45f2c58987
This patch is the first step towards implementing gestures. The pointer router allows the gesture detectors to hook in at the end of the pointer event propagation chain. Related to #145
Sky Base
AbstractNode
The node.dart file defines a class, AbstractNode, which
can be used to build mutable trees.
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When a subclass is changing the parent of a child, it should call either parent.adoptChild(child) or parent.dropChild(child) as appropriate. Subclasses should expose an API for manipulating the tree if you want to (e.g. a setter for a 'child' property, or an 'add()' method to manipulate a list).
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You can see the current parent by querying 'parent'.
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You can see the current attachment state by querying 'attached'. The root of any tree that is to be considered attached should be manually attached by calling 'attach()'. Other than that, don't call 'attach()' or 'detach()'. This is all managed automatically assuming you call the 'adoptChild()' and 'dropChild()' methods appropriately.
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Subclasses that have children must override 'attach()' and 'detach()' as described below.
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Nodes always have a 'depth' greater than their ancestors'. There's no guarantee regarding depth between siblings. The depth of a node is used to ensure that nodes are processed in depth order. 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. This is all managed automatically assuming you call 'adoptChild()' and 'dropChild()' appropriately.
Dependencies
No dependencies except for dart:sky and Dart's core libraries.