Flutter, what are Widgets, RenderObjects and Elements?

Ever wondered how Flutter takes those widgets and actually converts them to pixels on the screen?

Norbert
Norbert
Jun 30, 2018 · 5 min read

Let’s get started

The Opacity

The Widget

The Rendering

@override
RenderOpacity createRenderObject(BuildContext context) => new RenderOpacity(opacity: opacity);

@override
void updateRenderObject(BuildContext context, RenderOpacity renderObject) {
renderObject.opacity = opacity;
}

RenderOpacity

double get opacity => _opacity;
double _opacity;
set opacity(double value) {
_opacity = value;
markNeedsPaint();
}
@override
void paint(PaintingContext context, Offset offset) {
context.pushOpacity(offset, _alpha, super.paint);
}

To recap

That’s it? Kind of.

Where is the element created in the opacity example?

@override
SingleChildRenderObjectElement createElement() => new SingleChildRenderObjectElement(this);

The element creates the RenderObject, but in our case the Opacity widget creates its own RenderObject?

SingleChildRenderObjectElement(SingleChildRenderObjectWidget widget) : super(widget);
@override
void mount(Element parent, dynamic newSlot) {
super.mount(parent, newSlot);
_renderObject = widget.createRenderObject(this);
attachRenderObject(newSlot);
_dirty = false;
}

The End

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