具有拖放功能的 JavaFX TreeView 产生异常

Question

我是 JavaFX 的新手。我想实现一个具有拖放功能的 JavaFX TreeView。
我执行以下操作：我创建一个 TreeView 实例 (treeView) 和一个 ProgramRootTreeItem 类型的根节点（此类扩展 TreeItem）。
我向根节点添加了一些 ProgramTreeItem 类型的子节点（这也扩展了 TreeItem）。
在此之后，我将根节点添加到 treeView 并向其注册 cellFactory (DnDTreeCell)。
DnDTreeCell 实现有关拖放的所有事件处理。
我可以编译，但如果我执行，我会得到一个异常：

Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to pluginmanagement.ProgramTreeItem
at pluginmanagement.DnDTreeCell.updateItem(DnDTreeCell.java:15)

我完全不知道为什么会出现这个异常。 你能帮我吗？ 提前谢谢。

DnDTreeApp.java

package pluginmanagement;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.TreeCell;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeItem.TreeModificationEvent;
import javafx.scene.control.TreeView;
import javafx.scene.input.ClipboardContent;
import javafx.scene.input.DataFormat;
import javafx.scene.input.DragEvent;
import javafx.scene.input.Dragboard;
import javafx.scene.input.MouseEvent;
import javafx.scene.input.TransferMode;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import javafx.util.Callback;


public class DnDTreeApp extends Application {

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

    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Example Dynamic Tree");
        primaryStage.setResizable(true);
        final VBox box = new VBox();
        box.setFillWidth(false);
        Scene scene = new Scene(box);
        primaryStage.setScene(scene);
        box.getChildren().add(this.getExampleTree());
        primaryStage.show();
    }


    private TreeView<ProgramTreeItem> getExampleTree() {
        TreeView<ProgramTreeItem> treeView = new TreeView<ProgramTreeItem>();
        ProgramRootTreeItem root = buildTree();

        treeView.setCellFactory(new Callback<TreeView<ProgramTreeItem>, TreeCell<ProgramTreeItem>>() {
            @Override
            public TreeCell call(TreeView<ProgramTreeItem> param) {
                System.out.format("\ncall() - param: %s\n", param);
                TreeCell cell = new DnDTreeCell(param);
                System.out.format("\ncall() - return: %s\n", cell);
                return cell;
            }
        });

        treeView.setPrefSize(1000, 750);
        treeView.setShowRoot(true);
        treeView.setRoot(root);
        root.setExpanded(true);

        return treeView;
    }


    private ProgramRootTreeItem buildTree() {
        ProgramRootTreeItem root = new ProgramRootTreeItem();
        for(int a=1; a<4; a++) {
            for(int b=1; b<4; b++) {
                ProgramTreeItem child = new ProgramTreeItem("child_" + a + "_" + b);
                root.getChildren().add(child);
            }
        }
        return root;
    }

}

DnDTreeCell.java

package pluginmanagement;

import javafx.event.EventHandler;
import javafx.scene.control.TreeCell;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.input.ClipboardContent;
import javafx.scene.input.DataFormat;
import javafx.scene.input.DragEvent;
import javafx.scene.input.Dragboard;
import javafx.scene.input.MouseEvent;
import javafx.scene.input.TransferMode;


public class DnDTreeCell extends TreeCell<ProgramTreeItem> {
    private TreeView<ProgramTreeItem> parentTree;
    private ProgramTreeItem item;


    public DnDTreeCell(final TreeView<ProgramTreeItem> parentTree) {
        System.out.format("\nCTR - DnDTreeCell: [%s]\n", parentTree);
        this.parentTree = parentTree;

        // ON SOURCE NODE.
        setOnDragDetected(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                System.out.println("\nDrag detected on " + item);
                if (item == null) {
                    return;
                }
                Dragboard dragBoard = startDragAndDrop(TransferMode.MOVE);
                ClipboardContent content = new ClipboardContent();
                content.put(DataFormat.PLAIN_TEXT, item.toString());
                dragBoard.setContent(content);
                event.consume();
            }
        });
        setOnDragDone(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                System.out.println("\nDrag done on " + item);
                dragEvent.consume();
            }
        });
        // ON TARGET NODE.
    //            setOnDragEntered(new EventHandler<DragEvent>() {
    //                @Override
    //                public void handle(DragEvent dragEvent) {
    //                    System.out.println("Drag entered on " + item);
    //                    dragEvent.consume();
    //                }
    //            });
        setOnDragOver(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                System.out.println("\nDrag over on " + item);
                if (dragEvent.getDragboard().hasString()) {
                    String valueToMove = dragEvent.getDragboard().getString();
                    if (!valueToMove.matches(item.toString())) {
                        // We accept the transfer!!!!!
                        dragEvent.acceptTransferModes(TransferMode.MOVE);
                    }
                }
                dragEvent.consume();
            }
        });
    //            setOnDragExited(new EventHandler<DragEvent>() {
    //                @Override
    //                public void handle(DragEvent dragEvent) {
    //                    System.out.println("Drag exited on " + item);
    //                    dragEvent.consume();
    //                }
    //            });
        setOnDragDropped(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                String valueToMove = dragEvent.getDragboard().getString();
                ProgramTreeItem itemToMove = search((ProgramTreeItem) parentTree.getRoot(), valueToMove);
                ProgramTreeItem newParent = search((ProgramTreeItem) parentTree.getRoot(), item.toString());
                if (!newParent.getParent().equals(itemToMove)) {
                    // Remove from former parent.
                    System.out.format("\nDrag dropped on " + item + ";  ItemToMove: %s;  NewParent: %s\n", itemToMove, newParent);
                    itemToMove.getParent().getChildren().remove(itemToMove);
                    // Add to new parent.
                    newParent.getChildren().add(itemToMove);
                    newParent.setExpanded(true);
                } else {
                    System.out.format("\nDrop not Allowed !\n");
                }
                dragEvent.consume();
            }
        });
    }

    private ProgramTreeItem search(final ProgramTreeItem currentNode, final String valueToSearch) {
        ProgramTreeItem result = null;
        if (currentNode.toString().matches(valueToSearch)) {
            result = currentNode;
        } else if (!currentNode.isLeaf()) {
            for (Object ch : currentNode.getChildren()) {
                result = search((ProgramTreeItem)ch, valueToSearch);
                if (result != null) {
                    break;
                }
            }
        }
        return result;
    }

    @Override
    protected void updateItem(ProgramTreeItem item, boolean empty) {
        System.out.format("\nupdateItem - [%s]\n", item);
        super.updateItem(item, empty);
        this.item = item;
        String text = (item == null) ? null : item.toString();
        setText(text);
    }
}

ProgramRootTreeItem.java

package pluginmanagement;


public class ProgramRootTreeItem extends ProgramTreeItem {

    public ProgramRootTreeItem() {
        super("ROOT");
    }


    @Override
    public boolean isRoot() {
        return true;
    }

    @Override
    public String toString() {
        return "ProgramRootTreeItem_"+getUserObject();
    }
}

ProgramTreeItem.java

package pluginmanagement;

import javafx.scene.control.TreeItem;


public class ProgramTreeItem extends TreeItem {
    String userObj;

    public ProgramTreeItem(String data) {
        super(data);
        userObj = data;
    }

    public String getUserObject() {
        return userObj;
    }

    public boolean isRoot() {
        return false;
    }

    @Override
    public String toString() {
        return "ProgramTreeItem_" + getUserObject();
    }
}

Answer 1

您错误地使用了TreeView。

你被允许打电话的唯一原因

treeView.setRoot(root);

是您自定义的TreeItem 实现扩展了原始类型而不是指定类型参数。否则编译器会抱怨方法调用。

TreeItems 只定义了树的结构。传递给单元格的实际值存储在TreeItem 的value 属性中。您所有的自定义TreeItems 都存储Strings。

当TreeView 稍后尝试用来自TreeItems 的值填充它的单元格时，它会从TreeItems 获得Strings 但无法使用这些值作为第一个参数调用updateItem，因为DnDTreeCell 期望 ProgramTreeItem。

实际上，您的自定义 TreeItem 类不包含您无法从传统 TreeItem 轻松获得的信息：

public static boolean isRoot(TreeItem<?> item) {
    return item.getParent() == null;
}

我建议使用TreeItem<String>s 重写代码：

private TreeView<String> getExampleTree() {
    TreeView<String> treeView = new TreeView<>();
    TreeItem<String> root = buildTree();

    treeView.setCellFactory(new Callback<TreeView<String>, TreeCell<String>>() {
        @Override
        public TreeCell call(TreeView<String> param) {
            System.out.format("\ncall() - param: %s\n", param);
            TreeCell<String> cell = new DnDTreeCell();
            System.out.format("\ncall() - return: %s\n", cell);
            return cell;
        }
    });

    treeView.setPrefSize(1000, 750);
    treeView.setShowRoot(true);
    treeView.setRoot(root);
    root.setExpanded(true);

    return treeView;
}

private TreeItem<String> buildTree() {
    TreeItem<String> root = new TreeItem<>();
    for(int a=1; a<4; a++) {
        for(int b=1; b<4; b++) {
            TreeItem<String> child = new TreeItem<>("child_" + a + "_" + b);
            root.getChildren().add(child);
        }
    }
    return root;
}

public class DnDTreeCell extends TreeCell<String> {

    public DnDTreeCell() {

        // ON SOURCE NODE.
        setOnDragDetected(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                TreeItem<String> item = getTreeItem();
                if (isEmpty() || item == null || item.getParent() == null) {
                    return; // don't start drag on empty cell or root cell
                }
                Dragboard dragBoard = startDragAndDrop(TransferMode.MOVE);
                ClipboardContent content = new ClipboardContent();
                content.put(DataFormat.PLAIN_TEXT, item.getValue());
                dragBoard.setContent(content);
                event.consume();
            }
        });
        setOnDragDone(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                dragEvent.consume();
            }
        });
        setOnDragOver(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                if (!(dragEvent.getGestureSource() instanceof TreeCell)
                        || ((TreeCell) dragEvent.getGestureSource()).getTreeView() != getTreeView()) {
                    return; // only allow dragging from this TreeView
                }
                TreeItem<String> sourceItem = ((TreeCell<String>) dragEvent.getGestureSource()).getTreeItem();

                // prevent item from being added to own subtree
                TreeItem<String> item = getTreeItem();
                while (item != null && item != sourceItem) {
                    item = item.getParent(); // go up through the tree until dragged item or root is found
                }
                if (item == null) {
                    // We accept the transfer!!!!!
                    dragEvent.acceptTransferModes(TransferMode.MOVE);
                }
                dragEvent.consume();
            }
        });
        setOnDragDropped(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent dragEvent) {
                TreeItem<String> itemToMove = ((TreeCell<String>) dragEvent.getGestureSource()).getTreeItem();
                TreeItem<String> newParent = getTreeItem();
                // Remove from former parent.
                itemToMove.getParent().getChildren().remove(itemToMove);
                // Add to new parent.
                newParent.getChildren().add(itemToMove);
                newParent.setExpanded(true);
                dragEvent.setDropCompleted(true);
                dragEvent.consume();
            }
        });
    }

    @Override
    protected void updateItem(String item, boolean empty) {
        System.out.format("\nupdateItem - [%s]\n", item);
        super.updateItem(item, empty);
        setText(item == null ? "" : item);
    }
}