block可用于并行程式設計,線程同步多用信号量來實作,保證執行完一個block再執行下一個block,當所有block都執行完畢後,再利用group,通知執行後面的代碼,使用wait(阻塞線程),或notify(不阻塞線程)等待執行結束後再繼續。
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
static volatile BOOL flag = NO;
//一個定義為volatile的變量是說這變量可能會被意想不到地改變,這樣,編譯器就不會去假設這個變量的值了。精确地說就是,優化器在用到這個變量時必須每次都小心地重新讀取這個變量的值,而不是使用儲存在寄存器裡的備份。
static const int Length = 1000000000;
static int Data[Length];
static void initData() {
for (int i = 0; i < Length; i++) {
Data[i] = i+1;
}
}
#define Test_Mode 7
int main(int argc, char * argv[]) {
@autoreleasepool {
#if Test_Mode == 0
//local block
void (^myBlock) (void) = ^(void){
NSLog(@"I am a block");
};
myBlock = ^{
NSLog(@"I have been changed");
};
myBlock();
#elif Test_Mode == 1
//array block
void (^blockArray[2])(void) = {
^{
NSLog(@"block 1");
},
^{
NSLog(@"block 2");
},
};
blockArray[0]();
blockArray[1]();
//block該如何修改外部變量呢?有兩種辦法,第一種是可以修改 static 全局變量;第二種是可以修改用關鍵字 __block 修飾的變量。
#elif Test_Mode == 2
//block recursion(遞歸) (必須用static或__blcok修飾block)
void (^aBlock)(int);
static void (^ const recursionBlock)(int) = ^(int i){
if (i > 0) {
NSLog(@"recursionBlock %d", i);
recursionBlock(i - 1);
}
};
aBlock = recursionBlock;
aBlock(6);
// recursionBlock(5);
__block void (^recursionBlock2)(int) = ^(int i){
if (i > 0) {
NSLog(@"recursionBlock2 %d", i);
recursionBlock2(i - 1);
}
};
aBlock = recursionBlock2;
aBlock(10);
// recursionBlock2(8);
#elif Test_Mode == 3
//dispatch block
initData();//調用初始化函數
void (^dispatchBlcok)(void) = ^{
int sum = 0;
for (int i = 0; i < Length; i++) {
sum += Data[i];
}
NSLog(@"sum = %d", sum);
flag = YES;
};
dispatch_queue_t queue = dispatch_queue_create("dispatch block", 0);
dispatch_async(queue, dispatchBlcok);
while (!flag);//阻塞線程,直到執行完block(非必需)(用信号量實作更好)
#elif Test_Mode == 4
//semaphore(信号量)
initData();
//不用block修飾好像也可以。
__block dispatch_semaphore_t sem = dispatch_semaphore_create(0);//0表示資源未準備好,需要等待
dispatch_queue_t queue = dispatch_queue_create("semaphore block", 0);
dispatch_async(queue, ^{
int sum = 0;
for (int i = 0; i < Length; i++)
sum += Data[i];
NSLog(@"sum = %d", sum);
dispatch_semaphore_signal(sem);//準備好了,增加 semaphore 計數
});
NSLog(@"<<semaphore<<");
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);//可以繼續執行了,減少 semaphore 計數
#elif Test_Mode == 5
//多個信号量.按照 FIFO 順序執行并用 semaphore 線程同步。
initData();
__block int sum = 0;
__block dispatch_semaphore_t sem = dispatch_semaphore_create(0);
__block dispatch_semaphore_t taskSem = dispatch_semaphore_create(0);
dispatch_queue_t queue = dispatch_queue_create("semaphore block", 0);
dispatch_block_t task1 = ^{
int s = 0;
for (int
i = 0; i < Length; i ++) {
s += Data[i];
}
sum = s;
NSLog(@">> sfter add : %d", sum);
dispatch_semaphore_signal(taskSem);
};
dispatch_block_t task2 = ^{
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);
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
NSLog(@"after 5");
#elif Test_Mode == 6
//dispatch_apply 進行并發疊代。這麼做與 for 循環相比有什麼好處呢?答案是:并行,這裡的求和是并行的,并不是按照順序依次執行求和的。
initData();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
__block int sum = 0;
__block int *pArray = Data;
dispatch_apply(Length, queue, ^(size_t i) {
//循環體的執行是并發執行的。
sum += pArray[i];
NSLog(@"pArray :%zu", i);
});
NSLog(@">> sum :%d", sum);
#elif Test_Mode == 7
//<span style="font-family: Arial, Helvetica, sans-serif;">dispatch_queue</span>
initData();
__block int sum = 0;
__block dispatch_semaphore_t taskSem = dispatch_semaphore_create(0);
dispatch_queue_t queue = dispatch_queue_create("group queue", 0);
dispatch_group_t group = dispatch_group_create();
dispatch_block_t task1 = ^{
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 = ^{
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_group_async(group, queue, task1);
dispatch_group_async(group, queue, task2);
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);//保證group裡的所有任務執行完後再往下執行。會阻塞目前線程。
NSLog(@"after group wait");
dispatch_group_notify(group, queue, ^{//在group執行完所有任務後執行,具體執行的時間不确定。
NSLog(@"group over");
});
#endif
NSLog(@"<<<<<<Test Done>>>>>>");
return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}
}