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Summary: This is meant to show a possible route format for a persistent form of Yoga. Where previous layouts can remain intact while still taking advantage of incremental layout by reusing previous subtrees. ```c YGNodeRef YGNodeClone(const YGNodeRef node); ``` The core of this functionality is a new API to clone an existing node. This makes a new detached node with all the same values as the previous one. Conceptually this makes the original node "frozen" from that point on. It's now immutable. (This is not yet enforced at runtime in this PR but something we should add.) Since the original is frozen, we reuse the children set from the original node. Their parent pointers still point back to the original tree though. The cloned node is still mutable. It can have its styles updated, and nodes can be inserted or deleted. If an insertion/deletion happens on a cloned node whose children were reused, it'll first shallow clone its children automatically. As a convenience I also added an API to clear all children: ```c void YGNodeRemoveAllChildren(const YGNodeRef node); ``` During insert/delete, or as a result of layout a set of reused children may need to be first cloned. A kind of copy-on-write. When that happens, the host may want to respond. E.g. by updating the `context` such as by cloning any wrapper objects and attaching them to the new node. ```c typedef void (*YGNodeClonedFunc)(YGNodeRef oldNode, YGNodeRef newNode, YGNodeRef parent, int childIndex); void YGConfigSetNodeClonedFunc(YGConfigRef config, YGNodeClonedFunc callback); ``` This PR doesn't change any existing semantics for trees that are not first cloned. It's possible for a single node to exist in two trees at once and be used by multiple threads. Therefore it's not safe to recursively free a whole tree when you use persistence. To solve this, any user of the library has to manually manage ref counting or tracing GC. E.g. by replicating the tree structure in a wrapper. In a follow up we could consider moving ref counting into Yoga. Closes facebook/yoga#636 Reviewed By: emilsjolander Differential Revision: D5941921 Pulled By: sebmarkbage fbshipit-source-id: c8e93421824c112d09c4773bed4e3141b6491ccf
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