像 Block 这种闭包，就是怎么用怎么爽，怎么用怎么喜欢的编码方式了。
闭包很伟大，也是各个语言都实现了的基础语法。我这边对闭包的理解，就是：内部函数持有外部变量。
但是各个语言都有需要注意的点，如 iOS 里面的循环引用，Swift 里面的逃逸闭包，Python 里面的闭包变量延迟定义等。
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| As a local variable:
returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};
As a property:
@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);
As a method parameter:
- (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName;
As an argument to a method call:
[someObject someMethodThatTakesABlock:^returnType (parameters) {...}];
As a typedef:
typedef returnType (^TypeName)(parameterTypes);
TypeName blockName = ^returnType(parameters) {...};
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