Mac OS 开发 为什么这样写不行

"NSTask *capture = [[NSTask alloc] init];

capture.launchPath = @"/usr/sbin/screencapture";

capture.arguments = [NSArray arrayWithObjects:@"-i",@"-c", nil];

[capture setTerminationHandler: ^(NSTask *t) {

NSPasteboard *pboard = [NSPasteboard generalPasteboard];

if ([pboard pasteboardItems].count <= 0) return ;

NSPasteboardItem *pboardItem = [[pboard pasteboardItems] objectAtIndex:0];

NSString *pboardItemType = [[pboard types] objectAtIndex:0];

NSData *imageData = [pboardItem dataForType:pboardItemType];

NSImage *image = [[NSImage alloc] initWithData:imageData];

__weak __typeof(self) weakself= self;

dispatch_async(dispatch_queue_create(0, 0), ^{

dispatch_async(dispatch_get_main_queue(), ^{

weakself.iconView.image = image;

});

});

}];

[capture launch]; "

这样写报错 screencapture: cannot run two interactive screen captures at a time

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