重新验证JPanel时闪烁

Question

I have looked all round, and no one seems to have an answer for me. I have a JPanel where I add/remove labels from it while it;s running. The problem is that every time I add a new label to it I have to revalidate() it so the new label shows up. Every time I revalidate() , it I get a very annoying flicker, and all my labels ghost up to the top of my window for a millisecond then return to what they were doing. The only time I call revalidate() is after I add a label; I do not change the paint method or anything only did

public void paintComponent(Graphics page)
{
    super.paintComponent(page);
}

so that my page would not look bad. I could really use some help on this. I've made a asteroids game using JFrame , Jlabel and Jpanel game; it work great, but this flickering and component "ghosting quickly ghosting to top of panel" is just unbearable, and I know for a fact this is the revalidate() method.

Answer 1

When I was having problems with filckering Swing components it turned out to be concurrency issues. Since you are doing a game, which means you are doing animation it may be the case here.

Take care to modify Swing components only from AWT-EVENT thread . Either use SwingUtilities.invokeLater or do your animation using javax.swing.Timer .

Answer 2

As a concrete example, Asteroids doesn't flicker at all. Here are a few reasons:

• It runs on the event dispatch thread .

• It uses JPanel , which is double buffered by default.

• It paces the animation using javax.swing.Timer , updating the game's model in actionPerformed() .

• It overrides paintComponent() , but can safely omit super.paintComponent() because the action listener repaints the entire panel.

• The listener calls paintImmediately() to avoid any potential delay; substitute repaint() to see if there's any difference on your platform.

Addendum: As shown in this counter-example , revalidate() does not cause flickering. Instead of replacing labels, update the label's icon, as shown here .

Answer 3

Have you considered reusing them - ie using setVisible(false) when they're not needed?

Not sure if adding and removing labels is the best way of achieving what you want for a game. I'd personally be tempted to manage the drawing myself.

Answer 4

Using labels (or generally GUI components) to represent quickly animating, dynamically created/moved objects is not practical. Its like using a car to drive from the living room to the kitchen.

The Swing components come with a lot of nice features, eg they can determine the space they require and a layout manager can use that to automatically place them nicely. Unfortunately for a game, you neither need all the nice stuff, nor do you want to pay the processing overhead involved with it. Matters are further complicated by swings multi-platform capabilities which mandate that each component has a peer component (the peer deals with rendering the component to the active look).

To solve this, you do not represent individual game objects as swing components. Instead you use a single swing component (usually JPanel) as a canvas to paint your game objects on to. You simply call repaint() for that JPanel periodically (eg 30 or 60 times a second). Of course you will override paintComponent() on the JPanel to do paint all the game objects using your own code.

Edit: To outline how you do your own rendering

You need to decide how to organize your game related objects. A very simple skeleton could look like this (getters/setters omitted):

public class GameObject {
    public int x;
    public int y;
    public Image image;
}

In practice this class will have additional members (like a behavior that controls how the object moves, and maybe some states/counters for animation). It may also be abstract with a concrete subclass for each possible behavior (like ship, asteroid and bullet). All instances of GameObject are managed in some kind of collection, I assume you use a List.

When implementing the paintComponent it comes down to just loop through the list and render each objects's image:

public void paintComponent(Graphics g) {
    List<GameObject> list = ... // however you obtain it in your game
    for (GameObject gobject : list) {
        g.drawImage(gobject.image, gobject.x, gobject.y, (ImageObserver) null);
    }
}

More elaborate games may order the gameobjects to get control of how game objects overlap each other. But in principle its incredibly simple.