如何从 Web 服务器更新 JSON 文件

Question

我在 json 文件中有我的本地数据，例如：

 {
  "locations": [
    {
      "title": "The Pump Room",
      "place": "Bath",
      "latitude": 51.38131,
      "longitude": -2.35959,
      "information": "The Pump Room Restaurant in Bath is one of the city’s most elegant places to enjoy stylish, Modern-British cuisine.",
      "telephone": "+44 (0)1225 444477",
      "visited" : true
    },
    {
      "title": "The Eye",
      "place": "London",
      "latitude": 51.502866,
      "longitude": -0.119483,
      "information": "At 135m, the London Eye is the world’s largest cantilevered observation wheel. It was designed by Marks Barfield Architects and launched in 2000.",
      "telephone": "+44 (0)8717 813000",
      "visited" : false
    },
    {
      "title": "Chalice Well",
      "place": "Glastonbury",
      "latitude": 51.143669,
      "longitude": -2.706782,
      "information": "Chalice Well is one of Britain's most ancient wells, nestling in the Vale of Avalon between the famous Glastonbury Tor and Chalice Hill.",
      "telephone": "+44 (0)1458 831154",
      "visited" : true
    }
  ]
}

只要点击刷新按钮，我想更新网络服务器上的 json 文件吗？

总体思路是从服务器刷新本地数据并在没有互联网连接的情况下使用它

请帮忙...

Answer 1

最合适的解决方案取决于您的具体要求。例如：

• 您需要身份验证吗？
• 是否需要在后台模式下加载 JSON？
• 您的 JSON 是否很大，以至于将其加载到内存中对系统来说不是那么好？
• 您是否需要取消正在运行的请求，因为它可能会停止或耗时过长？
• 您是否需要同时并行加载多个请求？

还有更多。

如果您可以用“否”回答所有要求，那么最简单的方法就足够了：

你可以在异步版本中使用NSURLConnection的便捷类方法

+ (void)sendAsynchronousRequest:(NSURLRequest *)request queue:(NSOperationQueue *)queue completionHandler:(void (^)(NSURLResponse*, NSData*, NSError*))handler

您可以在此处找到更多信息： Using NSURL Connection

此外，Cocoa 和 Cocoa Touch 在NSURLConnection 和NSURLSession 中提供了更多先进技术来完成这项任务。您可以在官方文档URL Loading System Programming Guide阅读更多内容。

这里有一个如何使用异步便利类方法的简短示例：sendAsynchronousRequest:queue:completionHandler:：

// Create and setup the request
NSMutableURLRequest* urlRequest = [NSURLRequest requestWithURL:url];
[urlRequest setValue: @"application/json; charset=utf-8" forHTTPHeaderField:@"Accept"];

// let the handler execute on the background, create a NSOperation queue:
NSOperationQueue* queue = [[NSOperationQueue alloc] init];

[NSURLConnection sendAsynchronousRequest:urlRequest 
                                   queue:queue  
                       completionHandler:^(NSURLResponse* response, 
                                                  NSData* data, 
                                                 NSError* error)
{
    if (data) {        
        // check status code, and optionally MIME type
        if ( [(NSHTTPURLResponse*)(response) statusCode] == 200 /* OK */) {
            NSError* error;
            // here, you might want to save the JSON to a file, e.g.:
            // Notice: our JSON is in UTF-8, since we explicitly requested this 
            // in the request header:
            if (![data writeToFile:path_to_file options:0 error:&error]) {
                [self handleError:err];  // execute on main thread!
                return;
            }
           
            // then, process the JSON to get a JSON object:
            id jsonObject = [NSJSONSerialization JSONObjectWithData:data 
                                                            options:0 
                                                              error:&error];
            ... // additionally steps may follow
            if (jsonObject) {
                // now execute subsequent steps on the main queue:
                dispatch_async(dispatch_get_main_queue(), ^{
                    self.places = jsonObject;
                });
            }
            else {
                [self handleError:err];  // execute on main thread!
            }                
        }
        else {
             // status code indicates error, or didn't receive type of data requested
             NSError* err = [NSError errorWithDomain:...];
             [self handleError:err];  // execute on main thread!
        }                     
    }
    else {
        // request failed - error contains info about the failure
        [self handleError:error]; // execute on main thread!
    }        
}];

另见：[NSData] writeToURL:options:error

编辑：

handleError: 应按如下方式实现：

- (void) handlerError:(NSError*)error 
{
    dispatch_async(dispatch_get_main_queue(), ^{
        [self doHandleError:error];
    });
}

这可以确保，例如，当您显示 UIAlertView 时，您的 UIKit 方法将在主线程上执行。