笔记:Objective C – Concurrency, NS

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

.h

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

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

.m

@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