如何通过 ListChangeListener 检索 ObservableList 中的更改项

Question

有ListChangeListener.Change.wasUpdated() which is used to detect changes within elements for ObservableLists created via ObservableList.observableArrayList(Callback<E,Observable[]> extractor).

如何检索导致触发更改的确切项目？

编辑

可能问题不够清楚，我举个例子吧

class Foo {
    private StringProperty name = new SimpleStringProperty();
    public final StringProperty nameProperty() { return name; }
    public final String getName() { return name.get(); }
    public final void setName(String n) { name.set(n); }
    public Foo(String fooName) { setName(fooName); }
}

// Creates an ObservableList with an extractor
ObservableList<Foo> fooList = FXCollections.observableArrayList(foo -> new Observable[] { foo.nameProperty() });
Foo fooA = new Foo("Hello");
Foo fooB = new Foo("Kitty");
fooList.add(fooA);
fooList.add(fooB);

fooList.addListener(new ListChangeListener<Foo>() {
     public void onChanged(Change<Foo> c) {
         while (c.next()) {
            if (c.wasUpdated()) {
                // One or more of the elements in list has/have an internal change, but I have no idea which element(s)!
            }
        }
    }
});

fooB.setName("Mickey");

fooB.setName() 将触发 ListChangeListener 的变化，wasUpdated() 条件会 return true。但是，我无法知道它是 fooB 在监听器中发生了变化。

这可能看起来微不足道，但我有一个地图应用程序，其中的列表存储了地图必须呈现的内容。当其中一项改变其位置时（即 lat/long），我需要在地图上重新绘制。如果我不知道哪个项目改变了位置，我将不得不重新绘制我已有的所有内容。

Answer 1

直接来自 docs。

Typical usage is to observe changes on an ObservableList in order to hook or unhook (or add or remove a listener) or in order to maintain some invariant on every element in that ObservableList. A common code pattern for doing this looks something like the following:

 ObservableList theList = ...;

 theList.addListener(new ListChangeListener<Item>() {
     public void onChanged(Change<tem> c) {
         while (c.next()) {
             if (c.wasPermutated()) {
                     for (int i = c.getFrom(); i < c.getTo(); ++i) {
                          //permutate
                     }
                 } else if (c.wasUpdated()) {
                          //update item
                 } else {
                     for (Item remitem : c.getRemoved()) {
                         remitem.remove(Outer.this);
                     }
                     for (Item additem : c.getAddedSubList()) {
                         additem.add(Outer.this);
                     }
                 }
             }
         }
     });

 }

Answer 2

您可以获得已更改项目的索引，这会为您提供一些有用的信息：

import javafx.beans.Observable;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ObservableList;

public class ListUpdateTest {

    public static void main(String[] args) {

        ObservableList<Foo> fooList = FXCollections.observableArrayList(foo -> new Observable[] { foo.nameProperty() });
        Foo fooA = new Foo("Hello");
        Foo fooB = new Foo("Kitty");
        fooList.add(fooA);
        fooList.add(fooB);

        fooList.addListener((Change<? extends Foo> c) -> {
             while (c.next()) {
                if (c.wasUpdated()) {
                    int start = c.getFrom() ;
                    int end = c.getTo() ;
                    for (int i = start ; i < end ; i++) {
                        System.out.println("Element at position "+i+" was updated to: " +c.getList().get(i).getName() );
                    }
                }
            }
        });

        fooB.setName("Mickey");
    }

    public static class Foo {
        private StringProperty name = new SimpleStringProperty();
        public final StringProperty nameProperty() { return name; }
        public final String getName() { return name.get(); }
        public final void setName(String n) { name.set(n); }
        public Foo(String fooName) { setName(fooName); }
    }
}

请注意，您无法从列表更改事件中确定那些列表元素中发生更改的实际属性（因此，如果您的提取器指向两个或更多属性，则无法找到哪些属性发生了更改），并且没有办法得到以前的值。不过，这对于您的用例来说可能已经足够了。

如何通过 ListChangeListener 检索 ObservableList 中的更改项

How to retrieve the changed item in an ObservableList via ListChangeListener

java

javafx

observablelist

编辑