为什么我的while循环不能在paintComponent中工作？

Question

When I run this code, I see nothing but a blank(white) Panel and I would like to know why.

Here is my code:

Graph.java

public class Graph extends JPanel {
    private static final long serialVersionUID = -397959590385297067L;
    int screen=-1;
    int x=10;
    int y=10;
    int dx=1;
    int dy=1;       
    boolean shouldrun=true;
    imageStream imget=new imageStream();

        protected void Loader(Graphics g){

            g.setColor(Color.black);
            g.fillRect(0,0,x,y);
            x=x+1;
            y=y+2;

        }


        @Override
        protected void paintComponent(Graphics g){
            super.paintComponent(g);
                while(shouldrun){
                    Loader(g);   
                    try {
                        Thread.sleep(200);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }    
                }   
        }
}

Answer 1

Do not ever call Thread.sleep() on the Event Dispatch Thread !!!

This causes the thread that actually redraws the screen and makes controls responsive to stop doing anything .

For animations, use a Timer . Don't worry about writing the while loop yourself, just tell the Timer to fire every so often, and change the values of x and y inside that timer. Something like:

// this is an **inner** class of Graph
public class TimerActionListener implements ActionListener {
    @Override
    public void actionPerformed(ActionEvent e) {
        x += dx;
        y += dy;
    }
}

// snip

private final Timer yourTimer;

public Graph() {
    yourTimer = new Timer(2000, new TimerActionListener());
    timer.start();
}

@Override
protected void paintComponent(Graphics g){
    super.paintComponent(g);
    g.setColor(Color.black);
    g.fillRect(0,0,x,y);
}

Answer 2

You never change the state of shouldrun within the loop -- so it will never end.

Also, never call Thread.sleep(...) within a painting method. This method is for painting and can never be put to sleep, else the GUI will be put to sleep, will be frozen.

Answer 3

First of all, your paintComponent method should only handle all painting and nothing else (if possible). You should not implement your program loop within paintComponent.

The blank screen can be caused by a number of reasons. You can easily debug it manually by commenting off certain section of your codes and run it. See whether it is still blank.

At least from what I see here, your paintComponent will give your problems.

If you want an animation, you can:

1. Use a swing timer

2. Create a loop in a new thread (not Event Dispatch Thread). Your loop will look something like this:

As below:

while(running){
    update();
    render();
    try(
        Thread.sleep(1000/fps);
    )catch(InterruptedException ie){
        ie.printStackTrace();
    }
}

Note: To make a proper loop for animation, you will need more than that.