NSOperation是基于GCD的面相对象封装，这里把重点计一下。
NSoperation的dependency
任务依赖是NSoperation的重要功能，可以让GCD的任务同步更直观的实现出来，不过有些地方也需要注意。
NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"1-1: %@",[NSThread currentThread]);
}];
NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"2-1");
}];
[op1 addDependency:op2];
[op2 start];
[op1 start];
由于op1依赖于op2的完成，如果[op1 start]写在[op2 start]前面的话，
会抛出异常-[__NSOperationInternal _start:]: receiver is not yet ready to execute,
你可以这么写
NSOperationQueue *qq = [[NSOperationQueue alloc] init];
NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"1-1: %@",[NSThread currentThread]);
}];
NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"2-1");
NSLog(@"2-3");
}];
[op1 addDependency:op2];
[qq addOperation:op1];
[qq addOperation:op2];
就不需要关心触发顺序的问题了，因为NSOperationQueue会帮你管理
NSoperation的queuePriority
queuePriority是次于dependency的属性，在dependency没有指明的情况下，NSOperationQueue会依据NSOperation的queuePriority来决定执行先后。
异步任务的同步问题
看以下代码
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"start-1");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"end-1");
});
}];
NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"start-2");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"end-2");
});
}];
[op1 addDependency:op2];
[queue addOperation:op1];
[queue addOperation:op2];
输出
2018-04-23 09:27:57.391220+0800 testP[20559:439375] start-2
2018-04-23 09:27:57.391494+0800 testP[20559:439382] start-1
2018-04-23 09:27:59.583992+0800 testP[20559:439102] end-2
2018-04-23 09:27:59.584309+0800 testP[20559:439102] end-1
可以发现，operationqueue对异步任务是不能同步的，想要实现对异步任务的同步，就得重写NSOperation子类，也就是控制他的isFinished属性。
NSOperation是通过KVO isFinished/isExecuting等属性来判断任务的生命周期，重写这几个关键属性就可以了，上代码
@interface SubOperation:NSBlockOperation
{
// 这是我们用来代替isFinishe的属性
BOOL sub_isFinish;
}
-(void)stopOperation;
@end
@implementation SubOperation
// 重写isFinished，替换掉他
-(BOOL)isFinished{
return sub_isFinish;
}
/*
* 这里需要手动调用此方法，来告诉operation，需要调用isFinished来结束operation
* 为什么没有直接调，setvalueforkey的方法呢，试试看就知道了
*/
-(void)stopOperation{
[self willChangeValueForKey:@"isFinished"];
sub_isFinish = YES;
[self didChangeValueForKey:@"isFinished"];
}
@end
现在重新测试下，需要加上我们手动的方法，可以发现可以实现异步任务同步了
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
__block SubOperation *op1;
op1 = [SubOperation blockOperationWithBlock:^{
NSLog(@"start-1");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"end-1");
[op1 stopOperation];
});
}];
__block SubOperation *op2;
op2 = [SubOperation blockOperationWithBlock:^{
NSLog(@"start-2");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"end-2");
[op2 stopOperation];
});
}];
[op1 addDependency:op2];
[queue addOperation:op1];
[queue addOperation:op2];
2018-04-23 09:30:41.700824+0800 testP[20651:442268] start-2
2018-04-23 09:30:43.895327+0800 testP[20651:442214] end-2
2018-04-23 09:30:43.895927+0800 testP[20651:442271] start-1
2018-04-23 09:30:46.089130+0800 testP[20651:442214] end-1
