Weakifying and strongifying block variables in Objective-C

When using blocks (or closures) in Objective-C, it is often necessary to make a variable reference weak so as not to introduce a memory cycle with the variables that are used in the lexical scope of the block. As far as I know, libextobjc (which is now archived) is the library that popularized the terms "weakify" and "strongify" for referencing a weakly-held pointer to an object outside of the block, then capturing it strongly again inside.

Here is how you do it without using libextobjc:

typeof(self) __weak weakSelf = self;
someClosure = ^(NSString * _Nonnull someVariable) {
    typeof(weakSelf) __strong strongSelf = weakSelf;
    [strongSelf doSomethingWith:someVariable];
};

__weak and __strong are called Ownership Qualifiers. These tell ARC how to treat the variables to not introduce a reference cycle. This can't be automated, thus the need to specify ownership based on the usage.

typeof(self) produces the type of self but is a nonstandard C extension. You may also see __typeof(self) or even __typeof__(self). The extension is discussed in the GCC documentation here.

Hopefully this small tip helps you write better code!