演示-DropDemo

现在，我们将看一个使用自定义传输处理程序实现列表组件放置的演示。尽管 list 的默认传输处理程序实现了导出，但是由于我们正在创建自定义传输处理程序来实现导入，因此我们也必须重新实现导出。

从屏幕快照中可以看到，DropDemo包含一个可编辑的文本区域，一个列表和一个组合框，您可以使用该组合框选择列表的放置 Pattern。

这是DropDemo.java的ListTransferHandler实现。

此列表的传输处理程序支持复制和移动，并且默认情况下重新实现了列表提供的拖动支持。

public class ListTransferHandler extends TransferHandler {
    private int[] indices = null;
    private int addIndex = -1; //Location where items were added
    private int addCount = 0;  //Number of items added.
            
    /**
     * We only support importing strings.
     */
    public boolean canImport(TransferHandler.TransferSupport info) {
        // Check for String flavor
        if (!info.isDataFlavorSupported(DataFlavor.stringFlavor)) {
            return false;
        }
        return true;
   }

    /**
     * Bundle up the selected items in a single list for export.
     * Each line is separated by a newline.
     */
    protected Transferable createTransferable(JComponent c) {
        JList list = (JList)c;
        indices = list.getSelectedIndices();
        Object[] values = list.getSelectedValues();
        
        StringBuffer buff = new StringBuffer();

        for (int i = 0; i < values.length; i++) {
            Object val = values[i];
            buff.append(val == null ? "" : val.toString());
            if (i != values.length - 1) {
                buff.append("\n");
            }
        }
        
        return new StringSelection(buff.toString());
    }
    
    /**
     * We support both copy and move actions.
     */
    public int getSourceActions(JComponent c) {
        return TransferHandler.COPY_OR_MOVE;
    }
    
    /**
     * Perform the actual import.  This demo only supports drag and drop.
     */
    public boolean importData(TransferHandler.TransferSupport info) {
        if (!info.isDrop()) {
            return false;
        }

        JList list = (JList)info.getComponent();
        DefaultListModel listModel = (DefaultListModel)list.getModel();
        JList.DropLocation dl = (JList.DropLocation)info.getDropLocation();
        int index = dl.getIndex();
        boolean insert = dl.isInsert();

        // Get the string that is being dropped.
        Transferable t = info.getTransferable();
        String data;
        try {
            data = (String)t.getTransferData(DataFlavor.stringFlavor);
        } 
        catch (Exception e) { return false; }
                                
        // Wherever there is a newline in the incoming data,
        // break it into a separate item in the list.
        String[] values = data.split("\n");
        
        addIndex = index;
        addCount = values.length;
        
        // Perform the actual import.  
        for (int i = 0; i < values.length; i++) {
            if (insert) {
                listModel.add(index++, values[i]);
            } else {
                // If the items go beyond the end of the current
                // list, add them in.
                if (index < listModel.getSize()) {
                    listModel.set(index++, values[i]);
                } else {
                    listModel.add(index++, values[i]);
                }
            }
        }
        return true;
    }

    /**
     * Remove the items moved from the list.
     */
    protected void exportDone(JComponent c, Transferable data, int action) {
        JList source = (JList)c;
        DefaultListModel listModel  = (DefaultListModel)source.getModel();

        if (action == TransferHandler.MOVE) {
            for (int i = indices.length - 1; i >= 0; i--) {
                listModel.remove(indices[i]);
            }
        }
        
        indices = null;
        addCount = 0;
        addIndex = -1;
    }
}

接下来，我们看一下目标如何选择放置动作。