build method

@override Widget build(BuildContext context)

Describes the part of the user interface represented by this widget.

The framework calls this method when this widget is inserted into the tree in a given BuildContext and when the dependencies of this widget change (e.g., an InheritedWidget referenced by this widget changes). This method can potentially be called in every frame and should not have any side effects beyond building a widget.

The framework replaces the subtree below this widget with the widget returned by this method, either by updating the existing subtree or by removing the subtree and inflating a new subtree, depending on whether the widget returned by this method can update the root of the existing subtree, as determined by calling Widget.canUpdate.

Typically implementations return a newly created constellation of widgets that are configured with information from this widget's constructor and from the given BuildContext.

The given BuildContext contains information about the location in the tree at which this widget is being built. For example, the context provides the set of inherited widgets for this location in the tree. A given widget might be built with multiple different BuildContext arguments over time if the widget is moved around the tree or if the widget is inserted into the tree in multiple places at once.

The implementation of this method must only depend on:

• the fields of the widget, which themselves must not change over time, and
• any ambient state obtained from the context using BuildContext.dependOnInheritedWidgetOfExactType.

If a widget's build method is to depend on anything else, use a StatefulWidget instead.

See also:

• StatelessWidget, which contains the discussion on performance considerations.

Implementation

@override
Widget build(BuildContext context) => LayoutBuilder(
      builder: (context, constraints) {
        final slotWidth = constraints.maxWidth.isFinite
            ? constraints.maxWidth
            : MediaQuery.sizeOf(context).width;
        final margin = config.margin;
        final inner = slotWidth - margin.horizontal;
        // A slot narrower than its own margins would give a negative width,
        // and a zero design width an infinite scale. Collapse instead: an
        // absent card is better than a broken one.
        if (inner <= 0 || config.designWidth <= 0) {
          return const SizedBox.shrink();
        }

        // Divide by whichever is larger. Normally the card spans its frame and
        // the two agree, but a payload whose `canvasWidth` exceeds its
        // `designWidth` — hand-edited, or saved before the width was pinned —
        // would otherwise be drawn wider than `inner` by exactly that ratio
        // and spill out of the slot. Taking the max makes "never wider than
        // the slot" a property of the layout rather than of the data, and it
        // is a no-op in the normal case.
        final scale =
            inner / math.max(config.designWidth, config.canvas.width);

        return Padding(
          padding: EdgeInsets.only(
            top: margin.top,
            right: margin.right,
            bottom: margin.bottom,
            left: margin.left,
          ),
          child: ClipRRect(
            borderRadius: BorderRadius.circular(config.cornerRadius * scale),
            child: SizedBox(
              width: config.canvas.width * scale,
              height: config.canvas.height * scale,
              // The stage draws at authored logical pixels; scaling the whole
              // thing here is what keeps one uniform factor across children,
              // background, and typography alike.
              child: FittedBox(
                fit: BoxFit.fill,
                alignment: Alignment.topLeft,
                child: SizedBox(
                  width: config.canvas.width,
                  height: config.canvas.height,
                  child: CampaignCanvasStage(canvas: config.canvas),
                ),
              ),
            ),
          ),
        );
      },
    );