JScrollPane 中的画布仅在滚动条位于最极端点时重新绘制

Question

我正在为自己开发一个游戏项目，以提高我的 Java 技能，今天我遇到了一个我似乎无法借助互联网解决的问题。这个问题大部分已经解决了，但最后一点仍然是挑衅！

我有一个对象，它扩展了 Canvas 并创建了我想要显示的地图。我已将此 Canvas 添加到 JPanel，然后将其添加到 JScrollPane（添加到游戏的 JFrame 中）的视口。将 ChangeListener 添加到 JScrollPane 的视口中，它将在 JFrame 上执行 validate 方法，并在视口上执行 repaint 方法。
JFrame 本身有 JMenuBar，其中菜单结构中有一个“新游戏”选项（以及其他尚未实现的 JMenuItems），它将创建一个新的 Canvas 对象并替换旧的对象并验证（使用验证方法）重新绘制 JScrollPane 。

结果是，当我按下 New Game JMenuItem 时，我的窗口窗格中会绘制出一张新地图。它正确显示地图（画布对象），并显示滚动条。菜单位于画布和滚动窗格的顶部，这是正确的。
但是，当我稍微更改滚动条的位置（水平或垂直，使用滚动条或滚动条中的按钮）时，会导致 Canvas 对象在 JFrame 上进行平移。这意味着它可能会与滚动条、菜单栏（并且立即绘制在打开的菜单上）重叠，而视图之外的 Canvas 对象部分不会显示。仅当滚动条实际到达其极值点（无法再进一步）时，它才会按应有的方式重新绘制并验证整个场景。但显然，当我只是轻推滚动条或类似的东西时，我也希望它这样做。

我的问题是：我怎样才能使这项工作按预期进行？
我认为我需要对 ChangeListener 做一些事情，但我自己似乎无法找到解决方案。

正如我注意到你们大多数人都要求源代码，我已经为您提供了：

package otherFiles;


导入 java.awt.BorderLayout;
导入java.awt.Color；
导入java.awt.Graphics；
导入 java.awt.Graphics2D;
导入 java.awt.event.ActionEvent;
导入 java.awt.event.ActionListener;
导入 java.awt.geom.Ellipse2D;
导入 java.awt.geom.Line2D;
导入 java.awt.geom.Rectangle2D;
导入java.util.ArrayList；

导入 javax.swing.JComponent;
导入 javax.swing.JFrame;
导入 javax.swing.JMenu;
导入 javax.swing.JMenuBar;
导入 javax.swing.JMenuItem;
导入javax.swing.JPanel；
导入 javax.swing.JScrollPane;

公共类 ProblemDemonstrator {
        私有静态 JScrollPane dispArea = null;
    私有静态 JPanel mapPanel = null;
    私有静态 JFrame thisFrame = null;

private static final int FRAME_WIDTH = 300;
private static final int FRAME_HEIGTH = 300;
private static boolean mode = false;

public static void main(String[] args){
    JFrame mainMenu = myMenuFrame();

    mainMenu.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    mainMenu.setVisible(true);
}

public static JFrame myMenuFrame(){
    thisFrame = new JFrame("Problem Demonstrator");
    thisFrame.setSize(FRAME_WIDTH, FRAME_HEIGTH);
    thisFrame.setLayout(new BorderLayout());
    //Create the menu bar and corresponding menu's.
    JMenuBar menuBar = new JMenuBar();
    thisFrame.setJMenuBar(menuBar);
    menuBar.add(createFileMenu(),BorderLayout.NORTH);

    //Create a scroll-able game display area
    dispArea = new JScrollPane();
    dispArea.setSize(thisFrame.getSize());
    thisFrame.getContentPane().add(dispArea, BorderLayout.CENTER);

    return thisFrame;
}

private static JMenu createFileMenu() 
{
    JMenu menu = new JMenu("File");
    menu.add(createFile_NewGameItem()); //New game button
    return menu;
}

/**
 * The button for the creation of a new game.
 * @return The JMenuItem for starting a new game.
 */
private static JMenuItem createFile_NewGameItem() 
{
    JMenuItem item = new JMenuItem("New Game");
    class MenuItemListener implements ActionListener
    {
        @Override
        public void actionPerformed(ActionEvent arg0)
        {
            System.out.println("actionPerformed - New Game [JMenuItem]");
            if(mapPanel == null){
                mapPanel = createGameMap(mode);
                dispArea.setViewportView(mapPanel);

            }else{
                dispArea.getViewport().remove(mapPanel);
                mapPanel = createGameMap(mode);
                dispArea.setViewportView(mapPanel);
            }
            //'flip' mode
            if(mode){
                mode = false;
            }else{
                mode = true;
            }

            thisFrame.pack();
            thisFrame.setSize(FRAME_WIDTH, FRAME_HEIGTH);
            dispArea.repaint();
            thisFrame.validate();
        }
    }
    ActionListener l = new MenuItemListener();
    item.addActionListener(l);
    return item;
}

/**
 * This creates the displayable map that has to go in the JScrollPane
 * @param mode Just a variables to be able to create 'random' maps.
 * @return The displayable map, a JPanel
 */
private static JPanel createGameMap(boolean mode){

    /**
     * This is a quick version of the planets I generate for the map.
     * Normally this is a another class object, using another class called Planet
     * to set parameters like the location, size and color in it's constructor.
     * x = the x location on the map (center of the planet!)
     * y = the y location on the map (also center)
     * diam = the diameter of the planet
     */
    @SuppressWarnings("serial")
    class myPlanetComponent extends JComponent{
        private int x,y,diam;
        public myPlanetComponent(int x, int y, int diam){
            this.x = x;
            this.y =y;
            this.diam = diam;
        }

        //Paint a circle on with the centre on (x,y)
        public void paintComponent(Graphics g){
            Graphics2D g2 = (Graphics2D) g;
            g2.setColor(Color.BLUE);
            Ellipse2D.Double circle =
                new Ellipse2D.Double((x-diam/2), (y-diam/2), diam, diam  );
            g2.fill(circle);
            g2.setColor(Color.DARK_GRAY);
            g2.draw(circle); //I want a border around my planet
        }

        public int getX(){
            return this.x;
        }

        public int getY(){
            return this.y;
        }
    }

    /**
     * This is also a quick way version of how I create the map display
     * I intend to use in my game. It's a collection of planets with lines
     * between them, on a black background.
     */
    @SuppressWarnings("serial")
    class myMap extends JPanel{
        private boolean modeOne;
        private int sizeX, sizeY;
        public myMap(boolean mode){
            this.sizeX = 500;
            this.sizeY = 500;
            this.modeOne = mode;
            //JPanel map = new JPanel();
            this.setSize(this.sizeX, this.sizeY);
        }

        public int getSizeX(){
            return this.sizeX;
        }

        public int getSizeY(){
            return this.sizeY;
        }

        public void paintComponent(Graphics g){
            Graphics2D g2 = (Graphics2D) g;

            //Create the black background plane
            //this.setBackground(Color.BLACK); //Tried it with this, but won't give any bakcground
            int heightBG = this.getSizeX();
            int widthBG = this.getSizeY();
            Rectangle2D.Double SpaceBackGround = new Rectangle2D.Double(0,0,
                    heightBG, widthBG);
            g2.setColor(Color.BLACK);
            g2.fill(SpaceBackGround);

            //Normally, I import this list from somewhere else, but the result
            //is very similar.
            ArrayList<myPlanetComponent> planetsList = new ArrayList<myPlanetComponent>(5);
            //Need to be able to generate at least 2 different maps to demonstrate
            //the effects of using the New game button. Normally this list is randomly
            //generated somewhere else, but idea stays the same.
            if(modeOne){
                planetsList.add(new myPlanetComponent(20,20,20));
                planetsList.add(new myPlanetComponent(70,30,20));
                planetsList.add(new myPlanetComponent(130,210,20));
                planetsList.add(new myPlanetComponent(88,400,20));
                planetsList.add(new myPlanetComponent(321,123,20));
            }else{
                planetsList.add(new myPlanetComponent(40,40,20));
                planetsList.add(new myPlanetComponent(140,60,20));
                planetsList.add(new myPlanetComponent(260,420,20));
                planetsList.add(new myPlanetComponent(176,200,20));
                planetsList.add(new myPlanetComponent(160,246,20));
            }

            //for all planets
            for(int i=0; i<planetsList.size()-1; i++){
                //planet 1 coordinates

                myPlanetComponent p1 = planetsList.get(i);
                if(i == 0){
                    p1.paintComponent(g2); //Only draw all planets once
                }
                //start coordinates of the line
                int x1 = p1.getX();
                int y1 = p1.getY();

                //Be smart, and don't do things double!
                for(int j=i+1; j<planetsList.size(); j++){
                    myPlanetComponent p2 = planetsList.get(j);;
                    if( i == 0){
                        p2.paintComponent(g2); //Only Draw all planets once
                    }
                    //planet 2 coordinates, endpoint of the line
                    int x2 = p2.getX();
                    int y2 = p2.getY();
                    Line2D.Double tradeRoute =
                        new Line2D.Double(x1, y1, x2, y2);

                    g2.setColor(Color.GREEN);
                    g2.draw(tradeRoute);
                }
            }

        }
    }

    return new myMap(mode);

}

}

Answer 1

您的问题可能是由于您在同一程序中混合了 AWT 和 Swing（重量级和轻量级）组件，这是不应该做的事情，除非您明确需要这样做并且知道自己在做什么。我怀疑您是否需要 ChangeListener，因为 JScrollPanes 应该能够开箱即用地处理此类事情。您是否尝试过让您的类扩展 JPanel 或 JComponent 而不是 Canvas？

另外，在发布代码时，请考虑创建并发布一个 SSCCE，一个小的可编译的可运行程序，我们可以运行、测试、修改、并希望是正确的。如果您创建并发布此类代码，您可能会很快获得一个体面且完整的解决方案。

编辑：我创建了一个测试程序来查看 Canvas 对 JScrollPanes 的影响，正如我所想的那样——Canvas 覆盖了滚动条以及其他所有内容。要亲自查看，请编译并运行此代码，然后通过单击并拖动来调整 JFrame 的大小。 Canvas 是蓝色的，位于左侧的 JScrollPane 中，而 JPanel 是红色的，位于右侧的 JScrollPane 中。

import java.awt.*;

import javax.swing.*;

@SuppressWarnings("serial")
public class CanvasInScrollPane extends JPanel {
    private static final Dimension CANVAS_SIZE = new Dimension(300, 300);
    private static final Dimension APP_SIZE = new Dimension(500, 250);
    Canvas canvas = new Canvas();
    JPanel panel = new JPanel();

    public CanvasInScrollPane() {
        canvas.setPreferredSize(CANVAS_SIZE);
        canvas.setBackground(Color.blue);

        panel.setPreferredSize(CANVAS_SIZE);
        panel.setBackground(Color.red);

        setPreferredSize(APP_SIZE);
        setLayout(new GridLayout(1, 0, 5, 0));
        add(new JScrollPane(canvas));
        add(new JScrollPane(panel));
    }

    private static void createAndShowUI() {
        JFrame frame = new JFrame("CanvasInScrollPane");
        frame.getContentPane().add(new CanvasInScrollPane());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                createAndShowUI();
            }
        });
    }
}