QML Key Press Events break when multiple keys are pressed

[logic]

Hey everyone, I'm having an issue with my keyPress events. My problem happens when I hold down one key, and press another (not held) at the same time. Basically, think of playing Super Mario where you're running left, right, and jumping. Well, my character moves left, right and jumps... but if I'm moving right or left and press jump, my character stops moving left or right.

Now, I've looked into my code and can only conclude it's a problem in the key events. This is my code:

Keys.onPressed: {
 
                if (event.key == Qt.Key_Left) {
                    console.log("Left")
                    player.moveHoriz = -1;
                    player.moveBackward();
 
                }
                else if (event.key == Qt.Key_Right) {
                    console.log("Right")
                    player.moveHoriz = 1;
                    player.moveForward();
 
                }else if (event.key == Qt.Key_Up) {
                    console.log("Jump")
                    player.jump();
                }
            } Keys.onReleased: {
 
                if (event.isAutoRepeat)
                    return ;
 
                if (event.key == Qt.Key_Left ||
                    event.key == Qt.Key_Right) {
                    player.moveHoriz = 0;
 
                    return ;
                }
 
                //if (event.key == Qt.Key_Up){
                //    return ;
                //}
            }

Keys.onPressed: {

                if (event.key == Qt.Key_Left) {
                    console.log("Left")
                    player.moveHoriz = -1;
                    player.moveBackward();

                }
                else if (event.key == Qt.Key_Right) {
                    console.log("Right")
                    player.moveHoriz = 1;
                    player.moveForward();

                }else if (event.key == Qt.Key_Up) {
                    console.log("Jump")
                    player.jump();
                }
            } Keys.onReleased: {

                if (event.isAutoRepeat)
                    return ;

                if (event.key == Qt.Key_Left ||
                    event.key == Qt.Key_Right) {
                    player.moveHoriz = 0;

                    return ;
                }

                //if (event.key == Qt.Key_Up){
                //    return ;
                //}
            }

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If I run my game in a console, this is what is happening behind the scenes.



As you can see, my held-key event is broken upon jumping. I have to take my finger off of my held key (such as the right arrow) and press it again to start moving right once again. I know that QML is not the best tool for building games in, but I'm sure it can't be this limited in key event control. If it makes a difference, I'm on Ubuntu 14.04 - Linux Kernel 3.13.0-43-generic.

Thanks for the help.

[wysota]

Your app behaves the way you told it to. If you press a key, you get a key event with that key in the event's key property. Then you get more events for that key because of autoRepeat being active, hence your event handlers fire again. There is no logic in your code for handling keys which are already pressed when a new event arrives. A regular approach to handling key events in games is to have a set of flags which are raised or lowered based on incoming events.

SomeElement {
  id: root
  // ...
 
  property bool isMovingRight: false
 
  Keys.onRightPressed: {
    if(event.isAutoRepeat) return
    isMovingRight = true
  }
  Keys.onReleased: {
    if(event.isAutoRepeat) return    
    if(event.key === Qt.Key_Right) isMovingRight = false
  }
 
  states: [
    State {
      name: "characterMovingRight"
      when: root.isMovingRight
    }
  ]
}

SomeElement {
  id: root
  // ...

  property bool isMovingRight: false

  Keys.onRightPressed: {
    if(event.isAutoRepeat) return
    isMovingRight = true
  }
  Keys.onReleased: {
    if(event.isAutoRepeat) return    
    if(event.key === Qt.Key_Right) isMovingRight = false
  }

  states: [
    State {
      name: "characterMovingRight"
      when: root.isMovingRight
    }
  ]
}

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By the way:

I know that QML is not the best tool for building games in

Where exactly do you know that from? My impression is exactly opposite to that of your source of information.

[logic]

Okay, your explanation did help give me better results. I can now jump and move at the same time and continue moving. I utilized a Timer and set a flag on the player. While this does make things a lot smoother. My next issue is this (and I still find it's related to the key events). If I'm holding down the right key and my character goes to the right, AND THEN I hold the left key down, my character starts moving left (that's fine). If I let go of the left key, my flag resets to zero instead of catching that the right key is still held down and it should go back to 1 (-1 = left, 1 = right). It's only a real problem if I'm trying to slow my character down by "tapping" the opposite direction. I don't know how to accomplish this with the release events since I don't want a "tap" to keep me from continuing in the same direction. I appreciate the help, thank you very much.

Keys.onLeftPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = -1;
                player.startTimer();
            }
            Keys.onRightPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = 1;
                player.startTimer();
            }
            Keys.onUpPressed: {
 
                player.jump();
            }
 
 
 
 
            Keys.onReleased: {
 
                if (event.isAutoRepeat)
                    return ;
 
                if (event.key == Qt.Key_Left &&
                    event.key == Qt.Key_Right) {
                    player.moveHoriz = -player.moveHoriz;
                    return ;
                } else if (event.key == Qt.Key_Left || event.key == Qt.Key_Right){
                    player.moveHoriz = 0;
                }
 
                }
            }

Keys.onLeftPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = -1;
                player.startTimer();
            }
            Keys.onRightPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = 1;
                player.startTimer();
            }
            Keys.onUpPressed: {

                player.jump();
            }




            Keys.onReleased: {

                if (event.isAutoRepeat)
                    return ;

                if (event.key == Qt.Key_Left &&
                    event.key == Qt.Key_Right) {
                    player.moveHoriz = -player.moveHoriz;
                    return ;
                } else if (event.key == Qt.Key_Left || event.key == Qt.Key_Right){
                    player.moveHoriz = 0;
                }

                }
            }

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[wysota]

Flags for right and left should be exclusive. Setting one should clear the other.

By the way the condition from lines 24 and 25 will never evaluate to true.

Also try doing things more in declarative manner. In particular I don't like the timer of yours.

[logic]

Thank you. I actually did get it to work once I set exclusive flags. I feel stupid, my mistake. I don't know what to use other than a timer, though I don't have it starting upon key events at this point. I use the timer to control the incremental acceleration, but if there is a better way, I wouldn't mind hearing it.

Thank you for the help, though. I actually have the fundamentals of the controls I need now. Anyway, here's what my code looks like now and it's definitely working:

Keys.onLeftPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = -1
                player.moveLeft = true;
            }
            Keys.onRightPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = 1;
                player.moveRight = true;
            }
            Keys.onUpPressed: {
 
                player.jump();
            }
 
 
 
 
            Keys.onReleased: {
 
                if (event.isAutoRepeat)
                    return ;
 
                if(event.key == Qt.Key_Left){
                    player.moveLeft = false;
                }
                if(event.key == Qt.Key_Right){
                    player.moveRight = false;
                }
 
 
            }

Keys.onLeftPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = -1
                player.moveLeft = true;
            }
            Keys.onRightPressed: {
                if(event.isAutoRepeat) return;
                player.moveHoriz = 1;
                player.moveRight = true;
            }
            Keys.onUpPressed: {

                player.jump();
            }




            Keys.onReleased: {

                if (event.isAutoRepeat)
                    return ;

                if(event.key == Qt.Key_Left){
                    player.moveLeft = false;
                }
                if(event.key == Qt.Key_Right){
                    player.moveRight = false;
                }


            }

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In regards to where I had heard QML wasn't great for games, it was from a developer I knew who builds on Windows. I think he feels the performance is lackluster, but I don't know what his personal gripes with it might be. Anyway, it hasn't stopped me from working with it and that's why I came here to find out how to do what I needed to do. As I said, thanks for the help. I don't often come to forums, but this was more than helpful.

[wysota]

Instead of a timer you could use a Behavior or Animation (or both). But even with a timer you should state when the timer is active instead of manually starting and stopping it. Don't try replicating imperative approach in a declarative environment. This leads to clutter in code and less performant result.