如何在 iOS 中取消完成处理程序

Posted 2023-02-23

【中文标题】如何在 iOS 中取消完成处理程序

【英文标题】：How to cancel a completion handler in iOS

【发布时间】：2015-07-30 08:46:20

【问题描述】：

我想使用 mapkit 框架获取附近位置的数组。因此，当用户在文本字段中键入时，我会调用以下函数。

- (void)searchForLocations:(NSString *)string

    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(search:) object:nil];
    [self performSelectorInBackground:@selector(search:) withObject:string];


- (void)search :(NSString *)string

    MKLocalSearchRequest *request = [[MKLocalSearchRequest alloc] init];
    request.naturalLanguageQuery = string;
    MKCoordinateRegion region;
    MKCoordinateSpan span;
    span.latitudeDelta = 0.05;
    span.longitudeDelta = 0.05;

    region.span = span;
    region.center = newLocation.coordinate;

    request.region = region;

    MKLocalSearch *search = [[MKLocalSearch alloc]initWithRequest:request];

    [search startWithCompletionHandler:^(MKLocalSearchResponse
                                         *response, NSError *error) 
        if (response.mapItems.count == 0)
        
            NSLog(@"No Matches");
        
        else
        

            NSLog(@"name = %@", item.name);
            NSLog(@"Phone = %@", item.phoneNumber);
        
    ];

如您所见，如果有新的输入文本出现，我想取消之前的搜索。但是之前的搜索并没有取消。如何取消之前的搜索？

提前致谢。

【参考方案1】：

在 MKLocalSearch 上有一个 cancel method。你试过那个吗？

编辑：啊，对不起，我是愚蠢的。您需要以某种方式保留对旧搜索的引用才能取消它。将其放在搜索完成时可以清除（即设置为 nil）的属性中。每当你调用搜索函数时，取消之前的搜索函数（不需要“if”，nil 吞没所有），然后创建一个新的

@property (nonatiomic, strong) MKLocalSearch *previousSearch;

- (void)search :(NSString *)string

    [self.previousSearch cancel];
    MKLocalSearchRequest *request = [[MKLocalSearchRequest alloc] init];
    request.naturalLanguageQuery = string;
    MKCoordinateRegion region;
    MKCoordinateSpan span;
    span.latitudeDelta = 0.05;
    span.longitudeDelta = 0.05;

    region.span = span;
    region.center = newLocation.coordinate;

    request.region = region;

    MKLocalSearch *search = [[MKLocalSearch alloc]initWithRequest:request];

    [search startWithCompletionHandler:^(MKLocalSearchResponse
                                     *response, NSError *error) 
        self.previousSearch = nil;
        if (response.mapItems.count == 0)
        
            NSLog(@"No Matches");
        
        else
        

            NSLog(@"name = %@", item.name);
            NSLog(@"Phone = %@", item.phoneNumber);
        
    ];

    self.previousSearch = search;

【讨论】：

我不明白为什么它会在我的案例中显示以前的搜索结果。

【参考方案2】：

MKLocalSearch 对象有一个cancel 方法，您可以使用该方法取消挂起的搜索。您不能像尝试那样简单地取消选择器，因为该选择器将很快完成，MKLocalSearch 在后台调度。

您将需要一个属性来存储您的搜索对象，以便您可以判断它是否仍在搜索并在需要时取消搜索。

@property (strong,nonatomic) MKLocalSearch *localSearch;

- (void)search :(NSString *)string

    MKLocalSearchRequest *request = [[MKLocalSearchRequest alloc] init];
    request.naturalLanguageQuery = string;
    MKCoordinateRegion region;
    MKCoordinateSpan span;
    span.latitudeDelta = 0.05;
    span.longitudeDelta = 0.05;

    region.span = span;
    region.center = newLocation.coordinate;

    request.region = region;

    if (self.localSearch!=nil) 
        if (self.localSearch.searching) 
            [self.localSearch cancel];
            self.localSearch=nil;
        
    

    self.localSearch = [[MKLocalSearch alloc]initWithRequest:request];

    [search startWithCompletionHandler:^(MKLocalSearchResponse
                                         *response, NSError *error) 
        if (response.mapItems.count == 0)
        
            NSLog(@"No Matches");
        
        else
        

            NSLog(@"name = %@", item.name);
            NSLog(@"Phone = %@", item.phoneNumber);
        
    ];

【参考方案3】：

你可以使用 NSBlockOperation。 假设您用于 NSBlockOperation 的全局变量是 blockOperation。然后：

[search startWithCompletionHandler:^(MKLocalSearchResponse
                                     *response, NSError *error) 
    blockOperation = NSBlockOperation(block:  () -> Void in
         if (response.mapItems.count == 0) 
            NSLog(@"No Matches");
          else 
            NSLog(@"name = %@", item.name);
            NSLog(@"Phone = %@", item.phoneNumber);
         
    )
];

然后你可以使用这个blockOperation来取消你想要的任何地方，如下所示：

blockOperation.cancel()