Gap: A simple example to create your own RenderObject

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Photo by Alex Radelich on Unsplash

I love how Flutter is designed. The combination of immutables objects, the Widgets and mutables ones called RenderObjects is very powerful although not super intuitive for beginners.

We can do a lot just by composing widgets with StatelessWidgets and StatefulWidgets, but we cannot do everything, or not in an efficient way. That’s why you need to understand how to create your own RenderObject when it’s necessary.
In this post I will not explain what is the difference between a Widget and a RenderObject but I will show you how I created a simple RenderObject in order for you to do the same when you need it. If you want to know more about Widgets, Elements and RenderObjects before reading this, I advise you to read this excellent article of Norbert.

Last week I was fed up with adding gaps between my widgets in some Columns and Rows. I use SizedBoxes for that, but we need to specify the width or the height depending on whether we are inside a Column or a Row. I don’t find this very productive so I ended up creating my own RenderObject for that. I called the underlaying widget Gap!

Fun fact 🙃: Three days after I started to work on this widget, I saw this tweet in my TL:

I was very amused by the timing! I read the article of Stefan Matthias Aust where he explains how he created a Gap widget with pretty much the same implementation than mine. I will try to make a different analyze 😉.

The following code is the prototype of my widget. I think it’s simple enough to explain how to easily create a new Widget and its associatedRenderObject.

The RenderObject is the piece of code responsible of computing the layout and paint it to the screen. I start with the render layer, because it’s completely independent from the widget layer.

There are (for the moment) two kinds of RenderObjects: RenderBoxes and RenderSlivers.

To know which of them you must extend, it’s pretty simple: If your component needs to be aware of the scroll constraints (for a scroll effect or because it needs to be in a scroll container), the RenderObject must extend RenderSliver, otherwise it must extend RenderBox.
Pro tip 💪: In most of the cases you want a RenderBox 😉.

In our context, the parent of our widget will be a Flex widget, which only understands RenderBoxes, so we don’t even have a choice, our RenderObject must extend RenderBox.

Our RenderGap have only one property called mainAxisExtent. This is the extent of our gap in the direction of its parent: if the widget’s parent is a Column, then this is the height and if it’s a Row then it represents the width.

We set the mainAxisExtent at the creation and we can update it with the associated setter. This setter makes a call to markNeedsLayout, which tells the framework that it needs to re-run the performLayout method before rendering the frame.

But what it this performLayout method? As its name indicates, it’s the method responsible for computing the layout of our component. In the case of a RenderBox, it does so by setting its size property.
In this method we know that the constraints are already computed and we can use them to determine our size.

As I mentioned before, the parent of our widget is either a Column or a Row. Both of them extend Flex and the associated RenderObject of a Flex is a RenderFlex.
We want our RenderGap to only work if the parent is a RenderFlex, so this is the first thing we do in the performLayout method. Then we look at the direction of its parent to know how to compute the size. If it’s Axis.horizontal, then we are in a Row, and we want the size to have a width of the value of our mainAxisExtent, and a height of 0. But we need to constrain our requested size with the constraints imposed by the parent. If the Gap is the direct child of a Row, it will not be constrained horizontally (maxWidth will have an infinite value), but if there is a Flexible between the Row and the Gap, then the maxWidth constrain will have a finite value, and thus we have to take into account this constraint. This is the meaning of constraints.constrain(Size(mainAxisExtent, 0));.

Oh wait, didn’t I tell you that the parent of a Gap must be a Flex widget before? Then how a Flexible can be between a Row and a Gap??
Well, I wasn’t totally precise. I should have written that the direct parent of a RenderGap must be a RenderFlex but the direct parent of a Gap is not necessarily a Flex widget.
This can be possible because there are widgets which are not visible and thus don’t need to be tied up with a RenderObject.
This is the case for example with the Flexible widget. If we look at the code we can see that Flexible extends ParentDataWidget<Flex> and it does not create any RenderObject.

This is all for the RenderObject. We don’t even need to override the paint method because we don’t want to paint anything, we just want to tell the framework that our component takes some place and that’s it.

To fully respect the RenderObject contract we should also implement other methods such as computeMinIntrinsicWidth, but we will stop here for now.

We already have our RenderObject, now we need to create a specific widget in order to add it to the build method of another widget. Our widget must extend RenderObjectWidget since it’s associated with a RenderGap. If we don’t want to manage the children ourselves, we can use one of the three out-of-the-box classes depending on the number of children the widget can take:

  • For zero children the widget must extends LeafRenderObjectWidget.
  • For one child the widget must extends SingleChildRenderObjectWidget.
  • For two or more children the widget must extends MultiChildRenderObjectWidget.

In our case, the Gap widget has no children, that’s why Gap extends LeafRenderObjectWidget.

There are two methods to override when we extend RenderObjectWidget:

  • createRenderObject: this method is called the first time the widget is added to the widget tree. It must return a new instance of the associatedRenderObject populated with the properties set or computed from the widget.
  • updateRenderObject: this method is called every time the widget is updated. It must update the previous instance of the associated RenderObject with the properties set or computed from the widget.

In our case, the code is very simple since we have only one property to pass to the RenderObject.

Our RenderGap needs a mainAxisExtent, so our widget also needs to have a mainAxisExtent property in order to be set within a build method.

And that’s it, this is the only things we have to do for the widget layer.

This is a small example of this widget for testing it. Enjoy!

This was a very simple widget which uses directly a RenderObject behind the scene. I made it a package downloadable on pub.dev if you find it useful. The code of the package is a little more complex because I added some options to allow the Gap to set its own crossAxisExtent and a color. Feel free to look at the full code on GitHub.

The rendering world is beautiful, but it can be difficult to understand, you’ll have to start with simple use cases and add complexity step by step.

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