computeDryLayout method Null safety

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

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 return a dummy value of const Size(0, 0).

Implementation

@override
Size computeDryLayout(BoxConstraints constraints) {
  if (child != null) {
    final Size childSize = child!.getDryLayout(const BoxConstraints());

    // During [RenderObject.debugCheckingIntrinsics] a child that doesn't
    // support dry layout may provide us with an invalid size that triggers
    // assertions if we try to work with it. Instead of throwing, we bail
    // out early in that case.
    bool invalidChildSize = false;
    assert(() {
      if (RenderObject.debugCheckingIntrinsics && childSize.width * childSize.height == 0.0) {
        invalidChildSize = true;
      }
      return true;
    }());
    if (invalidChildSize) {
      assert(debugCannotComputeDryLayout(
        reason: 'Child provided invalid size of $childSize.',
      ));
      return Size.zero;
    }

    switch (fit) {
      case BoxFit.scaleDown:
        final BoxConstraints sizeConstraints = constraints.loosen();
        final Size unconstrainedSize = sizeConstraints.constrainSizeAndAttemptToPreserveAspectRatio(childSize);
        return constraints.constrain(unconstrainedSize);
      default:
        return constraints.constrainSizeAndAttemptToPreserveAspectRatio(childSize);
    }
  } else {
    return constraints.smallest;
  }
}