Why does QQmlComponent::create() return null?

Tom asso

Under what conditions does QQmlComponent::create(QQmlContext *) fail by returning nullptr? The documentation only says " Returns nullptr if creation failed" without further detail. My backend C++ code tries to instantiate a MessageDialog from the following .qml:

import QtQuick 2.0
import QtQuick.Dialogs 1.1

MessageDialog {
  id: messageDialog
  title: "My message"
  text: "Fill in this message from C++"
  onAccepted: {
      console.log("Knew you'd see it my way!")
      // Hide the dialog
      visible = false
  }

  Component.onCompleted: visible = true
}

And here's a fragment of my backend constructor:

    QQmlApplicationEngine engine;

    // Note that resource URL is a local file, not network,
    // no need to wait before calling create()? 
    QQmlComponent *component = 
          new QQmlComponent(&engine, 
                            QStringLiteral("ui-components/MessageDialog.qml"));

    // Following call returns nullptr - why???
    QObject *childItem = component->create();

Anyone know why? Thanks!

jsulm

@Tom-asso said in Why does QQmlComponent::create() return null?:

QStringLiteral("ui-components/MessageDialog.qml"));

You're using a relative path: are you sure the QML file is actually found?

Also, before calling create() - did you check https://doc.qt.io/qt-5/qqmlcomponent.html#isError and https://doc.qt.io/qt-5/qqmlcomponent.html#isNull ?

raven-worx

@Tom-asso said in Why does QQmlComponent::create() return null?:

Anyone know why?

QQmlComponent::errors()

Tom asso

@jsulm said in Why does QQmlComponent::create() return null?:

https://doc.qt.io/qt-5/qqmlcomponent.html#isNull

It was a relative path problem - thank you!
My application did not print out any Qt-level messages to stderr when create() failed - it was silent and mysterious. ;-) Is there any way to make Qt more verbose? For example if it had printed out "QML file not found" that would have saved me hours of trouble!

Thanks
Tom

fcarney

@Tom-asso said in Why does QQmlComponent::create() return null?:

Is there any way to make Qt more verbose?

I don't know if there is an automatic way, but you might start using asserts to check for null and other conditions. The asserts will only translate to code when in debug mode, in release they will disappear.

Q_ASSERT_X(component != nullptr, "createComponentFunction", QQmlComponent::errors()[0].toString()); // might need a lambda to get all errors in list
or 
Q_ASSERT(component != nullptr);

jsulm

@Tom-asso Did you try to call errors() after creating the instance but before calling create?

Tom asso

@jsulm - No, I didn't know about that function - looks like it may be very useful!

jsulm

@Tom-asso It's what @raven-worx suggested above...

CrouchEndTiger

Just to add to this.

I was having QQmlComponent::create() failing too, and it wasn't any problem with the paths.

It turned out to be the root type use to instantiate the component was an ApplicationWindow.

I'm not sure if this was clashing with my main QApplicationMainWindow class that I had elsewhere..?

Anyway changing the root type to something an Item fixed the problem for me.