Objective-C Declare vars with ({ … })

This is a GNU (non-standard) C language extension called a “statement expression”. The syntax is supported by gcc, clang, and several other compilers.

Basically, it lets you treat an arbitrary block as a single expression, whose value is the value of the last statement in the block.

This extension is mostly useful is macro definitions. In my opinion, the code you quoted in your question (from the showFromRect:inView: method in REMenu.m) would be better if it did not use statement expressions. Instead, the code in those statement expressions should be factored out into separate methods. For example:

    self.menuWrapperView = [self newMenuWrapperView];
    self.toolbar = [self newToolbar];

...

- (UIView *)newMenuWrapperView {
    UIView *view = [[UIView alloc] init];
    view.autoresizingMask = UIViewAutoresizingFlexibleWidth;
    if (!self.liveBlur || !REUIKitIsFlatMode()) {
        view.layer.shadowColor = self.shadowColor.CGColor;
        view.layer.shadowOffset = self.shadowOffset;
        view.layer.shadowOpacity = self.shadowOpacity;
        view.layer.shadowRadius = self.shadowRadius;
        view.layer.shouldRasterize = YES;
        view.layer.rasterizationScale = [UIScreen mainScreen].scale;
    }
    return view;
}

- (UIToolbar *)newToolbar {
    UIToolbar *toolbar = [[UIToolbar alloc] init];
    toolbar.barStyle = self.liveBlurBackgroundStyle;
    if ([toolbar respondsToSelector:@selector(setBarTintColor:)])
        [toolbar performSelector:@selector(setBarTintColor:) withObject:self.liveBlurTintColor];
    toolbar.autoresizingMask = UIViewAutoresizingFlexibleWidth;
    return toolbar;
}