GCD实例

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转自http://blog.csdn.net/jjunjoe/article/details/8743434，学习了，感谢

示例源码清单如下：

1、SingletonSample.h

// SingletonSample.h

// BlockSample

// Created by developer on 13-9-27.

#import <Foundation/Foundation.h>

@interface SingletonSample : NSObject

+ (id)ShareInstance;

@end

2、SingletonSample.m

// SingletonSample.m

// BlockSample

// Created by developer on 13-9-27.

#import "SingletonSample.h"

@implementation SingletonSample

+ (id)ShareInstance{

static SingletonSample *SharedInstance = nil;

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

SharedInstance = [[SingletonSample alloc] init];

});

return SharedInstance;

}

@end

3、BlockSampleTests.h

// BlockSampleTests.h

// BlockSampleTests

// Created by developer on 13-8-3.

#import <SenTestingKit/SenTestingKit.h>

@interface BlockSampleTests : SenTestCase

@end

4、BlockSampleTests.m

// BlockSampleTests.m

// BlockSampleTests

// Created by developer on 13-8-3.

#import "SingletonSample.h"

#import "BlockSampleTests.h"

static int global = 100;

static volatile BOOL flag = NO;

static const int Length = 100;

static int data[Length];

static void initData()

{

for(int i = 0; i < Length; i++)

data[i] = i + 1;

}

@implementation BlockSampleTests

- (void)setUp

{

[super setUp];

// Set-up code here.

}

- (void)tearDown

{

// Tear-down code here.

[super tearDown];

}

- (void)test1

{

// 默认打印一条语句"Hello, World!"

NSLog(@"Hello, World!");

}

- (void)testBlock1

{

// block 打印一条语句"Hello, World!"

void (^aBlock)(void) = ^(void){ NSLog(@"Hello, World!"); };

aBlock();

}

- (void)testBlock2

{

// block 打印一条语句"Hello, World!"

void (^aBlock)(void) = 0;

aBlock = ^(void) {

NSLog(@"Hello, World!");

};

aBlock();

}

- (void)testBlockArray

{

// block 数组

void (^blocks[2])(void) = {

^(void){ NSLog(@" >> This is block 1!"); },

^(void){ NSLog(@" >> This is block 2!"); }

};

blocks[0]();

blocks[1]();

}

- (void)testBlock4

{

block 是分配在 stack 上的，这意味着我们必须小心里处理 block 的生命周期。

比如如下的做法是不对的，因为 stack 分配的 block 在 if 或 else 内是有效的，但是到大括号 } 退出时就可能无效了：

dispatch_block_t block;

BOOL x = 1;

if (x) {

block = ^{ printf("true\n"); };

} else {

block = ^{ printf("false\n"); };

}

block();

}

- (void)testBlock5

{

考虑到 block 的目的是为了支持并行编程，对于普通的 local 变量，我们就不能在 block 里面随意修改（原因很简单，block 可以被多个线程并行运行，会有问题的）

而且如果你在 block 中修改普通的 local 变量，编译器也会报错。

那么该如何修改外部变量呢？有两种办法:

第一种是可以修改 static 全局变量；

第二种是可以修改用新关键字 __block 修饰的变量。

__block int blockLocal = 100;

static int staticLocal = 100;

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

NSLog(@" >> Sum: %d\n", global + staticLocal);

global++;

blockLocal++;

staticLocal++;

};

aBlock();

NSLog(@"After modified, global: %d, block local: %d, static local: %d\n", global, blockLocal, staticLocal);

}

- (void)testBlock6

{

我们也可以引用 static block 或 __block block。比如我们可以用他们来实现 block 递归

void (^aBlock)(int) = 0;

static void (^ const staticBlock)(int) = ^(int i) {

if (i > 0) {

NSLog(@" >> static %d", i);

staticBlock(i - 1);

}

};

aBlock = staticBlock;

aBlock(5);

// 2

__block void (^blockBlock)(int);

blockBlock = ^(int i) {

if (i > 0) {

NSLog(@" >> block %d", i);

blockBlock(i - 1);

}

};

blockBlock(5);

}

- (void)testBlock7

{

上面我们介绍了 block 及其基本用法，但还没有涉及并行编程。

block 与 Dispatch Queue 分发队列结合起来使用，是 iOS 中并行编程的利器。

// create dispatch queue

dispatch_queue_t queue = dispatch_queue_create("StudyBlocks",NULL);

dispatch_async(queue, ^(void) {

int sum = 0;

for(int i = 0; i < Length; i++)

sum += data[i];

NSLog(@" >> Sum: %d", sum);

flag = YES;

});

// wait util work is done.

while (!flag);

dispatch_release(queue);

}

- (void)testBlock8

{

在上面的例子中，我们的主线程一直在轮询 flag 以便知晓 block 线程是否执行完毕，这样做的效率是很低的，严重浪费 CPU 资源。

我们可以使用一些通信机制来解决这个问题，如：semaphore（信号量）。

semaphore 的原理很简单，就是生产-消费模式，必须生产一些资源才能消费，没有资源的时候，那我就啥也不干，直到资源就绪。

initData();

// Create a semaphore with 0 resource

__block dispatch_semaphore_t sem = dispatch_semaphore_create(0);

// create dispatch semaphore

dispatch_queue_t queue = dispatch_queue_create("StudyBlocks",NULL);

dispatch_async(queue, ^(void) {

int sum = 0;

for(int i = 0; i < Length; i++)

sum += data[i];

NSLog(@" >> Sum: %d", sum);

// signal the semaphore: add 1 resource

dispatch_semaphore_signal(sem);

});

// wait for the semaphore: wait until resource is ready.

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

dispatch_release(sem);

dispatch_release(queue);

}

- (void)testBlock9

{

下面我们来看一个按照 FIFO 顺序执行并用 semaphore 同步的例子：先将数组求和再依次减去数组。

initData();

__block int sum = 0;

// Create a semaphore with 0 resource

__block dispatch_semaphore_t sem = dispatch_semaphore_create(0);

__block dispatch_semaphore_t taskSem = dispatch_semaphore_create(0);

// create dispatch semaphore

dispatch_queue_t queue = dispatch_queue_create("StudyBlocks",NULL);

dispatch_block_t task1 = ^(void) {

int s = 0;

for (int i = 0; i < Length; i++)

s += data[i];

sum = s;

NSLog(@" >> after add: %d", sum);

dispatch_semaphore_signal(taskSem);

};

dispatch_block_t task2 = ^(void) {

dispatch_semaphore_wait(taskSem, DISPATCH_TIME_FOREVER);

int s = sum;

for (int i = 0; i < Length; i++)

s -= data[i];

sum = s;

NSLog(@" >> after subtract: %d", sum);

dispatch_semaphore_signal(sem);

};

dispatch_async(queue, task1);

dispatch_async(queue, task2);

// wait for the semaphore: wait until resource is ready.

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

dispatch_release(taskSem);

dispatch_release(sem);

dispatch_release(queue);

在上面的代码中，我们利用了 dispatch_queue 的 FIFO 特性，

确保 task1 先于 task2 执行，而 task2 必须等待直到 task1 执行完毕才开始干正事，主线程又必须等待 task2 才能干正事。

这样我们就可以保证先求和，再相减，然后再让主线程运行结束这个顺序。

}

- (void)testBlock10

{

使用 dispatch_apply 进行并发迭代：

对于上面的求和操作，我们也可以使用 dispatch_apply 来简化代码的编写：

initData();

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

__block int sum = 0;

__block int *pArray = data;

// iterations

dispatch_apply(Length, queue, ^(size_t i) {

sum += pArray[i];

});

NSLog(@" >> sum: %d", sum);

dispatch_release(queue);

注意这里使用了全局 dispatch_queue。

dispatch_apply 的定义如下：

dispatch_apply(size_t iterations, dispatch_queue_t queue, void (^block)(size_t));

参数 iterations 表示迭代的次数，void (^block)(size_t) 是 block 循环体。这么做与 for 循环相比有什么好处呢？

答案是：并行，这里的求和是并行的，并不是按照顺序依次执行求和的。

}

- (void)testBlock11

{

我们可以将完成一组相关任务的 block 添加到一个 dispatch group 中去，这样可以在 group 中所有 block 任务都完成之后，再做其他事情。

比如 testBlock9 中的示例也可以使用 dispatch group 实现

initData();

__block int sum = 0;

// Create a semaphore with 0 resource

__block dispatch_semaphore_t taskSem = dispatch_semaphore_create(0);

// create dispatch semaphore

dispatch_group_t group = dispatch_group_create();

dispatch_queue_t queue = dispatch_queue_create("StudyBlocks",NULL);

dispatch_block_t task1 = ^(void) {

int s = 0;

for (int i = 0; i < Length; i++)

s += data[i];

sum = s;

NSLog(@" >> after add: %d", sum);

dispatch_semaphore_signal(taskSem);

};

dispatch_block_t task2 = ^(void) {

dispatch_semaphore_wait(taskSem, DISPATCH_TIME_FOREVER);

int s = sum;

for (int i = 0; i < Length; i++)

s -= data[i];

sum = s;

NSLog(@" >> after subtract: %d", sum);

};

// Fork

dispatch_group_async(group, queue, task1);

dispatch_group_async(group, queue, task2);

// Join

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_release(taskSem);

dispatch_release(queue);

dispatch_release(group);

}

- (void)testBlock12

{

“带有自动变量值”在block中表现为“截获自动变量”

即保存自动变量的值

所以在执行block语法后，即使改写block中使用的自动变量的值也不会影响block执行时自动变量的值

该源码就在block语法后改写了block中自动变量val和fmt

但执行结果是：

val = 10

int val = 10;

const char *fmt = "val = %d\n";

void (^blk)(void) = ^{printf(fmt, val);};

val = 2;

fmt = "These values were changed. val = %d\n";

blk(); // 执行结果是：val = 10

}

- (void)testBlock13{

// 虽然我们把Block Objects 异步分派到了串行队列上，这个还是按照FIFO原则执行它的代码

__block dispatch_semaphore_t sem1 = dispatch_semaphore_create(0);

__block dispatch_semaphore_t sem2 = dispatch_semaphore_create(0);

__block dispatch_semaphore_t sem3 = dispatch_semaphore_create(0);

dispatch_queue_t firstSerialQueue = dispatch_queue_create("com.launch.GCD.serialQueue1", 0);

dispatch_async(firstSerialQueue, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"First interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem1);

});

dispatch_async(firstSerialQueue, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"Second interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem2);

});

dispatch_async(firstSerialQueue, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"Third interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem3);

});

dispatch_semaphore_wait(sem1, DISPATCH_TIME_FOREVER);

dispatch_semaphore_wait(sem2, DISPATCH_TIME_FOREVER);

dispatch_semaphore_wait(sem3, DISPATCH_TIME_FOREVER);

dispatch_release(firstSerialQueue);

dispatch_release(sem1);

dispatch_release(sem2);

dispatch_release(sem3);

}

- (void)testBlock14{

// 虽然我们把Block Objects 异步分派到了串行队列上，这个还是按照FIFO原则执行它的代码

// 但是我们可以创建多个串行队列，不同串行队列之间是并行的

__block dispatch_semaphore_t sem1 = dispatch_semaphore_create(0);

__block dispatch_semaphore_t sem2 = dispatch_semaphore_create(0);

__block dispatch_semaphore_t sem3 = dispatch_semaphore_create(0);

dispatch_queue_t firstSerialQueue1 = dispatch_queue_create("com.launch.GCD.serialQueue1", 0);

dispatch_queue_t firstSerialQueue2 = dispatch_queue_create("com.launch.GCD.serialQueue2", 0);

dispatch_queue_t firstSerialQueue3 = dispatch_queue_create("com.launch.GCD.serialQueue3", 0);

dispatch_async(firstSerialQueue1, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"First interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem1);

});

dispatch_async(firstSerialQueue2, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"Second interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem2);

});

dispatch_async(firstSerialQueue3, ^{

NSUInteger counter = 0;

for (counter=0; counter<5; counter++) {

NSLog(@"Third interation, counter=%d", counter);

}

dispatch_semaphore_signal(sem3);

});

dispatch_semaphore_wait(sem1, DISPATCH_TIME_FOREVER);

dispatch_semaphore_wait(sem2, DISPATCH_TIME_FOREVER);

dispatch_semaphore_wait(sem3, DISPATCH_TIME_FOREVER);

dispatch_release(firstSerialQueue1);

dispatch_release(firstSerialQueue2);

dispatch_release(firstSerialQueue3);

dispatch_release(sem1);

dispatch_release(sem2);

dispatch_release(sem3);

}

static dispatch_once_t onceToken;

void (^executedOnlyOnce)(void) = ^{

static NSUInteger numberOfEntries = 0;

numberOfEntries++;

NSLog(@"Executed %d time(s)", numberOfEntries);

};

- (void)testBlock15{

// 一个任务最多只执行一次

dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_once(&onceToken, ^{

dispatch_async(concurrentQueue, executedOnlyOnce);

});

dispatch_once(&onceToken, ^{

dispatch_async(concurrentQueue, executedOnlyOnce);

});

// 稍缓线程退出

NSUInteger count= 0;

while (count++ < 3) {

sleep(1);

}

- (void)testBlock16{

// 利用dispatch_once实现单例

@interface SingletonSample : NSObject

+ (id)ShareInstance;

@end

@implementation SingletonSample

+ (id)ShareInstance{

static SingletonSample *SharedInstance = nil;

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

SharedInstance = [[SingletonSample alloc] init];

});

return SharedInstance;

}

@end

SingletonSample *obj1 = [SingletonSample ShareInstance];

SingletonSample *obj2 = [SingletonSample ShareInstance];

NSLog(@"obj1=%@, obj2=%@", obj1, obj2);

STAssertEquals(obj1, obj2, nil);

}

- (void)testBlock17{

// 利用GCD延时后执行任务

double delayInsenconds = 2.0;

dispatch_time_t delayInNanoSeconds = dispatch_time(DISPATCH_TIME_NOW, delayInsenconds * NSEC_PER_SEC);

dispatch_queue_t concurrentQueue =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

NSLog(@"testBlock17");

dispatch_after(delayInNanoSeconds, concurrentQueue, ^(void){

NSLog(@"Now do work in dispatch_after");

});

// 稍缓线程退出

NSUInteger count= 0;

while (count++ < 5) {

sleep(1);

}

@end

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