(譯文)IOS block編程指南 4 聲明和建立blocks

 

Declaring and Creating Blocks (聲明和建立blocks)

Declaring a Block Reference （聲明一個block引用）

Block variables hold references to blocks. You declare them using syntax similar to that you use to declare a pointer to a function, except that you use ^ instead of *. The block type fully interoperates with the rest of the C type system. The following are all valid block variable declarations:

block變量維持了一個對block的引用。你聲明block使用了和聲明函數指針相同的語法，除了你使用「^」代替了「*」以外。block類型能夠和所有C類型系統相互操做。下列都是block變量的聲明：

void (^blockReturningVoidWithVoidArgument)(void);
int (^blockReturningIntWithIntAndCharArguments)(int, char);
void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);

 

Blocks also support variadic (...) arguments. A block that takes no arguments must specify void in the argument list.

Blocks are designed to be fully type safe by giving the compiler a full set of metadata to use to validate use of blocks, parameters passed to blocks, and assignment of the return value. You can cast a block reference to a pointer of arbitrary type and vice versa. You cannot, however, dereference a block reference via the pointer dereference operator (*)—thus a block's size cannot be computed at compile time.

You can also create types for blocks—doing so is generally considered to be best practice when you use a block with a given signature in multiple places:

blocks  也支持可變參數。一個沒有參數的block必須在參數列表中指定void。

blocks 被設計成對編譯器的徹底安全類型，它有一套完整的數據源設置來檢測block的合法性，經過傳給blocks參數，來分配返回值。你能夠給block建立任意的指針類型，反之亦然（PS：這句話，翻譯有疑問）。儘管如此，你不能經過解引用操做符（*）來解引用一個block——由於這樣在編譯的時候沒法計算block的大小。

你也能夠建立block做爲類型，當你要在多個地方使用同一block簽名的block的時候，這是一般狀況下最好的方法。

typedef float (^MyBlockType)(float, float);
 
MyBlockType myFirstBlock = // ... ;
MyBlockType mySecondBlock = // ... ;

 

Creating a Block（建立一個block）

You use the ^ operator to indicate the beginning of a block literal expression. It may be followed by an argument list contained within (). The body of the block is contained within {}. The following example defines a simple block and assigns it to a previously declared variable (oneFrom)—here the block is followed by the normal ; that ends a C statement.

你使用^操做指示一個block表達的開始。也許還會有一個（）包裹的參數列表。block的主體包含在{}中。下面的例子定義了一個簡單的block分配給一個已經存在的變量（oneFrom）——這裏的block是正常的，以C語言作結。

float (^oneFrom)(float);
 
oneFrom = ^(float aFloat) {
    float result = aFloat - 1.0;
    return result;
};

 

If you don’t explicitly declare the return value of a block expression, it can be automatically inferred from the contents of the block. If the return type is inferred and the parameter list is void, then you can omit the (void) parameter list as well. If or when multiple return statements are present, they must exactly match (using casting if necessary).

若是你沒有明確的聲明block表達式的返回值，系統能夠根據block的內容推斷。若是返回類型推斷好，且參數列表爲空，而後你也能夠忽略參數列表。若是須要不少返回值，他們須要寄去匹配（若是必要能夠進行類型轉換）。

Global Blocks（全局block）

At a file level, you can use a block as a global literal:  

在文件層面，你能夠把block做爲全局變量。

#import <stdio.h>
 
int GlobalInt = 0;
int (^getGlobalInt)(void) = ^{ return GlobalInt; };

 

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本文原創，轉載請註明出處：http://blog.csdn.net/zhenggaoxing/article/details/44308855