ios - 检查服务器上的文件是否已更新

Question

在我的应用程序中，在启动时，我检查数据库是否已在服务器上更改并且副本存储在本地。我正在使用对服务器的同步请求，并且正在根据HTTP响应中的上次修改日期时间字段进行检查

如果服务器上文件的最后修改日期时间>本地文件的最后修改日期时间，请问用户是否要更新数据库，如果他接受，我将下载数据库。

我将机器用作服务器，但是关闭机器时出现问题，启动时应用程序崩溃

谢谢你的帮助

您将在我的代码下面找到

#import "FirstViewController.h"

@interface FirstViewController ()

@end

@implementation FirstViewController


- (void)viewDidLoad
{
    [super viewDidLoad];



    // check connectivity
    if ([[Reachability reachabilityForInternetConnection] currentReachabilityStatus] == NotReachable) {
        [self displayConenctivityAlert];

    }else{

        [self checkDatabases];

    }



}




- (void) checkDatabases {


    bool res = [self fileUpdated];


    if (res){
        // Ask user if he would like to update the databases    
    }

}




-(void) displayConenctivityAlert{
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:[TSLanguageManager localizedString:@"NO_CONNECTED"] delegate:self cancelButtonTitle:[TSLanguageManager localizedString:@"OK"] otherButtonTitles:nil];

    [alert show];
}



- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
    NSLog(@"Error HTTP ...");
}




- (BOOL)fileUpdated {
    NSString *urlString = @"http://192.168.0.10:8888/fuel/stations.db";
    NSLog(@"Downloading HTTP header from: %@", urlString);
    NSURL *url = [NSURL URLWithString:urlString];


    //store locally data into the resource folder.
    NSString *documentsDirectory = [Utility documentsPath];



    NSString *cachedPath = [documentsDirectory stringByAppendingPathComponent:@"stations.db"];
    NSLog(@"Local URL / %@", cachedPath);
    NSFileManager *fileManager = [NSFileManager defaultManager];

    BOOL downloadFromServer = NO;
    NSString *lastModifiedString = nil;
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [request setHTTPMethod:@"HEAD"];
    NSHTTPURLResponse *response;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error: NULL];
    if ([response respondsToSelector:@selector(allHeaderFields)]) {
        lastModifiedString = [[response allHeaderFields] objectForKey:@"Last-Modified"];
    }

    NSDate *lastModifiedServer = nil;
    @try {
        NSDateFormatter *df = [[NSDateFormatter alloc] init];
        df.dateFormat = @"EEE',' dd MMM yyyy HH':'mm':'ss 'GMT'";
        df.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
        df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
        lastModifiedServer = [df dateFromString:lastModifiedString];
    }
    @catch (NSException * e) {
        NSLog(@"Error parsing last modified date: %@ - %@", lastModifiedString, [e description]);
    }

    NSLog(@"lastModifiedServer: %@", lastModifiedServer);

    NSDate *lastModifiedLocal = nil;
    if ([fileManager fileExistsAtPath:cachedPath]) {
        NSError *error = nil;
        NSDictionary *fileAttributes = [fileManager attributesOfItemAtPath:cachedPath error:&error];
        if (error) {
            NSLog(@"Error reading file attributes for: %@ - %@", cachedPath, [error localizedDescription]);
        }
        lastModifiedLocal = [fileAttributes fileModificationDate];
        NSLog(@"lastModifiedLocal : %@", lastModifiedLocal);
    }

    // Download file from server if we don't have a local file
    if (!lastModifiedLocal) {
        downloadFromServer = YES;
    }
    // Download file from server if the server modified timestamp is later than the local modified timestamp
    if ([lastModifiedLocal laterDate:lastModifiedServer] == lastModifiedServer) {
        downloadFromServer = YES;
    }

    return downloadFromServer;
}

@end

Answer 1

您的应用崩溃是因为完成didFinishLaunching花费的时间太长，系统监视程序杀死了您的应用。这是因为您正在用根 View Controller 的viewDidLoad发出同步http请求，该请求必须先完成，然后才能完成启动。您可以通过多种方式解决此问题，或者通过在NSURLConnection上调用sendAsynchronousRequest:queue:completionHandler异步执行HTTP请求。另一个选择是将此代码移出启动管道，也许是通过将代码移至 View Controller 的viewDidAppear来实现。这具有对 View 的每次返回执行检查的副作用，而不仅仅是初始加载。

简而言之，执行同步HTTP请求非常重要，因为您的UI会在请求完成之前挂起。在这种情况下，这特别糟糕，因为您将强制启动推迟到请求完成之前，这会导致服务器关闭时失败。