objective-c+中代码块（blocks）

原文地址:http://blog.sina.com.cn/s/blog_67419c420100vl8b.html

<wbr>在ios4之后，引入了代码块的特性，在gcd中会经常的用到，所以决定好好的看看代码块文档，把这块总结一下。从头开始讲解代码块。 </wbr>

1.声明和使用代码块

一般用^操作符声明一个块变量，并作为块的开始符。而块的本身用{}包括起来，就像下面那样。

int multiplier = 7;

int (^myBlock)(int) = ^(int num) {

return num * multiplier;

};

下面的图是详细的讲解：

<wbr><wbr><wbr>其实意思就是前半句声明了一个名字为myBlock的代码块，有一个int类型的参数，并返回一个int类型的值；后面的半句就是一个块的定义，然后赋值给myBlock。</wbr></wbr></wbr>

<wbr><wbr><wbr>如果我们像上面那样，声明一个块像一个变量一样，我们就可以像使用函数一样使用它，如下：</wbr></wbr></wbr>

int multiplier = 7;

int (^myBlock)(int) = ^(int num) {

return num * multiplier;

};

printf("%d", myBlock(3));

// prints "21"

2.直接使用block

在大多数情况下，我们不需要去声明一个块变量，我们直接写一个简单的代码块作为参数传递就行。下面的代码函数qsort_b的第三个参数就是一个代码块。

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" }

<wbr><wbr>一些cocoa frameworks的方法采用一个block作为一个参数，典型的是对一个集合对象进行操作，或者是在一个操作完成之后使用回调。下面的例子是NSArray类的方法<span style="font-family:Courier,Consolas,monospace; font-size:13px; color:#666666; line-height:normal"><a href="http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/NSArray.html#//apple_ref/occ/instm/NSArray/sortedArrayUsingComparator:" target="_self">sortedArrayUsingComparat<wbr>or:</wbr></a></span>怎样使用一个block。此方法使用一个block作为一个参数。</wbr></wbr>

NSArray *stringsArray = [NSArray arrayWithObjects:

@"string 1",

@"String 21",

@"string 12",

@"String 11",

@"String 02", nil];

static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |

NSWidthInsensitiveSearch<wbr> | NSForcedOrderingSearch;</wbr>

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 = [stringsArraysortedArrayUsingComparat<wbr>or:finderSortBlock];</wbr>

NSLog(@"finderSortArray: %@", finderSortArray);

block的一个强大的功能是可以修改同一作用雨的变量，我们只需要在变量的前面加上一个_block标识符。下面的例子和上面的相同，只是添加功能用于记录相同元素的个数。

NSArray *stringsArray = [NSArray arrayWithObjects:

@"string 1",

@"String 21", // <-

@"string 12",

@"String 11",

@"Strîng 21", // <-

@"Striñg 21", // <-

@"String 02", nil];

NSLocale *currentLocale = [NSLocale currentLocale];

__block NSUInteger orderedSameCount = 0;

NSArray *diacriticInsensitiveSort<wbr>Array = [stringsArray sortedArrayUsingComparat<wbr>or:^(id string1, id string2) {</wbr></wbr>

NSRange string1Range = NSMakeRange(0, [string1 length]);

NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSe<wbr>arch range:string1Range locale:currentLocale];</wbr>

if (comparisonResult == NSOrderedSame) {

orderedSameCount++;

}

return comparisonResult;

}];

NSLog(@"diacriticInsensitiveSort<wbr>Array: %@", diacriticInsensitiveSort<wbr>Array);</wbr></wbr>

NSLog(@"orderedSameCount: %d", orderedSameCount);

3.block变量的声明

block的声明和函数指针差不多，只是把*改为了^

void (^blockReturningVoidWithVo<wbr>idArgument)(void);</wbr>

int (^blockReturningIntWithInt<wbr>AndCharArguments)(int, char);</wbr>

void (^arrayOfTenBlocksReturnin<wbr>gVoidWithIntArgument[10])(int);</wbr>

也可以使用typedef去声明block，方便以后使用，如下：

typedef float (^MyBlockType)(float, float);

MyBlockType myFirstBlock = // ... ;

MyBlockType mySecondBlock = // ... ;

4.变量的作用域对于其在块中的影响：

_block int x = 123; // x lives in block storage

void (^printXAndY)(int) = ^(int y) {

x = x + y;

printf("%d %d\n", x, y);

};

printXAndY(456); // prints: 579 456

// x is now 579

extern NSInteger CounterGlobal;

static NSInteger CounterStatic;

{

NSInteger localCounter = 42;

__block char localCharacter;

void (^aBlock)(void) = ^(void) {

++CounterGlobal;

++CounterStatic;

CounterGlobal = localCounter; // localCounter fixed at block creation

localCharacter = 'a'; // sets localCharacter in enclosing scope

};

++localCounter; // unseen by the block

localCharacter = 'b';

aBlock(); // execute the block

// localCharacter now 'a'

}

5.使用blocks

（1）调用一个声明好的block

int (^oneFrom)(int) = ^(int anInt) {

return anInt - 1;

};

printf("1 from 10 is %d", oneFrom(10));

// Prints "1 from 10 is 9"

float (^distanceTraveled) (float, float, float) =

^(float startingSpeed, float acceleration, float time) {

float distance = (startingSpeed * time) + (0.5 * acceleration * time * time);

return distance;

};

float howFar = distanceTraveled(0.0, 9.8, 1.0);

// howFar = 4.9