depth property

The depth of this render object in the render tree.

The depth of nodes in a tree monotonically increases as you traverse down the tree: a node always has a depth greater than its ancestors. There's no guarantee regarding depth between siblings.

The depth of a child can be more than one greater than the depth of the parent, because the depth values are never decreased: all that matters is that it's greater than the parent. Consider a tree with a root node A, a child B, and a grandchild C. Initially, A will have depth 0, B depth 1, and C depth 2. If C is moved to be a child of A, sibling of B, then the numbers won't change. C's depth will still be 2.

The depth of a node is used to ensure that nodes are processed in depth order. The depth is automatically maintained by the adoptChild and dropChild methods.