iOS 多线程之NSOperation

多线程处理任务的过程中，频繁的创建/销毁线程会很大程度上影响处理效率，新起的线程数过多会降低系统性能甚至引起app崩溃。在 Java 和C#开发过程中可以使用线程池来解决这些问题，线程池缓存一些线程，在接到任务的时候，系统就在线程池中调度一个闲置的线程来处理这个任务，免去了频繁创建/销毁的过程。从NSOperation的使用过程就能体会到，它和线程池非常类似，下面我们就来介绍一下NSOperation的使用。

二、NSOperation简介

NSOperation是一个抽象类，实际开发中需要使用其子类NSInvocationOperation、NSBlockOperation。首先创建一个NSOperationQueue，再建多个NSOperation实例（设置好要处理的任务、operation的属性和依赖关系等），然后再将这些operation放到这个queue中，线程就会被依次启动。 苹果官网对于NSOperation的介绍 NSOperation及其子类中的常用方法如下：

//// NSOperation
@property (readonly, getter=isCancelled) BOOL cancelled;
@property (readonly, getter=isExecuting) BOOL executing;
@property (readonly, getter=isFinished) BOOL finished;
@property (readonly, getter=isReady) BOOL ready;

@property NSOperationQueuePriority queuePriority;
@property (readonly, copy) NSArray<NSOperation *> *dependencies;

@property (nullable, copy) NSString *name;
@property (nullable, copy) void (^completionBlock)(void);

- (void)start;
- (void)main;
- (void)cancel;

- (void)addDependency:(NSOperation *)op;
- (void)removeDependency:(NSOperation *)op;

- (void)waitUntilFinished;
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下面我们依次介绍NSInvocationOperation、NSBlockOperation的使用过程，并自定义一个继承于NSOperation的子类并实现内部相应的方法。

2.1 NSInvocationOperation

NSInvocationOperation继承于NSOperation，NSInvocationOperation的定义如下：

@interface NSInvocationOperation : NSOperation {
@private
    id _inv;
    id _exception;
    void *_reserved2;
}

- (nullable instancetype)initWithTarget:(id)target selector:(SEL)sel object:(nullable id)arg;
- (instancetype)initWithInvocation:(NSInvocation *)inv NS_DESIGNATED_INITIALIZER;property (readonly, retain) NSInvocation *invocation;

@property (nullable, readonly, retain) id result;

@end
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下面使用NSInvocationOperation来加载一张图片，示例方法如下：

- (void)loadImageWithMultiThread {
    /*创建一个调用操作
     object:调用方法参数
    */
    NSInvocationOperation *invocationOperation=[[NSInvocationOperation alloc]initWithTarget:self selector:@selector(loadImage) object:nil];
    //创建完NSInvocationOperation对象并不会调用，它由一个start方法启动操作，但是注意如果直接调用start方法，则此操作会在主线程中调用，一般不会这么操作,而是添加到NSOperationQueue中
//    [invocationOperation start];
    
    //创建操作队列
    NSOperationQueue *operationQueue=[[NSOperationQueue alloc]init];
    //注意添加到操作队后，队列会开启一个线程执行此操作
    [operationQueue addOperation:invocationOperation];
}
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2.2 NSBlockOperation

NSBlockOperation继承于NSOperation，NSBlockOperation的定义如下：

@interface NSBlockOperation : NSOperation {
@private
    id _private2;
    void *_reserved2;
}

+ (instancetype)blockOperationWithBlock:(void (^)(void))block;

- (void)addExecutionBlock:(void (^)(void))block;
@property (readonly, copy) NSArray<void (^)(void)> *executionBlocks;

@end
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下面我们来使用NSOperation，实现多个线程加载图片，示例代码如下：

//// 首先 定义一个OperationImage的Model

@interface OperationImage : NSObject

@property (nonatomic, assign) NSInteger index;
@property (nonatomic, strong) NSData *imgData;

@end

@implementation OperationImage

@end



//// 使用NSOperation实现多线程加载图片

#define ColumnCount    4
#define RowCount       5
#define Margin         10

@interface MultiThread_NSOperation1 ()

@property (nonatomic, strong) NSMutableArray *imageViews;

@end

@implementation MultiThread_NSOperation1

- (void)viewDidLoad {
    
    [super viewDidLoad];
    [self setTitle:@"NSOperation1"];
    [self.view setBackgroundColor:[UIColor whiteColor]];
    self.edgesForExtendedLayout = UIRectEdgeNone;
    
    [self layoutViews];
}

- (void)layoutViews {
    
    CGSize size = self.view.frame.size;
    CGFloat imgWidth = (size.width - Margin * (ColumnCount + 1)) / ColumnCount;
    
    _imageViews=[NSMutableArray array];
    for (int row=0; row<RowCount; row++) {
        for (int colomn=0; colomn<ColumnCount; colomn++) {
            UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(Margin + colomn * (imgWidth + Margin), Margin + row * (imgWidth + Margin), imgWidth, imgWidth)];
            imageView.backgroundColor = [UIColor cyanColor];
            [self.view addSubview:imageView];
            [_imageViews addObject:imageView];
        }
    }
    
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    button.frame = CGRectMake(15, (imgWidth + Margin) * RowCount + Margin, size.width - 15 * 2, 45);
    [button addTarget:self action:@selector(loadImageWithMultiOperation) forControlEvents:UIControlEventTouchUpInside];
    [button setTitle:@"加载图片" forState:UIControlStateNormal];
    [self.view addSubview:button];
}


#pragma mark - 多线程下载图片

- (void)loadImageWithMultiOperation {
    
    int count = RowCount * ColumnCount;
    
    NSOperationQueue *operationQueue = [[NSOperationQueue alloc]init];
    operationQueue.maxConcurrentOperationCount = 5;
    
    NSBlockOperation *tempOperation = nil;
    for (int i=0; i<count; ++i) {
        OperationImage *operationImg = [[OperationImage alloc] init];
        operationImg.index = i;
        
        ////1.直接使用操队列添加操作
        //[operationQueue addOperationWithBlock:^{
        //    [self loadImg:operationImg];
        //}];
        
        ////2.创建操作块添加到队列
        NSBlockOperation *blockOperation = [NSBlockOperation blockOperationWithBlock:^{
            [self loadImg:operationImg];
        }];
        if (i > 0) {// 设置依赖
            [blockOperation addDependency:tempOperation];
        }
        [operationQueue addOperation:blockOperation];
        tempOperation = blockOperation;
    }
}

#pragma mark - 将图片显示到界面

-(void)updateImage:(OperationImage *)operationImg {
    
    UIImage *image = [UIImage imageWithData:operationImg.imgData];
    UIImageView *imageView = _imageViews[operationImg.index];
    imageView.image = image;
}


#pragma mark - 请求图片数据

- (NSData *)requestData {
    
    NSURL *url = [NSURL URLWithString:@"https://store.storeimages.cdn-apple.com/8756/as-images.apple.com/is/image/AppleInc/aos/published/images/a/pp/apple/products/apple-products-section1-one-holiday-201811?wid=2560&hei=1046&fmt=jpeg&qlt=95&op_usm=0.5,0.5&.v=1540576114151"];
    NSData *data = [NSData dataWithContentsOfURL:url];
    return data;
}


#pragma mark - 加载图片

- (void)loadImg:(OperationImage *)operationImg {
    
    // 请求数据
    operationImg.imgData = [self requestData];
    
    // 更新UI界面（mainQueue是UI主线程）
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [self updateImage:operationImg];
    }];
    
    // 打印当前线程
    NSLog(@"current thread: %@", [NSThread currentThread]);
}

@end
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在加载网络图片的代码上打一个断点，查看断点信息，从的运行过程可以看出NSOperation底层涉及到对GCD的封装：

三、关于自定义封装NSOperation

我们用到的很多三方库都自定义封装NSOperation，如MKNetworkOperation、SDWebImage等。自定义封装抽象类NSOperation只需要重写其中的main或start方法，在多线程执行任务的过程中需要注意线程安全问题，我们还可以通过KVO监听isCancelled、isExecuting、isFinished等属性，确切的回调当前任务的状态。下面就是对NSOperation的自定义封装代码：

@interface MyOperation ()

//要下载图片的地址
@property (nonatomic, copy) NSString *urlString;
//执行完成后，回调的block
@property (nonatomic, copy) void (^finishedBlock)(NSData *data);

// 自定义变量，用于重写父类isFinished的set、get方法
@property (nonatomic, assign) BOOL taskFinished;

@end

@implementation MyOperation

+ (instancetype)downloadDataWithUrlString:(NSString *)urlString finishedBlock:(void (^)(NSData *data))finishedBlock {
    
    MyOperation *operation = [[MyOperation alloc] init];
    operation.urlString = urlString;
    operation.finishedBlock = finishedBlock;
    return operation;
}

// 监听/重写readonly属性的set、get方法
- (void)setTaskFinished:(BOOL)taskFinished {
    [self willChangeValueForKey:@"isFinished"];
    _taskFinished = taskFinished;
    [self didChangeValueForKey:@"isFinished"];
}

- (BOOL)isFinished {

    return self.taskFinished;
}

//- (void)main {
//
//    // 打印当前线程
//    NSLog(@"%@", [NSThread currentThread]);
//
//    //判断是否被取消,取消正在执行的操作
//    if (self.cancelled) {
//        return;
//    }
//
//    NSURLSessionTask *task = [NSURLSession.sharedSession dataTaskWithRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:self.urlString]] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
//        //回到主线程更新UI
//
//        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
//            self.finishedBlock(data);
//        }];
//    }];
//    [task resume];
//}

- (void)start {
    
    // 打印当前线程
    NSLog(@"%@", [NSThread currentThread]);
    
    //判断是否被取消,取消正在执行的操作
    if (self.cancelled) {
        return;
    }
    
    NSURLSessionTask *task = [NSURLSession.sharedSession dataTaskWithRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:self.urlString]] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        //回到主线程更新UI
        
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.finishedBlock(data);
        }];
        
    self.taskFinished = YES;
    }];
    [task resume];
}

@end
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调用MyOperation中的方法：

- (void)testMyOperation {
    
    _queue = [[NSOperationQueue alloc] init];
    _queue.maxConcurrentOperationCount = 3;
    
    MyOperation *temp = nil;
    for (NSInteger i=0; i<500; i++) {
        MyOperation *operation = [MyOperation downloadDataWithUrlString:@"https://www.so.com" finishedBlock:^(NSData * _Nonnull data) {
            NSLog(@"--- %d finished---", (int)i);
        }];
        if (temp) {
            [operation addDependency:temp];
        }
        temp = operation;
        [_queue addOperation:operation];
    }
}
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四、NSOperation中的依赖

用NSThread来实现多线程时，线程间的执行顺序很难控制，但是使用NSOperation时可以通过设置操作的依赖关系来控制执行顺序。假设操作A依赖于操作B，线程操作队列在启动线程时就会首先执行B操作，然后执行A。例如在第三节testMyOperation方法中，我们从第二个任务一次设置了关系：

MyOperation *temp = nil;
    for (NSInteger i=0; i<500; i++) {
        MyOperation *operation = [MyOperation downloadDataWithUrlString:@"https://www.so.com" finishedBlock:^(NSData * _Nonnull data) {
            NSLog(@"--- %d finished---", (int)i);
        }];
        if (temp) {
            [operation addDependency:temp];
        }
        temp = operation;
        [_queue addOperation:operation];
    }
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