Objective-C声明变量（{...}）

Question

我正在寻找REMenu lib的代码，并看到一个变量声明为（{...}）; ..看起来像'封闭'懒惰评估代码..我不知道..有人可以解释我？

self.menuWrapperView = ({ 
     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; 
     } 
     view; 
    }); 

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

Answer 1

这是称为“statement expression”的GNU（非标准）C语言扩展。该语法由gcc，clang和several other compilers支持。

基本上，它允许您将任意块​​视为单个表达式，其值是该块中最后一条语句的值。

此扩展功能最为有用的是宏定义。在我看来，你在你的问题中引用的代码（来自REMenu.m中的showFromRect:inView:方法）会更好，如果它使用语句表达式而不是。相反，这些语句表达式中的代码应该分解成单独的方法。例如：

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; 
}