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https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Blocks/Articles/bxOverview.html#//apple_ref/doc/uid/TP40007502-CH3-SW1
Block objects provide a way for you to create an ad hoc function body as an expression in C, and C-derived languages such as Objective-C and C++. In other languages and environments, a block object is sometimes also called a “closure”. Here, they are typically referred to colloquially as “blocks”, unless there is scope for confusion with the standard C term for a block of code.
A block is an anonymous inline collection of code that:
You can copy a block and even pass it to other threads for deferred execution (or, within its own thread, to a runloop). The compiler and runtime arrange that all variables referenced from the block are preserved for the life of all copies of the block. Although blocks are available to pure C and C++, a block is also always an Objective-C object.
Blocks represent typically small, self-contained pieces of code. As such, they’re particularly useful as a means of encapsulating units of work that may be executed concurrently, or over items in a collection, or as a callback when another operation has finished.
Blocks are a useful alternative to traditional callback functions for two main reasons:
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