The control before items are added, and after a single item is added.
The flags, struct, and class editors. Simply check the boxes in the flags dropdown for the combination you want. The struct and class editors are just a dialog with a nested MPG, the main difference being that for classes the "null" checkbox is available.
A red outline is shown around editors who's contents cannot be assigned back to the property. The background of the editor cell will be gray if the value differs among the objects selected by the MPG.
Getting all the keyboard input, focus, mouse capture, data binding, etc. of this thing working correctly was at time enormously frustrating. I could itemize the challenges, but honestly I'm so tired of this control that I don't really want to drag my memory through it again. If anyone is using WPF and is interested though, I'd be happy to discuss it and share the code.
Switching gears... Somewhere along the line I took a break and refactored my XNA input manager, since I wasn't quite happy with it. The primary reason for making one was to provide quick methods for checking if buttons were just pressed or released during the current frame. From that though, I also wanted to reign in the inconsistent input APIs. The keyboard, gamepad, and mouse classes all exposed their states in slightly different ways, and I wanted to have one, single flat state for all digital inputs, and all analog inputs. For digital, I combined all inputs exposed by the 3 devices into a single, large enum, and allow the user to query if an input is down, up, just pressed, or just released. This should make it easy to bind controls to any device, or any combination of devices.
Similarly for analog, I made an enumeration of all axes available, and normalize them to a range of -1.0 to 1.0. The user can then query the current sample, or for the delta from the last sample. This works great for all the gamepad axes, but was slightly awkward for the mouse. To make it work, the input manager recenters the pointer after every mouse sampling. This allows one set of code to correctly handle camera control from either the mouse or the gamepad (albeit with different sensitivities). For the gamepad, camera movement is proportional to the current sample. If the player is holding the stick steady at 1.0, the camera will move at a certain speed, a different speed at 0.5, etc. For the mouse, camera movement is typically handled with the mouse delta, rather than the mouse sample. Recentering the mouse, however, effectively turns the sample into a delta, and we get the expected behavior.
One final challenge was the aspect ratio of the mouse. Normalizing horizontal and vertical axes to -1 to 1 means that the movement along the axes has different magnitudes, as the screen is not square. To deal with this, I provide aspect corrected mouse axes, as well as the non-aspect ones (which are themselves useful to tracking pointer position in view space).
Now I still have the undo/redo stack to implement, then It's finally back to working with 4D things. The level format aught to be interesting, especially if I add spatial partitioning...