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;
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int (^myBlock)(int) = ^(int num) {
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return num * multiplier;
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};
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int（^myblocks）(int)=(^int num){return num * multiplier};
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。
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：
int multiplier = 7;
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int (^myBlock)(int) = ^(int num) {
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return num * multiplier;
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};
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printf("%d", myBlock(3));
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// prints "21"
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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" };
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qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
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char *left = *(char **)l;
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char *right = *(char **)r;
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return strncmp(left, right, 1);
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});
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// myCharacters is now { "Charles Condomine", "George", "TomJohn" }
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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
sortedArrayUsingComparator:怎样使用一个block。此方法使用一个block作为一个参数。
NSArray *stringsArray = [NSArray arrayWithObjects:
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@"string 1",
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@"String 21",
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@"string 12",
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@"String 11",
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@"String 02", nil];
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static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
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NSWidthInsensitiveSearch | NSForcedOrderingSearch;
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NSLocale *currentLocale = [NSLocale currentLocale];
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NSComparator finderSortBlock = ^(id string1, id string2) {
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NSRange string1Range = NSMakeRange(0, [string1 length]);
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return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
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};
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NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
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NSLog(@"finderSortArray: %@", finderSortArray);
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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:
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@"string 1",
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@"String 21", // <-
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@"string 12",
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@"String 11",
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@"Strîng 21", // <-
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@"Striñg 21", // <-
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@"String 02", nil];
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NSLocale *currentLocale = [NSLocale currentLocale];
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__block NSUInteger orderedSameCount = 0;
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NSArray *diacriticInsensitiveSortArray = [stringsArray sortedArrayUsingComparator:^(id string1, id string2) {
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NSRange string1Range = NSMakeRange(0, [string1 length]);
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NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSearch range:string1Range locale:currentLocale];
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if (comparisonResult == NSOrderedSame) {
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orderedSameCount++;
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}
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return comparisonResult;
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}];
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NSLog(@"diacriticInsensitiveSortArray: %@", diacriticInsensitiveSortArray);
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NSLog(@"orderedSameCount: %d", orderedSameCount);
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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 (^blockReturningVoidWithVoidArgument)(void);
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int (^blockReturningIntWithIntAndCharArguments)(int, char);
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void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);
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也可以使用typedef去声明block，方便以后使用，如下：
typedef float (^MyBlockType)(float, float);
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MyBlockType myFirstBlock = // ... ;
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MyBlockType mySecondBlock = // ... ;
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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
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void (^printXAndY)(int) = ^(int y) {
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x = x + y;
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printf("%d %d\n", x, y);
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};
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printXAndY(456); // prints: 579 456
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// x is now 579
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extern NSInteger CounterGlobal;
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static NSInteger CounterStatic;
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{
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NSInteger localCounter = 42;
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__block char localCharacter;
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void (^aBlock)(void) = ^(void) {
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++CounterGlobal;
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++CounterStatic;
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CounterGlobal = localCounter; // localCounter fixed at block creation
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localCharacter = ‘a‘; // sets localCharacter in enclosing scope
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};
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++localCounter; // unseen by the block
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localCharacter = ‘b‘;
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aBlock(); // execute the block
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// localCharacter now ‘a‘
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}
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5.使用blocks
（1）调用一个声明好的block
int (^oneFrom)(int) = ^(int anInt) {
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return anInt - 1;
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};
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printf("1 from 10 is %d", oneFrom(10));
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// Prints "1 from 10 is 9"
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float (^distanceTraveled) (float, float, float) =
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^(float startingSpeed, float acceleration, float time) {
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float distance = (startingSpeed * time) + (0.5 * acceleration * time * time);
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return distance;
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};
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float howFar = distanceTraveled(0.0, 9.8, 1.0);
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// howFar = 4.9
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