Pragmatic Coder

0.引子
通常一個(gè)TreeNode （DefaultMutableTreeNode）如果沒有子結(jié)點(diǎn)，它的節(jié)點(diǎn)顯示為不可展開的節(jié)點(diǎn)，有時(shí)候我們希望即使該節(jié)點(diǎn)暫時(shí)沒有裝載子結(jié)點(diǎn)仍然顯示為可展開的節(jié)點(diǎn)（如圖）。比如，當(dāng)加載子結(jié)點(diǎn)比較耗時(shí)或者需要?jiǎng)討B(tài)刷新，可以延時(shí)加載子結(jié)點(diǎn)，即在展開節(jié)點(diǎn)的時(shí)候再去裝載子結(jié)點(diǎn)。
1.由來及解決方案
該屬性取決于，DefaultTreeModel的isLeaf方法

??? /** 
     * Returns whether the specified node is a leaf node.
     * The way the test is performed depends on the
     * <code>askAllowsChildren</code> setting.
     *
     * @param node the node to check
     * @return true if the node is a leaf node
     *
     * @see #asksAllowsChildren
     * @see TreeModel#isLeaf
     */
publicboolean isLeaf(Object node) {
        if(asksAllowsChildren)
return !((TreeNode)node).getAllowsChildren();
        return ((TreeNode)node).isLeaf();
    }

???  /**
      * Sets whether or not to test leafness by asking getAllowsChildren()
      * or isLeaf() to the TreeNodes.  If newvalue is true, getAllowsChildren()
      * is messaged, otherwise isLeaf() is messaged.
      */
publicvoid setAsksAllowsChildren(boolean newValue) {
        asksAllowsChildren = newValue;
    }

??????  model.setRoot(root);

        /*
         * Since nodes are added dynamically in this application, the only true
         * leaf nodes are nodes that don't allow children to be added. (By
         * default, askAllowsChildren is false and all nodes without children
         * are considered to be leaves.)
         *
         * But there's a complication: when the tree structure changes, JTree
         * pre-expands the root node unless it's a leaf. To avoid having the
         * root pre-expanded, we set askAllowsChildren *after* assigning the
         * new root.
         */
        model.setAsksAllowsChildren(true);

而在調(diào)用model.nodeStructureChanged(node)前后要分別設(shè)置為false和true。這是因?yàn)閚odeStructureChanged會(huì)自動(dòng)的試圖展開節(jié)點(diǎn)node，如果isLeaf返回為false。試想，asksAllowsChildren返回true，則一個(gè)結(jié)點(diǎn)即使沒有子結(jié)點(diǎn)，只要設(shè)置了allowsChildren，對(duì)應(yīng)的treeNodeModel的isLeaf就會(huì)返回true。然而當(dāng)自動(dòng)展開節(jié)點(diǎn)的時(shí)候發(fā)現(xiàn)并沒有子結(jié)點(diǎn)，當(dāng)然就會(huì)把圖標(biāo)去掉了。而先把a(bǔ)sksAllowsChildren設(shè)為false，就不會(huì)去試圖展開該節(jié)點(diǎn)，然后設(shè)為true，該節(jié)點(diǎn)就會(huì)顯示為。???

??? /**
     * Determines whether or not this node is allowed to have children. 
     * If <code>allows</code> is false, all of this node's children are
     * removed.
     * <p>
     * Note: By default, a node allows children.
     *
     * @param	allows	true if this node is allowed to have children
     */
publicvoid setAllowsChildren(boolean allows) {
	if (allows != allowsChildren) {
	    allowsChildren = allows;
	    if (!allowsChildren) {
		removeAllChildren();
	    }
	}
    }

        node.removeAllChildren();
        DefaultTreeModel model = (DefaultTreeModel) cardTreeView.getModel();

        /*
         * To avoid having JTree re-expand the root node, we disable
         * ask-allows-children when we notify JTree about the new node
         * structure.
         */

        model.setAsksAllowsChildren(false);
        model.nodeStructureChanged(node);
        model.setAsksAllowsChildren(true);