上一节《iOS NSOperation 非并发执行》中已经讲了NSOperation中系统提供的子类NSBlockOperation和NSInvocationOperation的任务的非并发执行，以及添加到NSOperationQueue中进行并发执行。这节将NSOperation 子类实现以及并发执行。
NSOperation非并发子类实现，只需重盖main方法，将要执行的任务放到main方法中，在main方法中要做cancel判断的，如下例：
@interface MyNonConcurrentOperation : NSOperation @property (strong) id myData; - (id)initWithData:(id)data;
@end
@implementation MyNonConcurrentOperation - (id)initWithData:(id)data { if (self = [super init]) { _myData = data; } return self; } - (void)main { @try { BOOL isDone = NO; while (![self isCancelled] && !isDone) { // Do some work and set isDone to YES when finished } } @catch(...) { // Do not rethrow exceptions. } }
@end
NSOperation 并发子类实现，必须覆盖 start、isConcurrent、isExecuting和isFinished四个方法，main方法可选
@interface MyOperation : NSOperation { BOOL executing; BOOL finished; } - (void)completeOperation;
@end
@implementation MyOperation - (id)init { self = [super init]; if (self) { executing = NO; finished = NO; } return self; } - (BOOL)isConcurrent { return YES; } - (BOOL)isExecuting { return executing; } - (BOOL)isFinished { return finished; } - (void)start { // Always check for cancellation before launching the task. 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 { @try { // Do the main work of the operation here. [self completeOperation]; } @catch(...) { // Do not rethrow exceptions. } } - (void)completeOperation { [self willChangeValueForKey:@"isFinished"]; [self willChangeValueForKey:@"isExecuting"]; executing = NO; finished = YES; [self didChangeValueForKey:@"isExecuting"]; [self didChangeValueForKey:@"isFinished"]; }
@end
NSOperation 的属性添加了KVO机制，当我们覆盖属性的getter方法，按我们指定的状态返回，所以在状态改变时要发起KVO通知，通知监听次属性的对象。在start方法中使用NSThread 开启了新线程实现并发执行。
