Conceptual Overview（概览）
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.
block对象给你提供了创建C语言和C语言拓展语言，比如objective-C和C++中的 ad hoc函数的可能。在其他语言环境中，block对象有时也被称作“closure”，这里呢，通常用大白话称为：“block”，除非作用域中有复杂的标准C block代码。
Block Functionality（block 功能）
A block is an anonymous inline collection of code that:
Has a typed argument list just like a function
Has an inferred or declared return type
Can capture state from the lexical scope within which it is defined
Can optionally modify the state of the lexical scope
Can share the potential for modification with other blocks defined within the same lexical scope
Can continue to share and modify state defined within the lexical scope (the stack frame) after the lexical scope (the stack frame) has been destroyed
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.
一个block是一个匿名内联代码集：
- 像函数一样有一个类型参数列表。
- 有一种推断或声明的返回值类型
- 可以在它定义的词法范围内捕获状态
- 可以随意的在词法作用域里面修改状态
- 可以和相同作用域的block分享修改
- 可以在词法作用域（栈）被销毁后继续分享和修改词法作用域中的状态。
Usage（使用）
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:
-
They allow you to write code at the point of invocation that is executed later in the context of the method implementation.
Blocks are thus often parameters of framework methods.
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They allow access to local variables.
Rather than using callbacks requiring a data structure that embodies all the contextual information you need to perform an operation, you simply access local variables directly.
- 他们允许你在调用点（函数内容已经结束啦）之后写代码。因此呢block常常作为框架函数的参数出现。
- 他们允许你访问本地变量。相比于回调函数要求操作的所有上下文信息，block只需要直接访问本地变量就行了。