Dynamic Sliver FloatingActionButton

by Mar 30, 2018Flutter16 comments

In this post, I will walk through the process of creating FloatingActionButton that is pinned to the edge of FlexibleSpaceBar (shown below). Let’s get to it!

TLDR: I exported pub package, so you can achieve similar FAB behavior. Feel free to use and expand it. 🙂

The problem

Main problem with implementing pinned FloatingActionButton like this, is that there is no place inside the CustomScrollView or SliverList to put a Widget that would be on top of SliverAppBar and Slivers below it. In order to do it, we have to put our FloatingActionButton outside CustomScrollView and place it inside a Stack. Here is our starting point:

It looks like this:

. . .

Pinning FloatingActionButton

As you can see, we have a beautiful FloatingActionButton pinned to the edge of the AppBarbut only if the user will not scroll the content. Not the best use case for us. In order to pin FloatingActionButton to edge of AppBar, we need to use ScrollController to keep track of scrolling offset and change position of fab. First, let’s add ScrollController that will update our Widget’s state:

Then let’s add it to the CustomScrollView:

And then let’s change FloatingActionButton position depending on scrolling offset. To do that I extracted creating FloatingActionButton to a new method. A thing that is worth mentioning is that on the first build _scrollController is not connected to scrollViewyet, so we can’t directly ask it for offset.

Current result should look like this:

. . .

Hiding FloatingActionButton

As you can see, our FloatingActionButton is scrolling way out of the screen. Let’s hide it when it comes close to the top. To do that we could use Opacity widget, however visual aspect of that solution is not great since Fab would be just vanishing. In my opinion better solution would be to scale FloatingActionButton down, so let’s try to do that:

ScaleStart and ScaleEnd values can be changed depending on what behavior you expect. I find these values to satisfy my aesthetic needs 😀

. . .

And that’s it! Now we have nice FloatingActionButton placed on the edge of AppBar which looks like this:

Since this kind of view might be needed by other people, I exported it as a pub package. I designed it only for my use case so if you think, there are any bugs or missing features I would highly appreciate any pull requests 🙂

You can find source code created in this post here and current repo with the code here.

If you have any questions, feel free to leave a comment! 🙂

Cheers 🙂

Marcin Szałek

Founder of Fidev

Flutter enthusiast since Alpha release in 2017. Believes that sharing is caring, which lead him to start a technical blog dedicated fo Flutter in its early days. Loves to see beautiful designs become real apps and is willing to help make it happen. Enjoys sunny beaches far from home.

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