WPF: How to dynamically generate ContextMenu

ListBox seems to have it's own internal command for the Ctrl+A key combination, as Marco Zhou explains. We can also test this by attempting to place a breakpoint in the Execute and Preview Execute handlers. As you will see neither is reached when the ListBox has focus and the key combination is pressed. Even when we set the SelectionMode to Extended and we can watch the items be selected by the command the handlers still are not reached. Thankfully though, we can override an existing InputGesture by just re-assigning it. We can do this in the ListBox to get rid of it's custom Ctrl+A handling, and re-assign it to the ApplicationCommands.SelectAll command.

<ListBox Name="listBox"
         SelectionMode="Multiple">
    <ListBox.InputBindings>
        <KeyBinding Command="ApplicationCommands.SelectAll"
                    Modifiers="Ctrl"
                    Key="A" />
    </ListBox.InputBindings>            
    ...
</ListBox>

Once the KeyBinding is added to the ListBox, when it has focus it will now route Ctrl+A back to your existing SelectAll command and SelectAllExecuted.

The contextmenu plugin already has support for this. From the documentation you linked to:

items: Expects an object or a function, which should return an object. If a function is used it fired in the tree's context and receives one argument - the node that was right clicked.

So rather than give contextmenu a hard-coded object to work with, you can supply the following function. It checks the element that was clicked for a class named "folder", and removes the "delete" menu item by deleting it from the object:

function customMenu(node) {
    // The default set of all items
    var items = {
        renameItem: { // The "rename" menu item
            label: "Rename",
            action: function () {...}
        },
        deleteItem: { // The "delete" menu item
            label: "Delete",
            action: function () {...}
        }
    };

    if ($(node).hasClass("folder")) {
        // Delete the "delete" menu item
        delete items.deleteItem;
    }

    return items;
}

Note that the above will hide the delete option completely, but the plugin also allows you to show an item while disabling its behaviour, by adding _disabled: true to the relevant item. In this case you can use items.deleteItem._disabled = true within the if statement instead.

Should be obvious, but remember to initialise the plugin with the customMenu function instead of what you had previously:

$("#tree").jstree({plugins: ["contextmenu"], contextmenu: {items: customMenu}});
//                                                                    ^
// ___________________________________________________________________|

Edit: If you don't want the menu to be recreated on every right-click, you can put the logic in the action handler for the delete menu item itself.

"label": "Delete",
"action": function (obj) {
    if ($(this._get_node(obj)).hasClass("folder") return; // cancel action
}

Edit again: After looking at the jsTree source code, it looks like the contextmenu is being re-created every time it is shown anyway (see the show() and parse() functions), so I don't see a problem with my first solution.

However, I do like the notation you are suggesting, with a function as the value for _disabled. A potential path to explore is to wrap their parse() function with your own one that evaluates the function at disabled: function () {...} and stores the result in _disabled, before calling the original parse().

It won't be difficult either to modify their source code directly. Line 2867 of version 1.0-rc1 is the relevant one:

str += "<li class='" + (val._class || "") + (val._disabled ? " jstree-contextmenu-disabled " : "") + "'><ins ";

You can simply add a line before this one that checks $.isFunction(val._disabled), and if so, val._disabled = val._disabled(). Then submit it to the creators as a patch :)