缺少委托阻止调用将使应用程序崩溃。在使用@协议时,是否可以声明类似@可选的块?

我正在为可拖动对象编写类。

平移完成后,我想使用块来委派它。 它工作得很好,但在使用@协议时,我应该如何设置@可选之类的内容? 因为如果我不调用myObject.didEndPanning应用程序崩溃。

// MyClass.h

#import <Foundation/Foundation.h>

typedef void (^DelegateBlock)(void);

@interface AHDraggableObject : UIView

- (id)initDragableView:(UIView *)view inView:(UIView *)parentView withBorderOffset:(int)offset;

@property (nonatomic, strong) UIView *holder;
@property (nonatomic, weak) DelegateBlock didEndPanning;

@end

// MyClass.m

- (void)handlePanning:(UIPanGestureRecognizer *)sender
{
    CGPoint translation = [sender translationInView:_parentView];
    sender.view.center = CGPointMake(sender.view.center.x + translation.x, sender.view.center.y + translation.y);
    [sender setTranslation:CGPointMake(0, 0) inView:_parentView];

    [self checkBondaries:sender withOffset:_offset inView:_parentView];

    //  end of pan - invoke event
    if (sender.state == UIGestureRecognizerStateEnded)
    {
        _didEndPanning();
    }
}

呼叫

//ViewController.m
-(void)viewDidLoad {
    _drag = [[MRShape alloc] initDragableView:_testImageView inView:self.view withBorderOffset:40];
    _drag.didEndPanning = ^
    {
     // if didEndPanning call is missing, app crash !
    };
}