mouseDown方法中出现异常

我正在用NSView做拖放操作。

@try {
    NSPoint location; 
    NSSize size ;
    NSPasteboard *pb = [NSPasteboard pasteboardWithName:@"CameraIconContainer"];

    location.x =  ([self bounds].size.width - size.width)/2 - 21.0;
    location.y =  ([self bounds].size.height - size.height)/2 - 7.0;

    NSLog(@"mouseDown: location- (%f, %f)",location.x,location.y);


    NSDictionary *iconViewDict = [[NSDictionary alloc] initWithObjectsAndKeys:[cameraNo stringValue],@"cameraNo",nil];
    NSLog(@"iconViewDict - %@",iconViewDict);

    NSData *data = [NSKeyedArchiver archivedDataWithRootObject:iconViewDict];


    [pb declareTypes:[NSArray arrayWithObject:IconDragDataType] owner:self];
    [pb setData:data forType:IconDragDataType];
    [self dragImage:[NSImage imageNamed:@"camera_icon.png"] at:location offset:NSZeroSize event:e pasteboard:pb source:self slideBack:YES];
}
@catch (NSException * e) {
    NSLog(@"CameraIconView (-mouseDown:), error - %@",e);
}

大多数情况下,它运行良好,但问题是-有时它提出了这一点

exception:Invalid parameter 不

在mouseDown:方法中,由于它,拖动的图像会连续出现在屏幕上,即使选择了其他窗口,也不会消失。

有人能告诉我为什么会发生这种情况,我如何解决它吗?

谢谢,

米拉杰