JavaFX 学习：属性和字段

港控/mmm° 2023-09-29 18:35 12阅读 0赞

现在的编程体系，都是面向对象编程，其中分为两个部分：方法 + 字段，其中字段设置为 private，通过方法读取字段，外界程序不直接对字段操作。

在程序运行过程中，经常会对对象的字段进行更新，所以如何将更新的字段信息通知，这是要注意的问题。在 JavaFX 中，可以使用属性 Property 替代字段，使用属性的监听器通知所有监听者，在数据修改时通知监听者更新数据。每个基本类型字段的属性接口：\*\*\*Property，对应的属性实例有 2 种，分别是 Simple\*\*\*Property 和 ReadOnly\*\*\*Wrapper，第 2 个是属性的包装器。

属性示例，属性构成 Book 类中的字段：

package learnjavafx8.ch02;
    
    import javafx.beans.property.DoubleProperty;
    import javafx.beans.property.ReadOnlyStringProperty;
    import javafx.beans.property.ReadOnlyStringWrapper;
    import javafx.beans.property.SimpleDoubleProperty;
    import javafx.beans.property.SimpleStringProperty;
    import javafx.beans.property.StringProperty;
    
    /**************************************************************************************************
     @copyright  2003-2022
     @package    learnjavafx8.ch02
     @file       Book.java
     @date       2021/7/10 12:22
     @author     qiao wei
     @version    1.0
     @brief      Book. Test properties
     @history
    **************************************************************************************************/
    public class Book {
        
        /**********************************************************************************************
         * @class   Book
         * @date    2022-12-24 15:11
         * @author  qiao wei
         * @version 1.0
         * @brief   Default constructor
         * @param   
         * @return  
         * @throws
         *********************************************************************************************/
        public Book() {
            this("Unknown", 0.0d, "Unknown");
        }
        
        /**********************************************************************************************
         * @class   Book
         * @date    2022-12-24 15:10
         * @author  qiao wei
         * @version 1.0
         * @brief   Constructor
         * @param   
         * @return  
         * @throws
         *********************************************************************************************/
        public Book(String title, double price, String ISBN) {
            this.titleProperty = new SimpleStringProperty(this, "title", title);
            this.priceProperty = new SimpleDoubleProperty(this, "price", price);
            
            // Property wrapper. It can read/write
            this.ISBN = new ReadOnlyStringWrapper(this, "ISBN", ISBN);
        }
        
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:09
         @author     qiao wei
         @version    1.0
         @brief      Get title Property
         @return     The title property
        **********************************************************************************************/
        public final StringProperty getTitleProperty() {
            return titleProperty;
        }
        
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:09
         @author     qiao wei
         @version    1.0
         @brief      Get book title
         @return     The book title
        **********************************************************************************************/
        public final String getTitle() {
            return titleProperty.get();
        }
    
        public final void setTitle(String title) {
            titleProperty.set(title);
        }
        
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:10
         @author     qiao wei
         @version    1.0
         @brief      Get the book price property
         @return     The book price property
        **********************************************************************************************/
        public final DoubleProperty getPriceProperty() {
            return priceProperty;
        }
    
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:13
         @author     qiao wei
         @version    1.0
         @brief      Get the book price
         @return     The book price
        **********************************************************************************************/
        public double getPrice() {
            return priceProperty.get();
        }
        
        public final void setPrice(double price) {
            priceProperty.set(price);
        }
    
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:20
         @author     qiao wei
         @version    1.0
         @brief      获取只读型ISBN属性
         @return     ISBN属性
        **********************************************************************************************/
        public final ReadOnlyStringProperty getISBNProperty() {
            return ISBN.getReadOnlyProperty();
        }
        
        /**********************************************************************************************
         @class      Book
         @date       2022/1/16 22:21
         @author     qiao wei
         @version    1.0
         @brief      获取只读型ISBN数据
         @return     ISBN数据
        **********************************************************************************************/
        public final String getISBN() {
            return ISBN.get(); 
        }
        
        /**********************************************************************************************
         @date      2022/1/16 22:06
         @author    qiao wei
         @brief     Book title property
        **********************************************************************************************/
        private StringProperty titleProperty;
        
        /**********************************************************************************************
         @date      2022/1/16 22:06
         @author    qiao wei
         @brief     Price property
        **********************************************************************************************/
        private DoubleProperty priceProperty;
        
        /**********************************************************************************************
         @date      2022/1/16 22:06
         @author    qiao wei
         @brief     Book ISBN. Read only field
        **********************************************************************************************/
        private ReadOnlyStringWrapper ISBN;
    
    }

测试 Book 类：

package learnjavafx8.ch02;
    
    import javafx.beans.property.ReadOnlyProperty;
    //import javafx.beans.property.SimpleStringProperty;
    import javafx.beans.property.StringProperty;
    
    /**************************************************************************************************
     @copyright  2003-2022
     @package    learnjavafx8.ch02
     @file       BookTest.java
     @date       2022/1/26 16:21
     @author     qiao wei
     @version    1.0
     @brief      Test JavaFX property
     @history
    **************************************************************************************************/
    public class BookTest {
    
        public static void main(String[] args) {
            Book book = new Book("Harnessing JavaFX", 9.99d, "0123456789");
            System.out.println("After creating the Book object...");
    
            BookTest.printDetails(book.getTitleProperty());
    //        printDetails(book.getPriceProperty());
    //        printDetails(book.getISBNProperty());
    
    //        book.setTitle("JavaFX 8.0");
            /** book.setPrice(22.49d);*/
            StringProperty property = book.getTitleProperty();
            property.set("Test");
            
    
            System.out.println("\nTitle changed to JavaFX 8.0, and Price is 22.49" );
            System.out.println("After changing the Book object...\n");
    
            printDetails(book.getTitleProperty());
    //        printDetails(book.getPriceProperty());
    //        printDetails(book.getISBNProperty());
            
    //        TestClass tc = new TestClass("Harnessing JavaFX", 9.99d, "0123456789");
        }
        
        /**********************************************************************************************
         @class      BookTest
         @date       2022/1/26 16:21
         @author     qiao wei
         @version    1.0
         @brief      Print book's details
         @param      property book property
        **********************************************************************************************/
        private static void printDetails(ReadOnlyProperty<?> property) {
            // Get the object's property implementing from the property interface
            String name = property.getName();
            Object bean = property.getBean();
            Object value = property.getValue();
            
            String beanClassName = (null == bean) ? "null" : bean.getClass().getSimpleName();
            String propClassName = property.getClass().getSimpleName();
    
            System.out.print(propClassName);
            System.out.print("[Name = " + name);
            System.out.print(", Bean Class = " + beanClassName);
            System.out.println(", Value = " + value + "]");
    
            System.out.println("propClass = " + propClassName);
        }
    }

运行结果：

After creating the Book object...
    SimpleStringProperty[Name = title, Bean Class = Book, Value = Harnessing JavaFX]
    propClass = SimpleStringProperty
    
    Title changed to JavaFX 8.0, and Price is 22.49
    After changing the Book object...
    
    SimpleStringProperty[Name = title, Bean Class = Book, Value = Test]
    propClass = SimpleStringProperty

从结果上看，使用属性并没有带来什么新的特性，相较于 String，double 步骤多，反而麻烦。但是属性有 2 种事件监听，分别是失效事件 invalidated 和改变事件 changed，分别可以对数据的改变做出反应，通知数据的监听者，这是 String 等类型数据没有的，数据更新注意两者的区别。

// 无效事件方法，继承自Observable接口
    void addListener(InvalidationListener listener)
    
    // 改变事件方法，继承自ObservableValue接口
    void addListener(ChangeListener<? super T> listener)

invalidated 事件例子：

package learnjavafx8.ch02;
    
    import javafx.beans.Observable;
    import javafx.beans.property.IntegerProperty;
    import javafx.beans.property.SimpleIntegerProperty;
    
    /**************************************************************************************************
     * @copyright 2003-2022
     * @package   learnjavafx8.ch02
     * @file      InvalidationTest.java
     * @date      2022/11/19 11:30
     * @author    qiao wei
     * @version   1.0
     * @brief     
     * @history
     *************************************************************************************************/
    public class InvalidationTest {
    
        public static void main(String[] args) {
            IntegerProperty counter = new SimpleIntegerProperty(100);
            
            // Register an invalidation listener to counter property
            counter.addListener(InvalidationTest::invalidated);
    
            // Set the same value 100
            System.out.println("Before invalidating the counterProperty value-1");
            // 数据从100到100，没有触发invalidation event
            counter.set(100);
            System.out.println("Before invalidating the counterProperty value-1");
            
            // At this point counter property is invalid and further changes to its value will not 
            // generate invalidation events
            System.out.println("\nBefore changing the counter value-2");
            counter.set(102);
            System.out.println("counter's value is not changed, invalidating event is ");
            System.out.println("After changing the counter value-2");
    
            // Make the counter property valid by calling its get() method
            int value = counter.get();
            System.out.println("Counter value = " + value);
            
            // At this point counter property is valid and further changes to its value will generate
            // invalidation events.
    
            // Try to set the same value
            System.out.println("\nBefore changing the counter value-3");
            counter.set(102);
            System.out.println("After changing the counter value-3");
    
            // Try to set a different value
            System.out.println("\nBefore changing the counter value-4");
            counter.set(103);
            System.out.println("After changing the counter value-4");
        }
        
        public static void invalidated(Observable property) {
            System.out.println("Counter is invalid. The invalidation event is fired.");
        }
    }

运行结果：

Before invalidating the counterProperty value-1
    Before invalidating the counterProperty value-1
    
    Before changing the counter value-2
    Counter is invalid. The invalidation event is fired.
    counter's value is not changed, invalidating event is 
    After changing the counter value-2
    Counter value = 102
    
    Before changing the counter value-3
    After changing the counter value-3
    
    Before changing the counter value-4
    Counter is invalid. The invalidation event is fired.
    After changing the counter value-4

change 事件：

package learnjavafx8.ch02;
    
    import javafx.beans.property.IntegerProperty;
    import javafx.beans.property.SimpleIntegerProperty;
    import javafx.beans.value.ObservableValue;
    
    /**************************************************************************************************
     * @copyright 2003-2022
     * @package   learnjavafx8.ch02
     * @file      ChangeTest.java
     * @date      2020/5/2 18:51
     * @author    qiao wei
     * @version   1.0
     * @brief     Test change event declared from ObservableValue interface
     * @history
     *************************************************************************************************/
    public class ChangeTest {
    
        public static void main(String[] args) {
            changedFunction();
        }
        
        private static void changedFunction() {
            IntegerProperty property = new SimpleIntegerProperty(100);
            
            // Register change listener
            property.addListener(ChangeTest::changed);
            
            fireChangedEvent(property, 101, 1);
            fireChangedEvent(property, 102, 2);
            
            // New value is the same with old value, the change event is not fired
            fireChangedEvent(property, 102, 3);
            
            fireChangedEvent(property, 104, 4);
        }
        
        /**********************************************************************************************
         * @class   ChangeTest
         * @date    2023-01-19 12:59
         * @author  qiao wei
         * @version 1.0
         * @brief   Fire change event by reset property
         * @param   
         * @return  
         * @throws
         *********************************************************************************************/
        private static void fireChangedEvent(IntegerProperty prop, int newNumber, int counter) {
            System.out.println("\nBefore changing the counter value-" + counter);
            
            // Set new value
            prop.set(newNumber);
            System.out.println("After changing the counter value-" + counter);
        }
        
        /**********************************************************************************************
         * @class   ChangeTest
         * @date    2022/10/22 16:33
         * @author  qiao wei
         * @version 1.0
         * @brief   修改事件
         * @param   prop 触发事件的实例，在这里是changedFunction方法中的实例property
         * @param   oldValue 触发事件前的值
         * @param   newValue 触发事件后的值
         * @return  
         * @throws
         *********************************************************************************************/
        private static void changed(ObservableValue<? extends Number> prop,
                                    Number oldValue,
                                    Number newValue) {
            System.out.println("Number changed : ");
            System.out.println("Old = " + oldValue + ", New = " + newValue);
        }
    }

运行结果：

Before changing the counter value-1
    Number changed : 
    Old = 100, New = 101
    After changing the counter value-1
    
    Before changing the counter value-2
    Number changed : 
    Old = 101, New = 102
    After changing the counter value-2
    
    Before changing the counter value-3
    After changing the counter value-3
    
    Before changing the counter value-4
    Number changed : 
    Old = 102, New = 104
    After changing the counter value-4
    
    Process finished with exit code 0