AVAssetExportSession progress KVO

在使用AVAssetExportSession的

exportAsynchronouslyWithCompletionHandler接口时有个progress属性，打算用KVO去实现监听 

static void *ExportProcess = &ExportProcess;

[_exporter addObserver:self forKeyPath:@"progress" options:NSKeyValueObservingOptionNew context:ExportProcess];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{
    
    
    if (context == ExportProcess) {
        
        CGFloat process = [change[NSKeyValueChangeNewKey] floatValue];
        CGFloat oldProcess = [change[NSKeyValueChangeOldKey] floatValue];
        
        NSLog(@"the process is %f", process);
        NSLog(@"the oldProcess is %f", oldProcess);
    }
    
}

但是突然发现

This property is not key-value observable 这个 

process不支持 KVO 那只能用定时器监听了或者RCA