java - 如何绕过 JavaFX 的 TableView "placeholder"？

Question

JavaFX 的 TableView 有一个 placeholder属性基本上是一个 Node，只要它为空，它就会显示在 TableView 中。如果此属性设置为 null(其默认值)，它将显示为 Label 或其他一些基于文本的 Node，表示“表中没有内容”。

但是如果表格中有任何数据行，那么占位符 Node 就会消失并且 TableView 中的整个垂直空间会被行填充，包括空行如果没有足够的数据来填满整个表格。

这些空行是我想要的，即使表格是空的。换句话说，我根本不想使用占位符。有谁知道我该怎么做？

我宁愿不做一些笨拙的事情，比如在 TableView 中放一个看起来空的行，但实际上它应该是空的。

Answer 1

不幸的是，old issue在 fx9 和更高版本(包括 fx18)中仍未修复。

fx9 的原始皮肤修改(见水平线下方)似乎仍适用于 fx18。尽管可以稍微改进，因为 fx12 引入了对流和 tableHeaderRow 的访问。

在尝试将其调整为更新的 api 时，我想出了一个 - 同样肮脏 且未经测试的 - 方法。这个想法是覆盖实际的布局方法，即在布置表格的各个子项时调用的方法，注意占位符并将其替换为流程:

public class NoPlaceHolderSkin<T> extends TableViewSkin<T> {

    public NoPlaceHolderSkin(TableView<T> control) {
        super(control);
    }

    @Override
    protected void layoutInArea(Node child, double areaX, double areaY, double areaWidth,
            double areaHeight, double areaBaselineOffset, HPos halignment, VPos valignment) {
        if (child.getStyleClass().contains("placeholder")) {
            child.setVisible(false);
            child = getVirtualFlow();
            child.setVisible(true);
        }
        super.layoutInArea(child, areaX, areaY, areaWidth, areaHeight, areaBaselineOffset, halignment, valignment);
    }
}

---

因此在 fx9 的上下文中重新审视了黑客。有好有坏的变化:

• 皮肤已移至公共(public)包中，现在允许在不访问隐藏类的情况下对其进行子类化(好)
• 移动introduced a bug不允许安装自定义 VirtualFlow(已在 fx10 中修复)
• 将来某个时候将强烈禁止对隐藏成员的反射访问(阅读:不可能)

在挖掘过程中，我注意到这些 hack 有非常轻微的故障(注意:我没有针对 fx8 运行它们，所以这些可能是 fx8 与 fx9 的差异造成的!)

• 占位符/流的强制可见性工作正常，除非从空表开始(显示占位符)并在空表时放大表("new"区域看起来是空的)
• 将 itemCount 伪造为非空让行在按下导航键时消失(这可能不是一个大问题，因为用户往往不会导航空表)——这在 fx9 中肯定引入，在 fx8 中工作正常<

所以我决定继续执行可见性:出现轻微故障的原因是 layoutChildren 在认为占位符可见时不会对流进行布局。如果 super 没有，则通过在布局中包含流来处理。

自定义皮肤:

/**
 * TableViewSkin that doesn't show the placeholder.
 * The basic trick is keep the placeholder/flow in-/visible at all 
 * times (similar to https://stackoverflow.com/a/27543830/203657).
 * <p> 
 * 
 * Updated for fx9 plus ensure to update the layout of the flow as
 * needed.
 * 
 * @author Jeanette Winzenburg, Berlin
 */
public class NoPlaceHolderTableViewSkin<T> extends TableViewSkin<T>{

    private VirtualFlow<?> flowAlias;
    private TableHeaderRow headerAlias;
    private Parent placeholderRegionAlias;
    private ChangeListener<Boolean> visibleListener = (src, ov, nv) -> visibleChanged(nv);
    private ListChangeListener<Node> childrenListener = c -> childrenChanged(c);
    
    /**
     * Instantiates the skin.
     * @param table the table to skin.
     */
    public NoPlaceHolderTableViewSkin(TableView<T> table) {
        super(table);
        flowAlias = (VirtualFlow<?>) table.lookup(".virtual-flow");
        headerAlias = (TableHeaderRow) table.lookup(".column-header-background");
        
        // startet with a not-empty list, placeholder not yet instantiatet
        // so add alistener to the children until it will be added
        if (!installPlaceholderRegion(getChildren())) {
            installChildrenListener();
        }
    }


    /**
     * Searches the given list for a Parent with style class "placeholder" and
     * wires its visibility handling if found.
     * @param addedSubList
     * @return true if placeholder found and installed, false otherwise.
     */
    protected boolean installPlaceholderRegion(
            List<? extends Node> addedSubList) {
        if (placeholderRegionAlias !=  null) 
            throw new IllegalStateException("placeholder must not be installed more than once");
        List<Node> parents = addedSubList.stream()
                .filter(e -> e.getStyleClass().contains("placeholder"))
                .collect(Collectors.toList());
        if (!parents.isEmpty()) {
            placeholderRegionAlias = (Parent) parents.get(0);
            placeholderRegionAlias.visibleProperty().addListener(visibleListener);
            visibleChanged(true);
            return true;
        }
        return false;
    }


    protected void visibleChanged(Boolean nv) {
        if (nv) {
            flowAlias.setVisible(true);
            placeholderRegionAlias.setVisible(false);
        }
    }


    /**
     * Layout of flow unconditionally.
     * 
     */
    protected void layoutFlow(double x, double y, double width,
            double height) {
        // super didn't layout the flow if empty- do it now
        final double baselineOffset = getSkinnable().getLayoutBounds().getHeight() / 2;
        double headerHeight = headerAlias.getHeight();
        y += headerHeight;
        double flowHeight = Math.floor(height - headerHeight);
        layoutInArea(flowAlias, x, y,
                width, flowHeight,
                baselineOffset, HPos.CENTER, VPos.CENTER);
    }


    /**
     * Returns a boolean indicating whether the flow should be layout.
     * This implementation returns true if table is empty.
     * @return
     */
    protected boolean shouldLayoutFlow() {
        return getItemCount() == 0;
    }


    /**
     * {@inheritDoc} <p>
     * 
     * Overridden to layout the flow always.
     */
    @Override
    protected void layoutChildren(double x, double y, double width,
            double height) {
        super.layoutChildren(x, y, width, height);
        if (shouldLayoutFlow()) {
            layoutFlow(x, y, width, height);
            
        }
    }

    /**
     * Listener callback from children modifications.
     * Meant to find the placeholder when it is added.
     * This implementation passes all added sublists to 
     * hasPlaceHolderRegion for search and install the 
     * placeholder. Removes itself as listener if installed.
     * 
     * @param c the change 
     */
    protected void childrenChanged(Change<? extends Node> c) {
        while (c.next()) {
            if (c.wasAdded()) {
                if (installPlaceholderRegion(c.getAddedSubList())) {
                    uninstallChildrenListener();
                    return;
                }
                
            }
        }
    }


    /**
     * Installs a ListChangeListener on the children which calls
     * childrenChanged on receiving change notification. 
     * 
     */
    protected void installChildrenListener() {
        getChildren().addListener(childrenListener);
    }
    
    /**
     * Uninstalls a ListChangeListener on the children:
     */
    protected void uninstallChildrenListener() {
        getChildren().removeListener(childrenListener);
    }
    
    
}

使用示例:

public class EmptyPlaceholdersInSkin extends Application {

    private Parent createContent() {
        // initially populated
        //TableView<Person> table = new TableView<>(Person.persons()) {
        // initially empty
        TableView<Person> table = new TableView<>() {

            @Override
            protected Skin<?> createDefaultSkin() {
                return new NoPlaceHolderTableViewSkin<>(this);
            }
            
        };
        TableColumn<Person, String> first = new TableColumn<>("First Name");
        first.setCellValueFactory(new PropertyValueFactory<>("firstName"));
        
        table.getColumns().addAll(first);
        
        Button clear = new Button("clear");
        clear.setOnAction(e -> table.getItems().clear());
        clear.disableProperty().bind(Bindings.isEmpty(table.getItems()));
        Button fill = new Button("populate");
        fill.setOnAction(e -> table.getItems().setAll(Person.persons()));
        fill.disableProperty().bind(Bindings.isNotEmpty(table.getItems()));
        BorderPane pane = new BorderPane(table);
        pane.setBottom(new HBox(10, clear, fill));
        return pane;
    }


    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent()));
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }

    @SuppressWarnings("unused")
    private static final Logger LOG = Logger
            .getLogger(EmptyPlaceholdersInSkin.class.getName());

}