MahApps.Metro - GroupItemHelper

Applies to a grouped ListView or ListBox. It gives a group of items two things beyond standing there: a header that holds at the top while its own items pass under it, and a box in that header picking the whole group at once.

GroupItemHelper is new in develop and is not in 2.4.11.

Property	Type	Default
`IsHeaderSticky`	`bool`	`false`	hold the header at the top while the group scrolls under it
`CanSelectAllItems`	`bool`	`false`	let the box in the header stand for every item of the group
`GroupContainerStyle`	`Style`	`null`	the style for a group
`GroupContainerStyleWithCheckBox`	`Style`	`null`	the same with a box beside the name of the group
`IsGroupSelected`	`bool?`	`null`	whether the group is picked; `null` for some of it

Both of the first two are set on the list, not on the group, because a group is built and thrown away again as the list scrolls. They inherit, so the value on the list reaches every group in it.

<ListView ItemsSource="{Binding AlbumsView}"
          SelectionMode="Extended"
          Style="{StaticResource MahApps.Styles.ListView.Win10}"
          mah:GroupItemHelper.CanSelectAllItems="True"
          mah:GroupItemHelper.IsHeaderSticky="True"
          mah:ItemHelper.IsMultiSelectCheckBoxEnabled="True" />

MahApps.Styles.ListView.Win10 asks for both already, and MahApps.Styles.ListView.WinUI brings the two group styles along without the sticky header.

The header that stays

WPF lays a group out as one GroupItem holding a header with the items under it, and scrolls the two together, so the header of the group you are reading leaves the top of the view before its own items do.

Holding it means moving it down by as much as the group has already passed the top edge, and no further than the bottom of that group, so the next group pushes it out of the way rather than drawing over it. The move is a render transform, so nothing is laid out again while the list scrolls.

The list has to scroll by the pixel. A list scrolling by the row, which is what WPF does unless told otherwise, never lets a group past the top edge at all: it puts a whole row at the top and shifts the contents of the group instead, so there is nothing to hold. Set VirtualizingPanel.ScrollUnit to Pixel, which is what MahApps.Styles.ListView.Win10 does.

A style of its own has to name the element to hold PART_Header, and that element has to be free to draw over the items, which means the two sharing one cell of a grid with the header declared after them.

The box that picks a group

CanSelectAllItems lets the box in a group header stand for all of the items under it: ticking it picks them, clearing it lets them go, and a group only some of whose items are picked shows the third state. It needs a list that allows more than one item to be picked, since there is nothing for it to do otherwise.

IsGroupSelected is that answer as a property, true, false or null for a group that is partly picked, should you want to read or write it yourself.

The box only appears where ItemHelper's IsMultiSelectCheckBoxEnabled asks for boxes on the rows as well. That is what GroupContainerStyleWithCheckBox is for: the helper hands that style to a group instead of the plain GroupContainerStyle while both flags are asking for it.

Two styles rather than one with a trigger inside it, because a trigger naming an attached property of this library costs a thrown exception every time WPF builds the style from inside a page, where the mah: prefix means nothing.

ItemHelper carries the per-state brushes of the rows and the box saying what is picked. ListView and ListBox are the lists themselves.

The header that stays

The box that picks a group

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