computeDryLayout method Null safety

  1. @protected
Size computeDryLayout(
  1. BoxConstraints constraints
)

Computes the value returned by getDryLayout. Do not call this function directly, instead, call getDryLayout.

Override in subclasses that implement performLayout or performResize or when setting sizedByParent to true without overriding performResize. This method should return the Size that this RenderBox would like to be given the provided BoxConstraints.

The size returned by this method must match the size that the RenderBox will compute for itself in performLayout (or performResize, if sizedByParent is true).

If this algorithm depends on the size of a child, the size of that child should be obtained using its getDryLayout method.

This layout is called "dry" layout as opposed to the regular "wet" layout run performed by performLayout because it computes the desired size for the given constraints without changing any internal state.

When the size cannot be known

There are cases where render objects do not have an efficient way to compute their size without doing a full layout. For example, the size may depend on the baseline of a child (which is not available without doing a full layout), it may be computed by a callback about which the render object cannot reason, or the layout is so complex that it is simply impractical to calculate the size in an efficient way.

In such cases, it may be impossible (or at least impractical) to actually return a valid answer. In such cases, the function should call debugCannotComputeDryLayout from within an assert and and return a dummy value of const Size(0, 0).

Implementation

@protected
Size computeDryLayout(BoxConstraints constraints) {
  assert(debugCannotComputeDryLayout(
    error: FlutterError.fromParts(<DiagnosticsNode>[
      ErrorSummary('The ${objectRuntimeType(this, 'RenderBox')} class does not implement "computeDryLayout".'),
      ErrorHint(
        'If you are not writing your own RenderBox subclass, then this is not\n'
        'your fault. Contact support: https://github.com/flutter/flutter/issues/new?template=2_bug.md',
      ),
    ]),
  ));
  return Size.zero;
}