Mehdi pointed me to a great accordion on this site. It is a horizontal accordion that only uses images. In this post I’ll walk us through creating something similar.

This is what the accordion looks like:

There is a snazzy effect where the headline pops in from below when you open the item. Go look at it and play a bit, it’s pretty cool!

I started out by adding references to the highlighted 3 dll’s. All 3 are necessary!

Then I created my accordion and gave it some fixed size accordion items. I used this Xaml:

 

As you can see, I set the ExpandDirection to “Right”, so the accordion opens up in the same way as our sample. I have not set a specific Width on the Accordion, which means that the Accordion will only take what it needs.

I get a lot of questions on how the Width of the Accordion determines sizing on AccordionItems. You need to remember that the Items will always divide the space equally they get. In our example, setting a fixed Width of 500 on the Accordion would have yielded this result:

Now, our AccordionItems exist always of a header and content. In our sample though, we do not see a header at all. So, we will need to retemplate AccordionItem in order to remove that header. 
There is a headerTemplate property on Accordion, but keep in mind that that will allow you to determine the look of what goes into the header! In our case, we will completely remove the header from AccordionItem. So, I went into Blend and selected the first AccordionItem and opened up its template:

This created a styleResource that is being applied to the first item. 
In the template, I removed every element that I don’t care about. The AccordionButton is the little arrow that moves. Leaving me with this:

However, we do actually need a header, since it will be the element a user can click to navigate the accordion. So, let’s look at the Grid that hosts ExpandSite:

You can see that the grid has 2 rows and 2 columns. This is all that is needed to position the header and content in all the possible expandDirections. Just for fun, I’ll show you the VisualState “ExpandedRight”:

As you can see, it is happily placing items in the correct location and setting heights and widths on the grid cells. Since we do want the header to have a width, I’ll give the first column (where our header would have been) a minWidth. I will also remove all the Expand visualstates since I don’t need those and they only serve to bloat the Xaml at this point.

Since we removed AccordionButton, we have nothing to press and open the Item. But that’s not what we want anyway, we want to open items as we mouse over them. So I’ve introduced an eventhandler for the MouseEnter event on AccordionItem:

Now we’re getting somewhere! The items behave quite similar to our sample. Still need to figure out the header and content parts though. The ExpandSite will always contract to 0 width when it is collapsed (not selected). That can’t be what we want.

We removed the header because we did not want it, and want a part of the content to be visible. There are two animations that govern the expand and collapse movements. They are here:

Let me explain what is going on there. An AccordionItem can be in either Collapsed or Expanded state. I have no way of determining how big an item should be when it is expanded. Remember, it is the accordion that calculates such things at runtime (given the amount of items, and the width of the accordion itself). When the accordion sets a ‘targetsize’, we also calculate a percentage. When we are collapsed, that percentage is 0, when we are fully expanded, that percentage is 1.

The storyboards that you can see here are being started after the AccordionItem has been collapsed or expanded. In other words, they actually do the revealing or hiding of the content. The only thing they have to animate is the percentage, and this way I allow designers to come in and create different kind of animations (changing the keyspline for instance).

The important value here is 0, in the Collapsed storyboard. Turns out we don’t want it to collapse to 0, but to something like 0.2! If we change that storyboard, the item will still be partially visible, even though the accordion feels it is completely collapsed.

That was easy. So, let’s start working on some cool headline action. 
I’d like to have a headline come in from below as we expand.

So, let’s create a ContentControl to display our header:

And have Blend generate some nice animation effects:

You can see that the header starts out with Visibility=Collapsed, this way it won’t take up any space and mess up the AccordionItem. 
When the item expands, that visibility is toggled and a rendertransform is used to bring it into view.

Let’s finish it off with some images, to make our accordion look nice! I grabbed some images and created a somewhat better header:

I also changed the animations slightly, to be somewhat slower. The result looks like this:

I hope this little walkthrough helped.

See the live sample here, and download the project here. 
Let me know what you think!

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