Trying To Explain To Qt That I Need QGView To Follow My Player Sprite

[Grepur]

Hello

I'm trying to understand the Code I need to write in order to get the View window to follow the Player Sprite when it moves.
I've not found a clear example that explains 'the thinking behind the code'. Browser searches have helped me cobble together some Code which Qt6 accepts (no error messages) but also it's not working.

I'm working with player.h player.cpp, and the action's taking place in game.cpp .

I've got ItemChange code in Player.h :

#ifndef PLAYER_H
#define PLAYER_H

#include <QApplication>
#include <QObject>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QGraphicsPixmapItem>

class Player : public QObject , public QGraphicsPixmapItem {
    Q_OBJECT

public:
    Player(QGraphicsItem * parent=0);

signals:
    void positionChanged();

protected:
    void keyPressEvent(QKeyEvent *event) override;

    QVariant itemChange(GraphicsItemChange change, const QVariant &value) override {
    if (change == ItemPositionChange && scene()) {
        emit positionChanged();  // Emit signal when position changes
    }
    return QGraphicsPixmapItem::itemChange(change, value);
}

};

#endif // PLAYER_H

and I've got a Lambda expression in game.cpp :

#include "game.h"
#include <QImage>
#include <QBrush>
#include <QDebug>
#include <QGraphicsTextItem>
#include <QFont>

Game::Game(QWidget * parent) : QGraphicsView(parent) {

    // Create the scene
    scene = new QGraphicsScene();
    scene->setSceneRect(0,0,4800,2400);
    scene->setBackgroundBrush(QBrush(QImage(":/maps/map2.jpg")));
    setScene(scene);

    // Create player and set its position
    player = new Player();
    player->setPos(scene->width()/2, scene->height()/2);
    player->setFlag(QGraphicsItem::ItemIsFocusable);
    player->setFocus();
    scene->addItem(player);

    QGraphicsView *view = new QGraphicsView(scene);
    view->setFixedSize(1500, 1200);
    view->centerOn(player);


    // Connect the players's positionChanged signal to recentre the view on the player
    QObject::connect(player, &Player::positionChanged, [this, view]()
                     {
        view->centerOn(player);

    });

}

1. Are these Code Blocks in the right place?
2. Where am I hazy about what I need to say to the Programme to get the View following the Sprite around when it moves?

The only way I manage to learn is when I can look at Code and say it back to myself out loud, speaking in pseudocode, like a Director explaining the blocking in a Storyboard scene. Thanks for any help.
(I can be pretty thick sometimes, so please type slowly.)
Cheers

[JonB]

@Grepur
What is currently "not working"?
Although I have not looked at it in any detail, it seems reasonable to me.

[Pl45m4]

@Grepur said in Trying To Explain To Qt That I Need QGView To Follow My Player Sprite:

    QGraphicsView *view = new QGraphicsView(scene);
    view->setFixedSize(1500, 1200);
    view->centerOn(player);

What stands out to me is this:

Game is a QGraphicsView but you still create another independent instance ?!
Is there a reason to have two QGraphicViews?

[Grepur]

@JonB Hi. The Player sprite moves without the View following it; it doesn't keep the Player sprite in the centre of the View. The Player sprite can move out of the View; the View doesn't move.

[SGaist]

@Grepur @Pl45m4 is correct, you have created a secondary and invisible view that you center your sprite on. So you are not using the one you think.

I would recommend reconsidering your architecture.

[Grepur]

@Pl45m4 I'm not seeing where there are two instances of View that are the same, that one would be redundant. In one case, the Player sprite is made the focus, and in the Lambda expression, the View is intended to re-centre on the Player sprite each time the Player moves.

[SGaist]

@Grepur said in Trying To Explain To Qt That I Need QGView To Follow My Player Sprite:

@Pl45m4 I'm not seeing where there are two instances of View that are the same, that one would be redundant. In one case, the Player sprite is made the focus, and in the Lambda expression, the View is intended to re-centre on the Player sprite each time the Player moves.

Your Game class is a QGraphicsView. In its constructor your create a new QGraphicsView that you then use.

That's the second instance @Pl45m4 wrote about.

[JonB]

@Grepur
Have you understood @Pl45m4 post above?

In the constructor of Game, which itself is a QGraphicsView ,you create a new instance of a QGraphicsView. That is the one used in the connect(), but you never show it and it cannot be referenced once the constructor returns.

[Grepur]

@SGaist I don't know what 'reconsidering my architecture' means. I need to know how to see the situation, how to think about it, so that I'm clear on what needs to be done, and then I can do it correctly.

[Grepur]

@SGaist Hi. (Sorry for the delays in responses; I'm being delayed 600 seconds between posts today.)
I have deleted the second call to QGraphicsView, and now the lambda expression gets an error message about 'view' being an undeclared identifier. I'm trying to figure-out what that means now.
Thanks for the pointer.

[Grepur]

One of the error messages was fixed by my adding QGraphicsView * view in game.h .
I'm still snagged on the other one. Very new to lambdas. Barely understand what's expected or what are the rules for what information to place between the brackets.
I generally understand the signal/slot objective: connect Player, Player's changed Position... But I'm just blanking out at that point. Something about pairing the Change with the View, but all my guesses about what to put within those braces are rubbish.
I just need to know how to think about it. I'm lost down in the Code instead of up in the Overview where everything would make sense...

[SGaist]

That's the issue with your architecture.

You should take a pen and paper and draw what you want to achieve and the structure you currently have.

Your Game class should not be a QGraphicsView. Make it a QWidget, set a layout on it and then add that view to it.
If you're not yet fluent with lambdas, then let them on the side for now and use a member function for your slot.
Don't make things needlessly complicated while you are learning. Make it work and then refactor and improve.

[Grepur]

@SGaist Thank you. I'll do my best to put that advice to work.

[Pl45m4]

@Grepur

The (simple) fix is quite easy:
Remove your second view and everything related to it. Then replace every call to view with this (your actual Game class)... There might be some other structural flaws as QGraphicsView isn't the best place to hold all your game logic.
So you could split it into multiple classes like Game (backend) and GameView (your graphics view).

Assuming you're actually looking at the Game GraphicsView, change your lambda from

view->centerOn(player);

to

centerOn(player);

Also I wouldn't necessarily let your game/gameview hold a standard scene. You could introduce three classes (in addition to a QMainWindow for example):

• Game
• GameView (inheriting QGraphicsView as Game does now in your current situation)
• GameScene (inheriting QGraphicsScene, your game related custom scene, which you might want to modify other than having a standard QGraphicsScene)

[Grepur]

@Pl45m4 Thank you for those ideas. I'm feeling less wandering-the-wilderness now, thanks to the advice I'm getting here.