1.GCD

• 同步/異步和串行/并發(fā)
• dispatch_barrier_async
• dispatch_group

(1)同步/異步和串行/并發(fā)

• dispatch_sync(serial_queue, ^{//任務(wù)})讯壶；
• dispatch_async(serial_queue,^{//任務(wù)})核无；
• dispatch_sync(concurrent_queue,^{//任務(wù)});
• dispatch_async(concurrent_queue,^{//任務(wù)});
  同步串行

    - (void)viewDidLoad {
       [super viewDidLoad];
       dispatch_sync(dispatch_get_main_queue(), ^{
          [self doSomething];
      });
   }
  

  上面代碼會產(chǎn)生死鎖舟扎，產(chǎn)生死鎖的原因是，隊列引起的循環(huán)等待逐工。
  
  截圖1.png

- (void)viewDidLoad {
    [super viewDidLoad];
     dispatch_sync(serialQueue, ^{
        [self doSomething];
    });
}

上面的代碼是沒有問題的

截圖2.png

- (void)viewDidLoad {
    NSLog(@"1");
    dispatch_sync(global_queue(), ^{
        NSLog(@"2");
        dispatch_sync(global_queue(), ^{
             NSLog(@"3");
        });
        NSLog(@"4");
    });
    NSLog(@"5");
}
打印結(jié)果:12345

異步串行

- (void)viewDidLoad {
        [super viewDidLoad];
        dispatch_async(dispatch_get_main_queue(), ^{
          [self doSomething];
       });
 }

異步并發(fā)

   - (void)viewDidLoad {
     dispatch_async(global_queue,^{
        NSLog(@"1");
       [self performSelector:@selector(printLog) 
                         withObject:nil  
                          afterDelay:0];
          NSLog(@"3");
    })
  }
  - (void)printLog {
        NSLog(@"2");
  }
  打印結(jié)果: 13

2.dispatch_barrier_async()

(1)怎樣利用GCD實現(xiàn)多讀單寫？

截圖1.png

截圖2.png

dispatch_barrier_async(concurrent_queue,^{//寫操作})馅精；

#import <Foundation/Foundation.h>
@interface UserCenter : NSObject
- (id)objectForKey:(NSString *)key;
- (void)setObject:(id)obj forKey:(NSString *)key;
@end

#import "UserCenter.h"

@interface UserCenter()
{
    dispatch_queue_t  concurrent_queue;
    NSMutableDictionary *userCenterDic;
}
@end

@implementation UserCenter

- (instancetype)init{
    self = [super init];
    if (self) {
        // 通過宏定義DISPATCH_QUEUE_CONCURRENT 創(chuàng)建一個并發(fā)隊列
        concurrent_queue = dispatch_queue_create("read_write_queue", DISPATCH_QUEUE_CONCURRENT);
        // 創(chuàng)建數(shù)據(jù)庫容器
        userCenterDic = [NSMutableDictionary dictionary];
    }
    return self;
}

- (id)objectForKey:(NSString *)key{
    __block id obj;
    //同步讀取指定數(shù)據(jù)
    dispatch_sync(concurrent_queue, ^{
        obj = [userCenterDic objectForKey:key];
    });
    return obj;
}

- (void)setObject:(id)obj forKey:(NSString *)key{
    // 異步柵欄調(diào)用設(shè)置數(shù)據(jù)
    dispatch_barrier_async(concurrent_queue, ^{
        [userCenterDic setObject:obj forKey:key];
    });
}

@end

3.dispatch_group_async()

使用GCD實現(xiàn)這個需求:A岩馍、B、C三個任務(wù)并發(fā)伙菊，完成后執(zhí)行任務(wù)D?

截圖3.png

#import <Foundation/Foundation.h>

@interface GroupObject : NSObject

- (void)handle;

@end

#import "GroupObject.h"

@interface GroupObject()
{
    dispatch_queue_t concurrent_queue;
    NSMutableArray <NSURL *> *arrayURLs;
}
@end

@implementation GroupObject

- (instancetype)init
{
    
    self = [super init];
    if (self) {
        //創(chuàng)建并發(fā)隊列
        concurrent_queue = dispatch_queue_create("concurrent_queue",DISPATCH_QUEUE_CONCURRENT);
        arrayURLs = [NSMutableArray array];
    }
    return self;
}

- (void)handle{
    //創(chuàng)建一個group
    dispatch_group_t group = dispatch_group_create();
    // for循環(huán)遍歷各個元素
    for (NSURL *url in arrayURLs) {
        //異步組分派到并發(fā)隊列當中
        dispatch_group_async(group, concurrent_queue, ^{
            //根據(jù)url去下載圖片
            NSLog(@"%@",url);
        });
    }
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        //當添加到數(shù)組中的所有任務(wù)執(zhí)行完成之后會調(diào)用該block
        NSLog(@"所有圖片下載完成");
    });
}
@end

4.NSOperation

需要和NSOperationQueue配合使用來實現(xiàn)多線程方案

• 添加任務(wù)依賴
• 任務(wù)執(zhí)行狀態(tài)控制
• 最大并發(fā)量

(1)任務(wù)執(zhí)行狀態(tài)控制

• isReady 當前任務(wù)是否處于就緒狀態(tài)
• isExecuting 當前任務(wù)是否處于執(zhí)行中的狀態(tài)
• isFinished 當前任務(wù)是否已經(jīng)執(zhí)行完成
• isCancelled 當前任務(wù)是否已取消

如果只重寫main方法败玉，底層控制變更任務(wù)執(zhí)行狀態(tài)敌土，以及任務(wù)退出。
如果重寫了start方法运翼，自行控制任務(wù)狀態(tài)返干。