在JavaFx標籤中顯示更改值

Question

在JavaFX中，如何顯示使用「標籤」隨時間不斷變化的值？

Answer 1

有許多方法來實現這一目標，最方便的辦法是使用JavaFX的的數據綁定機制：

// assuming you have defined a StringProperty called "valueProperty" 
Label myLabel = new Label("Start"); 
myLabel.textProperty().bind(valueProperty); 

這樣一來，您的每一次valueProperty被調用它的設置方法改變，標籤的文本更新。

Answer 2

我喜歡塞巴斯蒂安的約束性答案。

對於多樣性，這裏是另一個基於時間修改標籤文本的示例。示例顯示標籤中的數字時鐘讀數，其標籤每秒使用Timeline更改。

import java.io.IOException; 
import java.net.URL; 
import javafx.animation.*; 
import javafx.event.*; 
import javafx.scene.control.Label; 
import javafx.util.Duration; 

import java.util.Calendar; 
import javafx.application.Application; 
import javafx.scene.Scene; 
import javafx.stage.Stage; 

public class DigitalClockSample extends Application { 
    public static void main(String[] args) { launch(args); } 
    @Override public void start(Stage stage) throws IOException { 
    stage.setScene(new Scene(new DigitalClock(), 100, 50)); 
    stage.show(); 
    } 
} 

/** 
* Creates a digital clock display as a simple label. 
* Format of the clock display is hh:mm:ss aa, where: 
* hh Hour in am/pm (1-12) 
* mm Minute in hour 
* ss Second in minute 
* aa Am/pm marker 
* Time is the system time for the local timezone. 
*/ 
class DigitalClock extends Label { 
    public DigitalClock() { 
    bindToTime(); 
    } 

    // the digital clock updates once a second. 
    private void bindToTime() { 
    Timeline timeline = new Timeline(
     new KeyFrame(Duration.seconds(0), 
     new EventHandler<ActionEvent>() { 
      @Override public void handle(ActionEvent actionEvent) { 
      Calendar time = Calendar.getInstance(); 
      String hourString = StringUtilities.pad(2, ' ', time.get(Calendar.HOUR) == 0 ? "12" : time.get(Calendar.HOUR) + ""); 
      String minuteString = StringUtilities.pad(2, '0', time.get(Calendar.MINUTE) + ""); 
      String secondString = StringUtilities.pad(2, '0', time.get(Calendar.SECOND) + ""); 
      String ampmString = time.get(Calendar.AM_PM) == Calendar.AM ? "AM" : "PM"; 
      setText(hourString + ":" + minuteString + ":" + secondString + " " + ampmString); 
      } 
     } 
    ), 
     new KeyFrame(Duration.seconds(1)) 
    ); 
    timeline.setCycleCount(Animation.INDEFINITE); 
    timeline.play(); 
    } 
} 

class StringUtilities { 
    /** 
    * Creates a string left padded to the specified width with the supplied padding character. 
    * @param fieldWidth the length of the resultant padded string. 
    * @param padChar a character to use for padding the string. 
    * @param s the string to be padded. 
    * @return the padded string. 
    */ 
    public static String pad(int fieldWidth, char padChar, String s) { 
    StringBuilder sb = new StringBuilder(); 
    for (int i = s.length(); i < fieldWidth; i++) { 
     sb.append(padChar); 
    } 
    sb.append(s); 

    return sb.toString(); 
    } 
} 

數字時鐘輸出樣本：

Answer 3

有關使用的SimpleDateFormat如何？ 不需要StringUtilities類！

private void bindToTime() { 
    Timeline timeline = new Timeline(
    new KeyFrame(Duration.seconds(0), 
     new EventHandler<ActionEvent>() { 
     @Override public void handle(ActionEvent actionEvent) { 
      Calendar time = Calendar.getInstance(); 
      SimpleDateFormat simpleDateFormat = new SimpleDateFormat("HH:mm:ss"); 
      setText(simpleDateFormat.format(time.getTime())); 
     } 
     } 
    ), 
    new KeyFrame(Duration.seconds(1)) 
); 
    timeline.setCycleCount(Animation.INDEFINITE); 
    timeline.play(); 
} 
} 

Answer 4

優秀的回覆，謝謝jewelsea爲您的輸入提供了很多幫助。

我更新了以前使用Java 8以更簡潔的格式發佈的DigitalClock。使用Java 8的新增功能，如Date API，當然還有lambdas。

import javafx.animation.Animation; 
import javafx.animation.KeyFrame; 
import javafx.animation.Timeline; 
import javafx.scene.control.Label; 
import javafx.util.Duration; 

import java.time.LocalTime; 
import java.time.format.DateTimeFormatter; 

public class DigitalClock extends Label 
{ 
    private static DateTimeFormatter SHORT_TIME_FORMATTER =  DateTimeFormatter.ofPattern("HH:mm:ss"); 

    public DigitalClock() 
    { 
     bindToTime(); 
    } 

    private void bindToTime() { 
     Timeline timeline = new Timeline(new KeyFrame(Duration.seconds(0), 
                 event -> setText(LocalTime.now().format(SHORT_TIME_FORMATTER))), 
             new KeyFrame(Duration.seconds(1))); 

     timeline.setCycleCount(Animation.INDEFINITE); 
     timeline.play(); 
    } 
}