笔记:Objective C – Concurrency, NSOperationQueue & Grand Central Dispatch

原文作者: Written By : Debasis Das (17-Jan-2015) 原文地址
文章会给出一些关于这一话题的代码示例.

什么是并发?
什么是NSOperationQueue
介绍Introduction:
NSInvocationOperation
NSBlockOperation
Operation
什么是Dispatch Queues
Types of Dispatch Queues
NSOperationQueue 和 DispatchQueues的区别
例子

什么是并发

在同一时间做这不同的事
充分利用CPU的性能
平行运行多个程序

何时需要使用多线程

运行时需要处理复杂的逻辑, 耗时任务
处理独立任务,同时彼此又内联

Operations and Operation Queue

OPeration 是 NSOperation(抽象类)的派生类
Cocoa框架提供两个能直接使用的Operation:
NSInvocationOperation (used to execute a method)
NSBlockOperation (used for executing one or more blocks concurrently)
operation要被执行必须加入 OperationQueue 队列中

NSInvocationOperation

NSInvocationOperation 不需要被子类化, 创建NSInvocationOperation的对象之后like this :

- (NSOperation*)taskWithData:(id)data  { 
 NSInvocationOperation* theOp = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(myTaskMethod:) object:data]; 
    return theOp;
// This is the method that does the actual work of the task.
} 

- (void)myTaskMethod:(id)data { 
// Perform the task.
}

NSBlockOperation

NSBlockOperation 能添加多个Task block, 也支持直接使用:
通过addDependency可以设置队列依赖关系

NSBlockOperation* theOp = [NSBlockOperation blockOperationWithBlock: ^{ NSLog(@"Beginning operation.\n"); // Do some work.  
}]:

NSOperation – Writing a Custom Operation

@interface KSCustomOperation : NSOperation
{
    BOOL executing;
    BOOL finished;
}

@property  (strong) NSDictionary *mainDataDictionary;
-(id)initWithData:(id)dataDictionary;
@end

@implementation KSCustomOperation
-(id)initWithData:(id)dataDictionary
{
    if (self = [super init])
    {
        _mainDataDictionary = dataDictionary;
        executing = NO;
        finished = NO;
        
    }
    return self;
}
-(void)start
{
    if ([self isCancelled]) { // Must move the operation to the finished state if it is canceled. 
[self willChangeValueForKey:@"isFinished"];
 finished = YES; 
[self didChangeValueForKey:@"isFinished"]; 
return; 
// If the operation is not canceled, begin executing the task. 
[self willChangeValueForKey:@"isExecuting"]; 
[NSThread detachNewThreadSelector:@selector(main) toTarget:self withObject:nil];
 executing = YES; 
[self didChangeValueForKey:@"isExecuting"];
}
}

-(void)main
{
    //This is the method that will do the work
    @try {
        NSLog(@"Custom Operation - Main Method isMainThread?? ANS = %@",[NSThread isMainThread]? @"YES":@"NO");
        NSLog(@"Custom Operation - Main Method [NSThread currentThread] %@",[NSThread currentThread]);
        NSLog(@"Custom Operation - Main Method Try Block - Do Some work here");
        NSLog(@"Custom Operation - Main Method The data that was passed is %@",_mainDataDictionary);
        for (int i = 0; i<5; i++)
        {
            NSLog(@"i%d",i);
            sleep(1); //Never put sleep in production code until and unless the situation demands. A sleep is induced here to demonstrate a scenario that takes some time to complete
        }
        [self willChangeValueForKey:@"isExecuting"];
        executing = NO;
        [self didChangeValueForKey:@"isExecuting"];
        
        [self willChangeValueForKey:@"isFinished"];
        finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        
    }
    @catch (NSException *exception) {
        NSLog(@"Catch the exception %@",[exception description]);
    }
    @finally {
        NSLog(@"Custom Operation - Main Method - Finally block");
    }
}

-(BOOL)isExecuting{
    return executing;
}

-(BOOL)isFinished{
    return finished;
}
@end

简单说明一下, 无论是自定义的Operation还是系统提供的 NSBlockOperation, NSInvocationOperation 当操作放入队列中开始执行的时候都是无法取消和暂停的.
只能取消和暂停还没有开始执行的队列.

DISPATCH QUEUES

dispatch queues 允许我们同步和异步执行我们的代码块
所有dispatch queues 都是先进先出原则
任务被加入dispatch queues中并不会马上执行, 和NSOperationqueen的区别
Dispatch Queue Types
串行(Serial)
并行(Concurrent) Global Dispatch Queue
主队列(Main Dispatch Queue) 主线程(串行)
我们可以通过dispatch group来管理队列之间的依赖关系