Struct Block Copy item path

An opaque type that holds an Objective-C block.

The generic type F must be a dyn Fn that implements the BlockFn trait (which means parameter and return types must be “encodable”), and describes the parameter and return types of the block.

For example, you may have the type Block<dyn Fn(u8, u8) -> i32>, and that would be a 'static block that takes two u8s, and returns an i32.

If you want the block to carry a lifetime, use Block<dyn Fn() + 'a>, just like you’d usually do with dyn Fn.

§Memory layout

This is intended to be an extern type, and as such the memory layout of this type is not guaranteed. That said, pointers to this type are always thin, and match that of Objective-C blocks. So the layout of e.g. &Block<dyn Fn(...) -> ... + '_> is defined, and guaranteed to be pointer-sized and ABI-compatible with a block pointer.

§Safety invariant

Calling this potentially invokes foreign code, so you must verify, when creating a reference to this, or returning it from an external API, that it doesn’t violate any of Rust’s safety rules.

In particular, blocks are sharable with multiple references (see e.g. Block::copy), so the caller must ensure that calling it can never cause a data race. This usually means you’ll have to use some form of interior mutability, if you need to mutate something from inside a block.