如果您将一个 jpanel 实例添加到内容 Pane ，就会调用 paintComponent 方法，对吧？
content_pane.add (fDrawingPanel);
那么为什么要在 run() 方法的第一行调用“repaint”？ (它说“绘制加载消息，但它应该已经被绘制了，因为之前调用了 paintComponent，并且一旦 fShow 为真，我们就不会将其设置回来为假，所以我认为这段代码应该被调用一次):
public class MediaTrackApplet extends JApplet
implements Runnable
{
// Need a reference to the panel for the
// thread loop.
DrawingPanel fDrawingPanel;
// Parameters to track the image
MediaTracker fTracker;
Image fImg;
int fImageNum = 0;
boolean fShow = false;
String fMessage ="Loading...";
/** Use a MediaTracker to load an image.**/
public void init () {
Container content_pane = getContentPane ();
// Create an instance of DrawingPanel
fDrawingPanel = new DrawingPanel (this);
// Add the DrawingPanel to the contentPane.
content_pane.add (fDrawingPanel);
// Get image and monitor its loading.
fImg = getImage (getCodeBase (), "m20.gif.jpg" );
fTracker = new MediaTracker (this);
// Pass the image reference and an ID number.
fTracker.addImage (fImg, fImageNum);
} // init
/** If the image not yet loaded, run the thread
* so the run() will monitor the image loading.
**/
public void start () {
if (!fTracker.checkID (fImageNum) ) {
Thread thread = new Thread (this);
thread.start ();
} else
// Unloading/reloading web page can will leave
// checkID true but fShow will be false.
fShow = true;
} // start
/** Use a thread to wait for the image to load
* before painting it.
**/
public void run () {
// Paint the loading message
repaint ();
// The wait for the image to finish loading
try {
fTracker.waitForID (fImageNum );
} catch (InterruptedException e) {}
// Check if there was a loading error
if (fTracker.isErrorID (fImageNum ))
fMessage= "Error";
else
fShow = true;
// Repaint with the image now if it loaded OK
repaint ();
} // run
}// class MediaTrackApplet
/** This JPanel subclass draws an image on the panel if
* the image is loaded. Otherwise, it draws a text message.
**/
class DrawingPanel extends JPanel {
MediaTrackApplet parent = null;
DrawingPanel (MediaTrackApplet parent) {
this.parent = parent;
}// ctor
public void paintComponent (Graphics g) {
super.paintComponent (g);
// Add your drawing instructions here
if (parent.fShow)
g.drawImage (parent.fImg,10,10,this);
else
g.drawString (parent.fMessage, 10,10);
} // paintComponent
} // class DrawingPanel
谢谢
最佳答案
Are you referring to the EDT?
是的。这个古老但过时的例子来自一个早于我们现代理解的时代，即 Swing GUI 对象应该仅在 event dispatch thread 上构造和操作。 . repaint() 的初始调用具有在 EventQueue 上发布新的 Runnable 的效果，同时允许后台线程完成加载图像对后续 repaint() 的预期。另见 Memory Consistency Properties和这个相关的example .
关于java - 为什么要在此代码中调用重绘？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9121784/