完成併發NSOperation塊不被每次調用

Question

我想運行一個併發的NSOperation。出於這個原因，我擴展了NSOperation類並重寫了開始和結束方法。它看起來是這樣的：

#import "AVFrameConversionOP.h" 


@implementation AVFrameConversionOP //extends NSOperation 

- (void)start 
{ 

    [self willChangeValueForKey:@"isFinished"]; 
    _isFinished = NO; 
    [self didChangeValueForKey:@"isFinished"]; 

    // Begin 
    [self willChangeValueForKey:@"isExecuting"]; 
    _isExecuting = YES; 
    [self didChangeValueForKey:@"isExecuting"]; 

    if (_isCancelled == YES) { 
     NSLog(@"** OPERATION CANCELED **"); 
     [self willChangeValueForKey:@"isFinished"]; 
     _isFinished = YES; 
     [self didChangeValueForKey:@"isFinished"]; 
     return; 
    } 

    [self performTask]; 
    [self finish]; 

} 

- (void)finish 
{ 

    [self willChangeValueForKey:@"isExecuting"]; 
    [self willChangeValueForKey:@"isFinished"]; 

    _isExecuting = NO; 
    _isFinished = YES; 

    [self didChangeValueForKey:@"isExecuting"]; 
    [self didChangeValueForKey:@"isFinished"]; 

    NSLog(@"operationfinished, _isFinished is %d", _isFinished); 

    if (_isCancelled == YES) 
    { 
     NSLog(@"** OPERATION CANCELED **"); 
    } 
} 

- (void)performTask 
{ 
    //The task is performed here 
    sleep(0.1); //This is just for demonstration purposes 
} 
@end 

我在viewDidLoad方法定義的塊完成此的NSOperation主視圖控制器：

- (void)viewDidLoad { 

NSLog(@"viewDidLoad!"); 
[super viewDidLoad]; 

static int counter = 0; 


_frameConvOP = [[AVFrameConversionOP alloc] init]; //Initializing the modified NSOperation class 
__weak typeof(self) weakSelf = self; 
_frameConvOP.completionBlock =^{ 
    counter++; 
    //NSLogs don't seem to work here 
    /*if(counter>1){ 
     exit(-1); //Never happens because counter only ever becomes =1 
    }*/ 

}; 
[[VideoPreviewer instance] setNSOperation:_frameConvOP]; //This is where I send the NSOperation object to the class where I'm using it 

} 

我開始的NSOperation做[frameConvOP start]內一個循環。每次幀被解碼時都會被連續調用，這就是說它經常被調用。我可以看到，NSOperation的finish和start方法正在通過日誌頻繁調用。而且這些方法中的函數也被正確調用。但是，阻塞完成內部的NSLogs不會被記錄。我在完成塊中放置了一個退出（-1），並且應用程序崩潰了，但NSLogs從不出現。所以我把一個靜態計數器放在那裏，它只增加到1.所以完成塊只被調用一次。

任何人都可以解釋爲什麼完成塊只被調用一次？是否因爲我（錯誤地）處理_isFinished和_isExecuting的KVO？當_isFinished設置爲YES時，應該調用完成塊，並且據我所知，我正在完成塊中執行該操作。結束塊被調用的頻率很高，因爲它應該是。

任何幫助，指導或方向表示讚賞。請讓我知道，如果我有任何錯誤，或者如果我在這篇文章中不太清楚。

感謝，

Answer 1

從NSOperation class reference：

的操作對象爲單次對象，也就是說，它一旦執行其任務，不能用於再次執行。

您不允許在單個操作對象上多次調用-start。

另一件事，在您的-finish方法中，您需要正確嵌套調用-willChange...和-didChange...。也就是說，你需要調用-didChange...在-willChange...調用的相反順序：

[self willChangeValueForKey:@"isExecuting"]; 
[self willChangeValueForKey:@"isFinished"]; 

_isExecuting = NO; 
_isFinished = YES; 

[self didChangeValueForKey:@"isFinished"]; 
[self didChangeValueForKey:@"isExecuting"];