自定义NSOperation

NSInvocationOperation和NSBlockOperation是系统默认的子类实现，属于同步operation。

实现类似于

@interface NSInvocationOperation ()

@property (getter=isExecuting) BOOL executing;
@property (getter=isFinished) BOOL finished;

@end

@implementation NSInvocationOperation
@synthesize finished = _finished;
@synthesize executing = _executing;

- (void)main {
    @synchronized (self) {
      if (self.isCancelled) {
            self.finished = YES;
            self.executing = NO;
            return;
        }
       self.finished = NO;
        self.executing = YES;
        if (self.executionBlock) {
            self.executionBlock();
        } 
        self.finished = YES;
        self.executing = NO;
    }
}

- (void)cancel {
    @synchronized (self) {
        [super cancel];
        if (self.isExecuting) {
            self.executing = NO;
            self.finished = YES;
        }
    }
}

- (void)setFinished:(BOOL)finished {
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)setExecuting:(BOOL)executing {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (BOOL)isAsynchronous {
    return NO;
}

@end

如果operation进行网络请求或者异步的任务，就需要自定义异步任务

@class BBAAsyncOperation;
typedef void (^BBAAsyncOperationExecutionBlockType)();

@interface BBAAsyncOperation : NSOperation

@property (nonatomic, copy) BBAAsyncOperationExecutionBlockType executionBlock;
- (void)done;

@end

#import "BBAAsyncOperation.h"

@interface BBAAsyncOperation ()

@property (assign, getter = isExecuting) BOOL executing;
@property (assign, getter = isFinished) BOOL finished;

@end

@implementation BBAAsyncOperation
@synthesize executing = _executing;
@synthesize finished = _finished;

// 异步任务会出发start方法
- (void)start {
    @synchronized (self) {
        if (self.isCancelled) {
            self.finished = YES;
            return;
        }
        
        self.finished = NO;
        self.executing = YES;
        
        if (self.executionBlock) {
            self.executionBlock();
        } else {
            self.executing = NO;
            self.finished = YES;
        }
    }
}
// 自定义方法，异步任务完成，回调，结束任务
- (void)done {
    @synchronized (self) { 
        if (self.isExecuting) {
            self.finished = YES;
            self.executing = NO;
        }
    }
}

- (void)cancel {
    @synchronized (self) {
        [super cancel];
        if (self.isExecuting) {
            self.executing = NO;
            self.finished = YES;
        }
    }
}

// 父类中的属性声明为readonly，不能触发KVO，这里需要手动实现KVO
- (void)setFinished:(BOOL)finished {
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)setExecuting:(BOOL)executing {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (BOOL)isAsynchronous {
 // 返回YES
    return YES;
}

@end

1：对属性executing和finished的访问需要保证同步，这里使用@synchronized (self) {
}实现

2：自定义需要实现start，isAsynchronous，setFinished：，setExecuting:方法。

官方文档描述：
Methods to Override
If you are creating a concurrent operation, you need to override the following methods and properties at a minimum:
start
asynchronous
executing
finished