Declaring and Using a Block (定义和使用block)
You use the^operator to declare a block variable and to indicate the beginning of a block literal. The body of the block itself is contained within{}, as shown in this example (as usual with C,;indicates the end of the statement):
你使用^操作定义一个block值同时表示了一个block的开始。block的主题被{}包围，就像例子中展示的那样（就像通常的C中，分号；申明了一个语句的结束）：
int multiplier = 7;
int (^myBlock)(int) = ^(int num) {
return num * multiplier;
};
The example is explained in the following illustration:
这个例子在下面的说面中得到了解释：
Notice that the block is able to make use of variables from the same scope in which it was defined.
If you declare a block as a variable, you can then use it just as you would a function:
注意block可以使用定义在block定义作用用中的变量。
如果你把block定义成了一个变量，你可以像使用函数一样使用block。
int multiplier = 7;
int (^myBlock)(int) = ^(int num) {
return num * multiplier;
};
printf("%d", myBlock(3));
// prints "21"
Using a Block Directly（直接使用block）
In many cases, you don’t need to declare block variables; instead you simply write a block literal inline where it’s required as an argument. The following example uses theqsort_bfunction.qsort_bis similar to the standardqsort_rfunction, but takes a block as its final argument.
在很多情况下你不需要把block定义成一个变量；相反的你可以直接写出一个block作为参数。下面的例子用到了qsort_b函数，qsort_b很类似标准的qsort_r函数，是指使用一个block作为一最后一个参数。
char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
char *left = *(char **)l;
char *right = *(char **)r;
return strncmp(left, right, 1);
});
// myCharacters is now { "Charles Condomine", "George", "TomJohn" }
Blocks with Cocoa（cocoa中的block）
Several methods in theCocoaframeworkstake a block as an argument, typically either to perform an operation on a collection of objects, or to use as a callback after an operation has finished. The following example shows how to use a block with theNSArraymethod. The method takes a single argument—the block. For illustration, in this case the block is defined as anlocal variable:
Cocoa 框架中有一些方法使用block作为一个参数，特别是对多个对象进行操作，，或者在一个操作结束之后回调。下面的例子展示了NSArray怎样在方法中使用block。这个方法使用了一个参数——block。为了说明，这个block被定义为类型的一个局部变量：
NSArray *stringsArray = @[ @"string 1",
@"String 21",
@"string 12",
@"String 11",
@"String 02" ];
static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSComparator finderSortBlock = ^(id string1, id string2) {
NSRange string1Range = NSMakeRange(0, [string1 length]);
return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
};
NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
NSLog(@"finderSortArray: %@", finderSortArray);
/*
Output:
finderSortArray: (
"string 1",
"String 02",
"String 11",
"string 12",
"String 21"
)
*/
__block Variables（_block 类型变量）
A powerful feature of blocks is that they can modify variables in the same lexical scope. You signal that a block can modify a variable using the__blockstorage type modifier. Adapting the example shown inBlocks with Cocoa, you could use a block variable to count how many strings are compared as equal as shown in the following example. For illustration, in this case the block is used directly and usescurrentLocaleas a read-only variable within the block:
block的另一个强大的特性是可以修改同一词法范围的变量。你可以把一个block想修改的变量声明成_block类型。改写Blocks with Cocoa中的例子，你可以在下面的例子中使用block变量统计有多少字符串是相同的。为了说明需要，例子中直接使用block，同时把currentLocate作为一个block中的只读变量。
NSArray *stringsArray = @[ @"string 1",
@"String 21", // <-
@"string 12",
@"String 11",
@"Strng 21", // <-
@"Strig 21", // <-
@"String 02" ];
NSLocale *currentLocale = [NSLocale currentLocale];
__block NSUInteger orderedSameCount = 0;
NSArray *diacriticInsensitiveSortArray = [stringsArray sortedArrayUsingComparator:^(id string1, id string2) {
NSRange string1Range = NSMakeRange(0, [string1 length]);
NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSearch range:string1Range locale:currentLocale];
if (comparisonResult == NSOrderedSame) {
orderedSameCount++;
}
return comparisonResult;
}];
NSLog(@"diacriticInsensitiveSortArray: %@", diacriticInsensitiveSortArray);
NSLog(@"orderedSameCount: %d", orderedSameCount);
/*
Output:
diacriticInsensitiveSortArray: (
"String 02",
"string 1",
"String 11",
"string 12",
"String 21",
"Str\U00eeng 21",
"Stri\U00f1g 21"
)
orderedSameCount: 2
*/
This is discussed in greater detail inBlocks and Variables.
更详细的讨论请看：Blocks and Variables.
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