为什么update()方法不执行update()或update()方法？

Question

当我执行repaint()时，类中的update()或update()方法没有被调用，我就遇到了这个问题。下面是代码：

public class BufferedDisplay extends Canvas implements Runnable {

// Contains all the images in order, ordered from background to foreground
private ArrayList<ImageStruct> images;
// Tracks the last insert ID of the last image for a particular layer
private TreeMap<Integer, Integer> insertIDs;
// Image that holds the buffered Image
private Image offscreen;

public BufferedDisplay() {
    images = new ArrayList<ImageStruct>();
    insertIDs = new TreeMap<Integer, Integer>();
}

public void addImageStruct(ImageStruct is) {
    int layer = is.getLayer();
    // Index to insert the image at
    int index = -1;
    if(insertIDs.containsKey(layer)) {
        index = insertIDs.get(layer)+1;
        insertIDs.put(layer, index);
    }
    else {
        index = images.size();
        insertIDs.put(layer, index);
    }
    if(index>-1) {
        images.add(index, is);
    }
}

public void run() {
    try
    {
        while(true)
        {
            System.out.print("\nSleeping... ");
            System.out.print("ArraySize:"+images.size()+" ");
            Thread.sleep(1000);
            System.out.print("Slept. ");
            repaint();
        }
    }
    catch(Exception e)
    {
        System.out.println("Display Error: ");
        e.printStackTrace();
        System.exit(-1);
    }
}

// Overrides method so the background isn't automatically cleared
public void update( Graphics g )
{
    System.out.print("Updating... ");
    paint(g);
}

public void paint( Graphics g )
{
    System.out.print("Painting... ");
    if(offscreen == null)
        offscreen = createImage(getSize().width, getSize().height);
    Graphics buffer = offscreen.getGraphics();
    buffer.setClip(0,0,getSize().width, getSize().height);
    g.setColor(Color.white);
    paintImages(buffer);
    g.drawImage(offscreen, 0, 0, null);
    buffer.dispose();
}

public void paintImages( Graphics window )
{
    for(ImageStruct i : images) {
        i.draw(window);
    }
}
}

这个类在以下内容中实现：

public class Game extends JPanel{
// A reference to the C4Game class
private C4Game game;
// A reference to the BufferedDisplay class
private BufferedDisplay display;
// The Image used to initialize the game board
private Image tile;
private int tileSize = 75;
private int tileLayer = 5;
// Thread that controls animation for the BufferedDisplay
Thread animation;

public Game(C4Game game) {
    this.game = game;
    display = new BufferedDisplay();
    try {
        tile = ImageIO.read(new File("img\\tile.png"));
    } catch (IOException e) {
        System.out.println("ERROR: ");
        e.printStackTrace();
    }
    ((Component)display).setFocusable(true);
    add(display);
    animation = new Thread(display);
    animation.start();
    initBoard();
}

public void initBoard() {
    for(int x = 0; x<game.numRows()*tileSize; x+=tileSize) {
        for(int y = 0; y<game.numCols()*tileSize; y+=tileSize)  {
            System.out.println("Placing " +x +" " +y +"...");
            display.addImageStruct(new ImageStruct(tile, tileLayer, x, y, tileSize, tileSize));
        }
    }
}
}

然后，...Which在JFrame中实现。

public class TetraConnect extends JFrame{

    public TetraConnect() {
    super("TetraConnect", 800, 600);
    try {
        setIconImage(Toolkit.getDefaultToolkit().createImage("img/icon.png"));
        ms = new MainScreen(this);
        add(ms);
        ms.updateUI();
        C4Game c4g = new C4Game(5,6);
        Game g = new Game(c4g);
        add(g);
        g.updateUI();
    }
    catch(Exception e) {
        System.out.println("Init. Error: ");
        e.printStackTrace();
        System.exit(-1);
    }
}

当我运行它时，输出是：

Slept.
Sleeping... Slept.
Sleeping... Slept.
Sleeping... Slept.

以此类推。我也无法在画布上看到任何图像(我假设它们从一开始就没有绘制过)。它似乎完全跳过了重新绘制方法；调试语句“更新.”和“重新油漆.”永远不要出现。然而，重绘似乎也在执行；循环没有问题地重复。为什么repaint方法不调用update()或update()方法？

Answer 1

正如@camickr在评论中所指出的，您使用的是重量级AWT画布。您确实应该使用轻量级的Swing组件。而不是：

public class BufferedDisplay extends Canvas implements Runnable {

我建议：

 public class BufferedDisplay extends JPanel implements Runnable {

在这小小的改变之后，我会做以下几件事：

在重写组件的默认绘图时，应该重写paintComponent()方法。

因此，与其：

public void paint( Graphics g )
{

它应该是：

protected void paintComponent( Graphics g )
{

这可能会解决你的问题。

此外，您不应该必须重写update()方法。相反，只需在paint组件方法中省略对super.paintCompenent(g)的调用。默认情况下，这应该会导致背景被单独保留。

Answer 2

确保BufferedDisplay对象已经添加到容器中(例如，Frame)，并且容器本身是可见的。如果组件没有显示，那么对repaint()的调用将不会导致调用update()。

这只是一般性的建议。如果您发布了一个可以编译和运行的自给示例，那么可能更容易找出问题所在。