Adding breadcrumb navigation to a WPF MVVM app using Caliburn

Question

I have an existing WPF application following the MVVM pattern with the Caliburn-Micro framework. I have a new requirement to add an interactive breadcrumb to the UI. While I can render the breadcrumb without issue, I am stumped trying to determine how to handle activation when one of the crumbs is clicked by the user.

I have an n-level object graph where each object is a Conductor<T>.Collection.OneActive.

The UI is able to handle basic "navigation" by calling ActivateItem with the item for each conductor to activate. At the each level, I display the list of items and when one is clicked, it is activated. As long as the activated item has child items, the UI displays its list of child items. When one of those is clicked, that child is activated and so on.

I am able to build the breadcrumb trail by crawling this object graph from the active root item down to active child item and so on but I don't know how to re-activate an item when the breadcrumb is clicked because I have a reference to the item, not the conductor that contains/manages that item.

Any ideas?

Answer 1

Just had an idea that may be of some use. It sounds like the conductors are hierarchical so:

Use the event aggregator to fire a simple message type with a property that contains the viewmodel you want to activate

public class BreadcrumbNavigate
{
    public IScreen Screen { get; private set; }

    public BreadcrumbNavigate(IScreen screen) { Screen = screen; }
}

The conductors should all be listening for these type of messages; when they receive a message they should only respond to it if they are currently active, meaning only the leaf node of the breadcrumb trail will respond to the first message.

If the VM in the message is not a child of the conductor, the conductor should close itself and re-publish the aggregator message so that the next active conductor can handle it (not sure how well this will work since it depends on a conductor closing itself then propagating a message but like I said, it's just an idea).

Eventually the correct conductor somewhere up the tree will handle the message, and that conductor will have a child VM which matches the message - this is the navigation target and that's where the message propagation stops

Edit:

The above won't work due to conductors being active when they have active children - so scrap that!

Can you post more information about your setup?

As far as you've described it, it sounds like a simple breadcrumb nav scenario; in order to go back along the breadcrumb you just need to use TryClose() on the active item of the VM that you want to jump back to:

// on the VM that you want to navigate to...

public void NavigateToMe() 
{
    // Check if we have a child, if so try and close it...will close all children etc
    if (ActiveItem != null)
    {
        ActiveItem.TryClose();
    }
}

Obviously you can just wrap this in a helper method such as

public void NavigateTo(IConductActiveItem vm)
{
    var iClose = vm.ActiveItem as IClose;
    if(iClose != null)
    {
        iClose.TryClose();
    }
}

So all you need in the breadcrumb is a reference to the VM, on checking it has a closeable child and calling TryClose() all child VMs/conductors should be closed below the selected child - obviously this will only work if the child of the VM is always an item conductor

this is assuming you are just using the following object graph:

Conductor -> Conductor -> Conductor -> Conductor