How to implement CRUD Master Details on the same screen under MVVM

Question

I have a MVVM (Prism) application that I need to implement a master details screen wheer the master is a listview and the details is displayed next to it. Read-only seems easy enough (haven't done it yet but I've got my head around WPF binding) but edit/add confuses me.

How to I make it so the master is not updated until the details is saved? How do I make it so you can't change the master's current selection while in edit/add mode?

I've been googling a plenty but have not found any meaty examples of this.

Thanks.

PS: This view is a child view on a larger screen. This is why I want both master and detail together.

Answer 1

You certainly can do this, though in my opinion such a UI design fails to harness the full power of WPF. Old WinForms UIs usually didn't update most of the application until data was saved to SQL Server (or wherever) because they didn't have real business objects and a powerful binding system like WPF. Trying to copy WinForms limitations within WPF seems like a step backward to me. Why not show the latest data everywhere it is visible in the UI, including in the master view? Also, why not allow the user to edit multiple items before saving, for example marking any edited but unsaved item with an animated marker in the master view? Combine these with a generalized undo and you have a better design and more intuitive for the user.

However if your business requirements make it absolutely necessary, here is how to do it:

Preventing changes to data from being visible outside the detail until it is saved

Upon entry into your "edit/add mode", make a copy of the data objects and set your detail view's DataContext to the copy instead of the live object. When the data is "saved", copy the data from the shadow copy back into the live object and set your detail view's DataContext back where it should be.

Preventing the master's current selection from changing while in edit/add mode

Two possibilities:

1. During edit/add mode, change the master view to disallow mouse hit testing or keyboard focus

2. When edit/add mode begins, capture the "current selection" then add an event handler that watches for "current selection" changes and immediately changes the selection back to what it was. When edit/add mode ends, remove the handler. This handler can be conveniently coded using a lambda expression and using a closure on a local variable to store the current selection.

Answer 2

Thanks for the answer. Now I've re-read my message, I see it is rather vague. I have a screen that edits an object which contains multiple lists of other child objects. I've implemented these as different tabs in a tab control. One of these tabs edits the comments, so I wanted to display a list of comments with an edit panel for the current selection next to the list. The user could then use add, edit or delete buttons to update the list. I wanted to do this in a pure(ish) MVVM way.

I came up with the following design which seems to work with minimal hacks.

The View includes a list of the child objects simply as a ListView bound to an observable collection within the ViewModel. I included a child object buffer – this is used to buffer changes until they are ready to be saved back to the list (or thrown away).

The View also includes an edit panel bound to the buffer object in the ViewModel. The buffer is updated whenever the list view’s current selection changes using a deep copy. I tried using data binding on the Selecteditem property but the set was never called, so a small code-behind method was added to force the property to be updated when the selection was changed.

The list view and edit view are mutually exclusive. In theory you could hide the disabled one, perhaps using a flip screen. As a general pattern, it is better for my app to have both visible at the same time as the edit panel may show extra information not shown in the list view. The choice as to which panel is enabled is controlled by binding IsEnabled to a ViewModel property like IsEditCommentMode.

Commands to manage the list have to be added, these are New, Editand Delete. Note that Add and Edit will set set up the buffer then set IsEditCommentMode to true. These list management commands are only available when IsEditCommentMode is false. The edit panel implements Save and Cancel commands, they are only be enabled when IsEditCommentMode is true. When Save is executed, it should copy from the buffer to the list (either add or update) and fire the change notification. Finally, it should set IsEditCommentMode to false.

This all works well and does not seem to violate any MVVM tenents (in my humble but often flawed opinion).