如何在Java Applet中立即顯示圖像？

Question

我在Java Applet中創建了一個簡單的內存游戲。 卡有問題。 在第一次出現時，圖像需要一些額外的加載時間。 如何解決？ 我需要在卡片翻轉后立即顯示圖像。

我顯示加載屏幕，直到應用程序不在AppStates.READY或AppStates.WAIT_FOR_START狀態，但它沒有幫助。

Memo.cs - 帶圖像加載的主類

public class Memo extends JApplet {

    //...   

    public void init() {
        //...   
        try {
            SwingUtilities.invokeAndWait(new Runnable() { @Override public void run() { createGUI(); }});
        } 
        catch(Exception e) {
            e.printStackTrace();
        }
    }

    private void createGUI() {
        final Model model = new Model(...);
        final View view = new View(model);

        getContentPane().add(view, BorderLayout.CENTER);
        setBackground(backgroundColor);
        setPreferredSize(new Dimension(width, height));

        model.setLoading(loadImages(loadingPath, format, 1));
        model.setCardsImages(loadImages(cardImagePath, format, 13));
        //...
        model.setAppState(AppStates.PROCESS);

        model.startNewGame();
        view.repaint();
    }

    private Image[] loadImages(String path1, String path2, int count) {
        Image[] imgs = new Image[count];        
        for(int i = 0; i < count; ++i) {
            imgs[i] = getImage(getCodeBase(), path1 + i + path2);
        }
        return imgs;
    }
}

Model.cs - 保存圖像和應用程序狀態，init板

public class Model {

    //...

    private AppStates appState;
    private Image[] cardsImages;

    public Model(...) {
        //...
        appState = AppStates.INIT;
    }

    public void startNewGame() {
        setAppState(AppStates.PROCESS);

        //... - init board - table with images' id

        setAppState(AppStates.WAIT_FOR_START);
    }

    public void setCardsImages(Image[] cardsImages) {
        this.cardsImages = cardsImages;
    }

    public Image getCardsImage(int v) {
        return cardsImages[v];
    }

    public AppStates getAppState() {
        return appState;
    }

    public void setAppState(AppStates appState) {
        this.appState = appState;
        //...
    }

    //...
}

View.cs - 顯示板

public class View extends JPanel {

    //...

    private Model model;

    public View(Model model) {
        this.model = model;
        //...
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        if(model != null) {
            switch (model.getAppState()) {

            //...

            case WAIT_FOR_START:            
            case READY:
                //...
                drawBoard(g2d, model.getBoard(), model.getStates(), model.getFrontTypes());
                break;

            }
        }
        repaint();
    }

    private void drawBoard(Graphics2D g2d, int[][] board, int[][] states, int[][] frontTypes) {
        if(board != null && states != null && board.length > 0 && states.length > 0) {
            for(int x = 0; x < board.length; x++) {
                for(int y = 0; y < board[x].length; y++) {  
                    if(states[x][y] != Model.HIDE) {
                        Image img = null;

                        //...
                        img = model.getCardsImage(board[x][y]);

                        g2d.drawImage(
                                img,
                                model.getFirstCardX() + x * model.getCardDistance() + x * model.getCardWidth(), 
                                model.getFirstCardY() + y * model.getCardDistance() + y * model.getCardHeight(), 
                                model.getCardWidth(), 
                                model.getCardHeight(),
                                null);

                        //...
                    }
                }
            }
        }
    }
}

Answer 1

來自JavaDocs ..

public Image getImage(URL url)

..

無論圖像是否存在，此方法始終立即返回。 當此applet嘗試在屏幕上繪制圖像時，將加載數據。

請改用ImageIO.read(URL) 。 此方法將“阻止”（它將停在該代碼行），直到讀取圖像。 所以當應用程序。 去使用圖像，它已經完全加載。

解決問題的另一種方法（不是你想要的方式）是改變..

g2d.drawImage(
    img,
    model.getFirstCardX() + x * model.getCardDistance() + x * model.getCardWidth(), 
    model.getFirstCardY() + y * model.getCardDistance() + y * model.getCardHeight(), 
    model.getCardWidth(), 
    model.getCardHeight(),
    null);

至..

g2d.drawImage(
    img,
    model.getFirstCardX() + x * model.getCardDistance() + x * model.getCardWidth(), 
    model.getFirstCardY() + y * model.getCardDistance() + y * model.getCardHeight(), 
    model.getCardWidth(), 
    model.getCardHeight(),
    this);

如果圖像是異步加載的（例如，使用Applet.getImage(URL) ），GUI會在更多可用時通知重繪。

Answer 2

你可以利用Swing的雙緩沖，並在一切准備就緒時立即顯示。

如果您需要在中間進行，可以保留預先制作的副本並在需要時顯示。

Answer 3

預取圖像的一種方法是使用MediaTracker 。 使用此類可確保在給定點加載圖像。 以下是文檔中的優秀演示：

import java.applet.Applet;
import java.awt.Color;
import java.awt.Image;
import java.awt.Graphics;
import java.awt.MediaTracker;

public class ImageBlaster extends Applet implements Runnable {
    MediaTracker tracker;
    Image bg;
    Image anim[] = new Image[5];
    int index;
    Thread animator;

    // Get the images for the background (id == 0)
    // and the animation frames (id == 1)
    // and add them to the MediaTracker
    public void init() {
        tracker = new MediaTracker(this);
        bg = getImage(getDocumentBase(),
                "images/background.gif");
        tracker.addImage(bg, 0);
        for (int i = 0; i < 5; i++) {
            anim[i] = getImage(getDocumentBase(),
                    "images/anim"+i+".gif");
            tracker.addImage(anim[i], 1);
        }
    }

    // Start the animation thread.
    public void start() {
        animator = new Thread(this);
        animator.start();
    }

    // Stop the animation thread.
    public void stop() {
        animator = null;
    }

    // Run the animation thread.
    // First wait for the background image to fully load
    // and paint.  Then wait for all of the animation
    // frames to finish loading. Finally, loop and
    // increment the animation frame index.
    public void run() {
        try {
            tracker.waitForID(0);
          tracker.waitForID(1);
        } catch (InterruptedException e) {
            return;
        }
        Thread me = Thread.currentThread();
        while (animator == me) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                break;
            }
            synchronized (this) {
                index++;
                if (index >= anim.length) {
                    index = 0;
                }
            }
            repaint();
        }
    }

    // The background image fills the frame so we
    // don't need to clear the applet on repaints.
    // Just call the paint method.
    public void update(Graphics g) {
        paint(g);
    }

    // Paint a large red rectangle if there are any errors
    // loading the images.  Otherwise always paint the
    // background so that it appears incrementally as it
    // is loading.  Finally, only paint the current animation
    // frame if all of the frames (id == 1) are done loading,
    // so that we don't get partial animations.
    public void paint(Graphics g) {
        if ((tracker.statusAll(false) & MediaTracker.ERRORED) != 0) {
            g.setColor(Color.red);
            g.fillRect(0, 0, size().width, size().height);
            return;
        }
        g.drawImage(bg, 0, 0, this);
        if (tracker.statusID(1, false) == MediaTracker.COMPLETE) {
            g.drawImage(anim[index], 10, 10, this);
        }
    }
}

如果您可以保證這將始終在Java 1.4或更高版本上運行，則可以使用[ImageIO.read][2]來獲取圖像。 它會等到圖像加載完畢。