[Sylvia Winkler]

Hi,

my Felgo App crashes every time I use PhysicsWorld+BoxCollider and then navigate back from the NavigationStack. This behaviour only appears when I use those two components combined (PhysicsWorld and BoxCollider). My guess is that the deconstruction of entities is causing troubles (see StackTrace below). It crashes in the LiveDemo on my computer and on my phone (iOS) as well.

This is a minimal version of the code that crashes:

import Felgo 3.0
import QtQuick 2.0

App {

    NavigationStack {
        Page {
            id: main

            SimpleButton {
                text: "next scene"
                onClicked: main.navigationStack.push(component)
            }
        }
    }

    Component {
        id: component

        Page {
            GameWindowItem {
                activeScene: subScene

                EntityManager {
                    id: entityManager
                    entityContainer: subScene
                }

                Scene {
                    id: subScene

                    PhysicsWorld {
                        gravity: Qt.point(0, 25)
                    }

                    EntityBase {
                        id: player
                        entityType: "player"
                        width: 25
                        height: 25

                        Rectangle {
                            anchors.fill: parent
                            color: "blue"
                        }

                        BoxCollider {
                            id: playerCollider
                            anchors.fill: parent
                        }
                    }
                }
            }
        }
    }
}

An excerpt from the crash report:
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 net.vplay.apps.QMLLive 0x000000010b4cbc4a Box2DBody::~Box2DBody() + 42
1 net.vplay.apps.QMLLive 0x000000010b4c533f QQmlPrivate::QQmlElement<Box2DBody>::~QQmlElement() + 47
2 org.qt-project.QtCore 0x000000010dd980e0 QObjectPrivate::deleteChildren() + 224
3 org.qt-project.QtCore 0x000000010dd97ec4 QObject::~QObject() + 1700
4 org.qt-project.QtQuick 0x000000010bf32647 QQuickItem::~QQuickItem() + 871

I am using MacOS, QtCreator 4.10.2, Felgo 3.3.0.

What can I do to avoid this problem?

Thanks for your help!

Sylvia

[AlexFelgo Team]

Hi Sylvia,

we recommend not to destroy the PhysicsWorld, as this can cause unexpected behaviour, e.g. the crash you experience. Our usual recommendation is to have one instance of your PhysicsWorld in your root component, and re-parent it to your desired component when required. Here is a quick example of how this could look like:

import Felgo 3.0
import QtQuick 2.0

App {

    // those aliases as child of the root item make the id's available globally
    property alias physicsContainer: physicsContainer
    property alias physicsWorld: physicsWorld
    
    Item {
        id: physicsContainer
        anchors.fill: parent

        PhysicsWorld {
            id: physicsWorld
            gravity: Qt.point(0, 25)
        }
    }

    NavigationStack {
        Page {
            id: main

            SimpleButton {
                text: "next scene"
                onClicked: main.navigationStack.push(component)
            }
        }
    }

    Component {
        id: component

        Page {
            // depending on your app, there might be better spots where to do the re-parenting and start/stop your game
            onPushed: {
                physicsWorld.parent = subScene
            }
            onPopped: {
                physicsWorld.parent = physicsContainer
            }
            GameWindowItem {
                activeScene: subScene

                EntityManager {
                    id: entityManager
                    entityContainer: subScene
                }

                Scene {
                    id: subScene

                    EntityBase {
                        id: player
                        entityType: "player"
                        width: 25
                        height: 25

                        Rectangle {
                            anchors.fill: parent
                            color: "blue"
                        }

                        BoxCollider {
                            id: playerCollider
                            anchors.fill: parent
                        }
                    }
                }
            }
        }
    }
}

Let me know if such an approach works for you or if there are any more questions.

Cheers,
Alex

[Sylvia Winkler]

Hi Alex,

thanks for the fast and helpful answer! That totally solved my issue.

Have a great week,

Sylvia