iOS开发之多线程（4）—— Operation

2020-07-09 本文已影响0人看影成痴

文集

iOS开发之多线程（1）—— 概述
 iOS开发之多线程（2）—— Thread
iOS开发之多线程（3）—— GCD
iOS开发之多线程（4）—— Operation
iOS开发之多线程（5）—— Pthreads
iOS开发之多线程（6）—— 线程安全与各种锁

版本

Xcode 11.5
Swift 5.2.2

简介

我们先来讨论相关理论知识点, 代码部分统一放到后文.

Operation基于GCD封装, 完全面向对象. 对应于GCD, Operation也有任务和队列的概念, 只不过在Operation中任务(block)被称为操作(operation).

Operation支持以下关键功能:

操作之间添加依赖

使用KVO监听操作的执行状态

对操作进行优先级排序

取消操作

操作

在GCD中, 任务代码只能写在block中, 并且需要放到队列(dispatch queue)中去执行.
而在Operation中, 操作对象可以单独执行, 也可以添加到队列 (operation queue)去执行.

Operation是一个抽象类, 代表一个任务. 通常我们使用它的子类NSInvocationOperation或NSBlockOperation来编写任务代码. 当然也可以直接使用Operation, 不过需要重写main方法, 在main里面编写任务代码.

3种创建方式:

NSInvocationOperation (swift不支持): 此类调用选择器方法(selector), 在方法里面编写任务代码.

NSBlockOperation (swift对应BlockOperation): 此类采用block方式, 在block中编写任务代码.

NSOperation (swift对应Operation): 需要重写main方法, 在main里面编写任务代码.

2种执行方式:

不添加到队列, 手动调用operation的start方法.

添加到队列, 系统自动调用start方法.

具体如何使用请看下小节.

同步 & 异步

在GCD中, 同步和异步分别对应dispatch_sync和dispatch_async方法.
在Operation中, 没有这种方法.

苹果文档
如果计划手动执行操作对象，而不是将其添加到队列中，则可以将操作设计为以同步或异步方式执行。默认情况下，操作对象是同步的。当start()直接从代码中调用同步操作的方法时，该操作将在当前线程中立即执行。
默认情况下，操作对象以同步方式执行-也就是说，它们在调用其start方法的线程中执行其任务。
为了获得最佳性能，您应该将操作设计为尽可能异步，使应用程序在执行操作时可以自由地做其他工作。

如果不使用队列, operation默认以同步方式执行. 但我们有办法使之异步执行: 新建一个新线程, 然后在新线程里面调用start方法.
如果使用队列, 系统默认以异步方式执行. 但我们可以使用waitUntilAllOperationsAreFinished (operation queue方法)进行等待, 以确保操作完成后才继续往下执行.

注意: 与GCD的同步执行不同, 这里虽然设置了等待, 但是一般不在当前线程执行, 而是新开一个线程来执行, 但会保证队列里的所有任务完成后才继续往下执行.

队列

队列 (Operation Queue)有两种: 主队列和非主队列 (自定义队列).

主队列通过mainQueue获得, 主队列里的任务都是放到主线程执行 (不包括使用addExecutionBlock:添加的额外操作, 因其可能在其他线程执行).
非主队列 (自定义队列) 即一般 alloc init 出来的队列, 默认在子线程中异步执行. 通多设置最大并发数(maxConcurrentOperationCount)来控制队列是串行还是并发.

添加操作(任务)到队列有四种方式:

addOperation:
添加一个现有的Operation (或者其子类).
addOperations:waitUntilFinished:
可添加多个现有的Operation (或者其子类), 可设置等待所有操作完成后方可继续往下执行.
addOperationWithBlock:
直接添加一个block
addBarrierBlock:
添加栅栏, 顺带一个任务. 等栅栏前的所有任务都执行完, 再执行本栅栏的任务, 起到隔离同步等待的目的.

串行 & 并行

主队列是串行队列. 自定义队列默认是并发队列, 但可通多设置最大并发数(maxConcurrentOperationCount)来控制队列是串行还是并发.

maxConcurrentOperationCount
-1, 默认值, 并发队列;
=0, 不执行任何操作;
=1, 串行队列;
<0, 除-1默认值外, 其他负值均报错;
>1, 并发队列, 如果数值过大, 最终并发数由系统决定.

基本用法

1. 不使用队列

#pragma mark - NSInvocationOperation (调用selector方法)

// 使用 NSInvocationOperation
- (void)useInvocationOperation {
    
    NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(doInvocationOperation) object:nil];
    [operation start];
}

// 任务
- (void)doInvocationOperation {
    
    NSLog(@"%s, thread:%@", __func__, [NSThread currentThread]);
}

//---------------------------------------------------------------------------------------------------
//log:
//-[ViewController doInvocationOperation], thread:<NSThread: 0x6000017ec2c0>{number = 1, name = main}


#pragma mark - NSBlockOperation (使用block)

// 使用 NSBlockOperation
- (void)useBlockOperation {
    
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        
        NSLog(@"%s, thread:%@", __func__, [NSThread currentThread]);
    }];
    
    [operation start];
}

//---------------------------------------------------------------------------------------------------
//log:
//-[ViewController useBlockOperation]_block_invoke, thread:<NSThread: 0x600000760100>{number = 1, name = main}


#pragma mark - 自定义Operation (重写main)

- (void)useCustomOperation {
    
    MyOperation *operation = [[MyOperation alloc] init];
    [operation start];
}

//---------------------------------------------------------------------------------------------------
//log:
//-[MyOperation main], thread:<NSThread: 0x600000618280>{number = 1, name = main}

MyOperation.m

@implementation MyOperation

- (void)main {
    
    NSLog(@"%s, thread:%@", __func__, [NSThread currentThread]);
}

@end

在自定义Operation中, 调用start方法后, 系统会执行多项安全检查, 最终会调用main方法.

Swift

//MARK: - 使用 BlockOperation (block)
@objc func useBlockOperation() {
    
    let operation = BlockOperation.init {
        print("\(#function), thread:\(Thread.current)")
    }
    operation.start()
}
    
//---------------------------------------------------------------------------------------------------
//log:
//useBlockOperation(), thread:<NSThread: 0x600003f78000>{number = 1, name = main}
    

//MARK: - 使用 自定义Operation (重写main)
@objc func useCustomOperation() {
    
    let operation = CustomOperation.init()
    operation.start()
}
    
//---------------------------------------------------------------------------------------------------
//log:
//main(), thread:<NSThread: 0x600003ec8cc0>{number = 1, name = main}

class CustomOperation: Operation {
    
    override func main() {
        print("\(#function), thread:\(Thread.current)")
    }
}

2. 使用队列

2.1 四种添加操作(任务)方法

    NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];

    // 1 addOperation:
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op1 thread:%@", [NSThread currentThread]);
    }] ;
    [operationQueue addOperation:op1];

    // 2 addOperations:waitUntilFinished:
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op2 thread:%@", [NSThread currentThread]);
    }] ;
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op3 thread:%@", [NSThread currentThread]);
    }] ;
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op4 thread:%@", [NSThread currentThread]);
    }] ;
    [operationQueue addOperations:@[op2, op3, op4] waitUntilFinished:YES];
    NSLog(@"queue finished");

    // 3 addOperationWithBlock:
    [operationQueue addOperationWithBlock:^{
        NSLog(@"block thread:%@", [NSThread currentThread]);
    }];

    // 4 addBarrierBlock:
    [operationQueue addBarrierBlock:^{
        NSLog(@"barrier thread:%@", [NSThread currentThread]);
    }];

//---------------------------------------------------------------------------------------------------
//log:
//op2 thread:<NSThread: 0x600002ec7c80>{number = 6, name = (null)}
//op4 thread:<NSThread: 0x600002eaf180>{number = 4, name = (null)}
//op1 thread:<NSThread: 0x600002ed77c0>{number = 5, name = (null)}
//op3 thread:<NSThread: 0x600002eaf140>{number = 3, name = (null)}
//queue finished
//block thread:<NSThread: 0x600002ec7c80>{number = 6, name = (null)}

Swift

    let operationQueue = OperationQueue.init()
        
    // 1 addOperation(_ op: Operation)
    let op1 = BlockOperation.init {
        print("op1, thread:\(Thread.current)")
    }
    operationQueue.addOperation(op1)
    
    
    // 2 addOperations(_ ops: [Operation], waitUntilFinished wait: Bool)
    let op2 = BlockOperation.init {
        print("op2, thread:\(Thread.current)")
    }
    let op3 = BlockOperation.init {
        print("op3, thread:\(Thread.current)")
    }
    let op4 = BlockOperation.init {
        print("op4, thread:\(Thread.current)")
    }
    operationQueue.addOperations([op2, op3, op4], waitUntilFinished: true)
    
    
    // 3 addOperation(_ block: @escaping () -> Void)
    operationQueue.addOperation {
        print("block, thread:\(Thread.current)")
    }
    

    // 4 addBarrierBlock(_ barrier: @escaping () -> Void)
    operationQueue.addBarrierBlock {
        print("barrier, thread:\(Thread.current)")
    }
        
//---------------------------------------------------------------------------------------------------
log:
op3, thread:<NSThread: 0x600002399200>{number = 4, name = (null)}
op1, thread:<NSThread: 0x6000023c34c0>{number = 5, name = (null)}
op2, thread:<NSThread: 0x600002398f80>{number = 6, name = (null)}
op4, thread:<NSThread: 0x6000023d42c0>{number = 7, name = (null)}
block, thread:<NSThread: 0x600002398f80>{number = 6, name = (null)}
barrier, thread:<NSThread: 0x600002398f80>{number = 6, name = (null)}

2.2 其他属性方法

maxConcurrentOperationCount

- (void)setOperationCount {
    
    NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
    
    NSLog(@"start");
    
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op1 thread:%@", [NSThread currentThread]);
    }] ;
    
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op2 thread:%@", [NSThread currentThread]);
    }] ;
    
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op3 thread:%@", [NSThread currentThread]);
    }] ;
    
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op4 thread:%@", [NSThread currentThread]);
    }] ;
    
    /**
     -1, 默认值, 并发队列;
     =0, 不执行任何操作;
     =1, 串行队列;
     <0, 除-1默认值外, 其他负值均报错;
     >1, 并发队列, 如果数值过大, 最终并发数由系统决定.
     */
    operationQueue.maxConcurrentOperationCount = 1;
    [operationQueue addOperations:@[op1, op2, op3, op4] waitUntilFinished:YES];
    
    NSLog(@"end");
}

//---------------------------------------------------------------------------------------------------
log:
start
op1 thread:<NSThread: 0x600000b181c0>{number = 5, name = (null)}
op2 thread:<NSThread: 0x600000b75f80>{number = 6, name = (null)}
op3 thread:<NSThread: 0x600000b75f80>{number = 6, name = (null)}
op4 thread:<NSThread: 0x600000b75f80>{number = 6, name = (null)}
end

我们看到:
由于是串行队列, 没有添加依赖和设置优先级, 所以会按照1~4顺序执行;
又因为设置了等待(waitUntilFinished:YES), 所以end最后才打印;
开不开新线程, 由系统决定.

添加依赖

addDependency:

设置优先级

typedef NS_ENUM(NSInteger, NSOperationQueuePriority) {
    NSOperationQueuePriorityVeryLow = -8L,
    NSOperationQueuePriorityLow = -4L,
    NSOperationQueuePriorityNormal = 0,
    NSOperationQueuePriorityHigh = 4,
    NSOperationQueuePriorityVeryHigh = 8
};

取消队列里所有任务

- (void)cancelAllOperations;

2.3 线程间通信

- (void)callMainQueue {
    
    NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
    
    [operationQueue addOperationWithBlock:^{
        
        NSLog(@"block, thread:%@", [NSThread currentThread]);
        
        // call main queue
        [NSOperationQueue.mainQueue addOperationWithBlock:^{
            
            NSLog(@"main, thread:%@", [NSThread currentThread]);
        }];
    }];
}

//---------------------------------------------------------------------------------------------------
log:
block, thread:<NSThread: 0x600003ee0580>{number = 6, name = (null)}
main, thread:<NSThread: 0x600003ea0240>{number = 1, name = main}

demo
https://github.com/LittleLittleKang/KKThreadsDemo