objC的block是基于c语言开发，可以在c，c＋＋和objc中使用。
声明和使用一个block：
block中会用到一个特殊的操作符^。整个block体都放在｛｝中。
例如：
1 int multiplier = 7;
2 int (^myBlock)(int) = ^(int num) {
3 return num * multiplier;
2 int (^myBlock)(int) = ^(int num) {
3 return num * multiplier;
4 };
借着这张E文的图，我们可以看到定义一个block是多么的简单。
从左到右依次是：myBlock要返回的类型，这里是int。myBlock就是我们block的名字，而^操作符表明此变量是个block。之后则是这个block需要输入的参数的类型。
赋值号后面的是这个block需要的参数，这里是num。大扩号中的是block体，也就是会执行到的代码块。我们看到，最后会返回一个整型值和定义的一样。
定义一个bock就是这么简单。在之后的代码中就可以使用这个block变量，就像使用一个函数一样。看例子：
1 int multiplier = 7;
2 int (^myBlock)(int) = ^(int num) {
3 return num * multiplier;
4 };
5
6 printf("%d", myBlock(3));
2 int (^myBlock)(int) = ^(int num) {
3 return num * multiplier;
4 };
5
6 printf("%d", myBlock(3));
7 // prints "21"
直接使用一个bock：
很多时候就不需要定义block变量了。可以直接使用上例中赋值号右边的block块。我们来看这个例子：
1 char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
2
3 qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
4 char *left = *(char **)l;
5 char *right = *(char **)r;
6 return strncmp(left, right, 1);
7 });
8
2
3 qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
4 char *left = *(char **)l;
5 char *right = *(char **)r;
6 return strncmp(left, right, 1);
7 });
8
9 // myCharacters is now { "Charles Condomine", "George", "TomJohn" }
在这里例子中，block块直接当作函数实参的一部分传入了函数中。
在Cocoa中使用到的block：
这里不多说。直接上例子：
1 NSArray *stringsArray = [NSArray arrayWithObjects:
2 @"string 1",
3 @"String 21",
4 @"string 12",
5 @"String 11",
6 @"String 02", nil];
7
8 static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
9 NSWidthInsensitiveSearch | NSForcedOrderingSearch;
10 NSLocale *currentLocale = [NSLocale currentLocale];
11
12 NSComparator finderSortBlock = ^(id string1, id string2) {
13
14 NSRange string1Range = NSMakeRange(0, [string1 length]);
15 return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
16 };
17
18 NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
19 NSLog(@"finderSortArray: %@", finderSortArray);
20
21 /*
22 Output:
23 finderSortArray: (
24 "string 1",
25 "String 02",
26 "String 11",
27 "string 12",
28 "String 21"
29 )
2 @"string 1",
3 @"String 21",
4 @"string 12",
5 @"String 11",
6 @"String 02", nil];
7
8 static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
9 NSWidthInsensitiveSearch | NSForcedOrderingSearch;
10 NSLocale *currentLocale = [NSLocale currentLocale];
11
12 NSComparator finderSortBlock = ^(id string1, id string2) {
13
14 NSRange string1Range = NSMakeRange(0, [string1 length]);
15 return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
16 };
17
18 NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
19 NSLog(@"finderSortArray: %@", finderSortArray);
20
21 /*
22 Output:
23 finderSortArray: (
24 "string 1",
25 "String 02",
26 "String 11",
27 "string 12",
28 "String 21"
29 )
30 */
__block变量：
block一个牛X的特性就是，block可以修改在同一个lexical scope（直译就是词法范围，好吧，我承认其他译法目前还没发现。等下上例子大家体会吧）中的__block变量。这些变量必须用__block修饰。看清楚block的前面实两道下划线！例子如下：
1 NSArray *stringsArray = [NSArray arrayWithObjects:
2 @"string 1",
3 @"String 21", // <-
4 @"string 12",
5 @"String 11",
6 @"Strîng 21", // <-
7 @"Striñg 21", // <-
8 @"String 02", nil];
9
10 NSLocale *currentLocale = [NSLocale currentLocale];
11 __block NSUInteger orderedSameCount = 0;
12
13 NSArray *diacriticInsensitiveSortArray = [stringsArray sortedArrayUsingComparator:^(id string1, id string2) {
14
15 NSRange string1Range = NSMakeRange(0, [string1 length]);
16 NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSearch range:string1Range locale:currentLocale];
17
18 if (comparisonResult == NSOrderedSame) {
19 orderedSameCount++;
20 }
21 return comparisonResult;
22 }];
23
24 NSLog(@"diacriticInsensitiveSortArray: %@", diacriticInsensitiveSortArray);
25 NSLog(@"orderedSameCount: %d", orderedSameCount);
26
27 /*
28 Output:
29
30 diacriticInsensitiveSortArray: (
31 "String 02",
32 "string 1",
33 "String 11",
34 "string 12",
35 "String 21",
36 "Str\U00eeng 21",
37 "Stri\U00f1g 21"
38 )
39 orderedSameCount: 2
2 @"string 1",
3 @"String 21", // <-
4 @"string 12",
5 @"String 11",
6 @"Strîng 21", // <-
7 @"Striñg 21", // <-
8 @"String 02", nil];
9
10 NSLocale *currentLocale = [NSLocale currentLocale];
11 __block NSUInteger orderedSameCount = 0;
12
13 NSArray *diacriticInsensitiveSortArray = [stringsArray sortedArrayUsingComparator:^(id string1, id string2) {
14
15 NSRange string1Range = NSMakeRange(0, [string1 length]);
16 NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSearch range:string1Range locale:currentLocale];
17
18 if (comparisonResult == NSOrderedSame) {
19 orderedSameCount++;
20 }
21 return comparisonResult;
22 }];
23
24 NSLog(@"diacriticInsensitiveSortArray: %@", diacriticInsensitiveSortArray);
25 NSLog(@"orderedSameCount: %d", orderedSameCount);
26
27 /*
28 Output:
29
30 diacriticInsensitiveSortArray: (
31 "String 02",
32 "string 1",
33 "String 11",
34 "string 12",
35 "String 21",
36 "Str\U00eeng 21",
37 "Stri\U00f1g 21"
38 )
39 orderedSameCount: 2
40 */
大家看到变量orderedSameCount在后面的变量中做了自增操作。