        exit(1)
+
+    if not which("gdb"):
+        print("'gdb' binary not found")
+        exit(1)
+
+    print("Starting gdb - please wait...")
+    subprocess.call(["gdb", "--silent", "--command", selfName, "--args"] + args)
+    exit(0)
+
+
+def printq6string(val):
+    d = val['d']
+    data = d['ptr'].reinterpret_cast(gdb.lookup_type('char').pointer())
+    data_len = d['size'] * gdb.lookup_type('unsigned short').sizeof
+    return data.string('utf-16', 'replace', data_len)
+
+
+def breakpointHandler(event):
+    frame = gdb.newest_frame()
+
+    if frame.name() == "QQmlPropertyBinding::createBindingLoopErrorDescription" or \
+       frame.name() == "QQmlAbstractBinding::printBindingLoopError":
+
+        print("===== Binding loop detected =====")
+        print("\nBacktrace:")
+
+        i = 0
+        while True:
+            frame = frame.older()
+
+            if frame == None or not frame.is_valid():
+                break
+
+            if frame.name() == "QQmlBinding::update":
+                currentBinding = frame.read_var("this")
+                eval_string = "(*(QQmlBinding*)(%s)).expressionIdentifier()" % str(currentBinding)
+                identifier = printq6string(gdb.parse_and_eval(eval_string))
+                print("#" + str(i) + " - " + identifier)
+                i+=1
+
+        print()
+
+        gdb.execute("continue")
+
+
+def main():
+    gdb.events.stop.connect(breakpointHandler)
+
+    bp1 = gdb.Breakpoint("QQmlPropertyBinding::createBindingLoopErrorDescription")
+    bp2 = gdb.Breakpoint("QQmlAbstractBinding::printBindingLoopError")
+
+    gdb.execute("run")
+
+
+if __name__ == "__main__":
+    main()
+

Diff for: binding-loops/test.qml

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+import QtQuick 2.0
+
+Rectangle {
+    width: childrenRect.width
+    Text {
+        text: parent.width > 10 ? "Hello World" : "Hi"
+    }
+}