Description

While working on #7557 I realized that the pure higher order component could lead to some potentially unexpected results when used with compose. I decided to document this possible flaw with a unit test and create this pull. I say "possible" because it actually may already be a known thing and it just is my misunderstanding of how pure is intended to function.

Currently pure examines the prototype of the incoming Wrapped component and if the prototype is a class component it extends it so that a shallow compare of state can be used to determine whether a component is re-rendered. Otherwise it only results in a shallow compare of just props.

Now if the expectation is that when pure is used in compose (as the left most function), that the original component does not re-render on shallow compares of state (i.e. MyComp in the test I added), then it's fine as is and there's no bug. However if the expectation is that the original component DOES re-render on shallow compares of state then this is a bug. Also this could just be a usability thing that the developer would have to be aware of when using pure with compose.

I'm only bringing this to your attention in "case" its not something you're already aware of as it could lead to some unexpected wrapped component behaviour if you're not.

How has this been tested?

• see unit test demonstrating the potential concern.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.