JavaFX ObservableList的回调和提取器 [英] Callback and extractors for JavaFX ObservableList

问题描述

这是代码:

package sample;

import javafx.beans.Observable;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ObservableList;
import javafx.util.Callback;

import java.util.List;

/**
 * Created by IDEA on 28/07/15.
 */
public class ListUpdateTest {
    public static void main(String[] args) {
        Callback<IntegerProperty, Observable[]> extractor = (IntegerProperty p) -> {
            System.out.println("The extractor is called.");
            Observable[] res = new Observable[]{p};
            System.out.println("Result from extractor: " + res);
            return res;
        };
        ObservableList<IntegerProperty> list = FXCollections.observableArrayList(extractor);
        System.out.printf("Before adding");
        IntegerProperty p1 = new SimpleIntegerProperty(10);
        IntegerProperty p2 = new SimpleIntegerProperty(20);
        list.addAll(p1, p2);
        System.out.println("After adding");
        list.addListener(new ListChangeListener<IntegerProperty>() {
            @Override
            public void onChanged(Change<? extends IntegerProperty> c) {
                System.out.println("List is " + c.getList());
                while (c.next()) {
                    if (c.wasUpdated()) {
                        System.out.println("An update is detected.");
                        int start = c.getFrom();
                        int end = c.getTo();
                        System.out.println(
                                String.format("Updated range: %d to %d",
                                        start, end)
                        );
                        List<? extends IntegerProperty> updatedElementsList;
                        updatedElementsList = c.getList().subList(start, end);
                        System.out.println("Updated elements: " + updatedElementsList);
                    }
                }
            }
        });
        p1.set(100);
    }
}

extractor的作用是什么，为什么我们在这里需要它?

What is extractor for and why do we need it here?

推荐答案

无论从列表中添加或删除项目时，ObservableList都会引发更改事件，无论它是否是使用提取程序创建的.

An ObservableList will fire change events when items are added and removed from the list, whether or not it is created with an extractor.

但是，如果列表中的项目是可观察属性或包含对可观察属性的引用，则只有当这些属性用提取器构造并返回包含对这些属性的引用的数组的构造器时，这些属性才会更改，列表将触发更新.

However, if items in the list either are, or contain references to, observable properties, the list will fire updates when those properties change only if it is constructed with an extractor which returns an array containing references to those properties.

这应该证明区别:

import javafx.beans.Observable;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ObservableList;


public class ListExtractorDemo {
    public static void main(String[] args) {
        ObservableList<IntegerProperty> listWithoutExtractor = 
                FXCollections.observableArrayList();

        ObservableList<IntegerProperty> listWithExtractor = 
                FXCollections.observableArrayList(p -> new Observable[]{p});

        listWithoutExtractor.addListener(createListener("listWithoutExtractor"));
        listWithExtractor.addListener(createListener("listWithExtractor"));

        IntegerProperty p1 = new SimpleIntegerProperty(1);
        IntegerProperty p2 = new SimpleIntegerProperty(2);

        // both lists will fire change events when items are added or removed:
        listWithoutExtractor.addAll(p1, p2);
        listWithExtractor.addAll(p1, p2);

        // only the list with the extractor will fire a change event when the observable value of an element changes:
        p2.set(3);
    }

    private static ListChangeListener<IntegerProperty> createListener(String listId) {
        return (Change<? extends IntegerProperty> c) -> {
            while (c.next()) {
                if (c.wasAdded()) {
                    System.out.println(listId + " added: "+c.getAddedSubList());
                }
                if (c.wasRemoved()) {
                    System.out.println(listId + " removed: "+c.getRemoved());
                }
                if (c.wasUpdated()) {
                    System.out.println(listId + " updated");
                }
            }
        };
    }
}

@kleopatra对

A typical use case is in @kleopatra's answer to JavaFX 2.0 Choice Box Issue. How to update a choiceBox, which represents a list of objects, when an object is updated?

这篇关于JavaFX ObservableList的回调和提取器的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！