后台线程混乱

Question

我试图提高我的应用程序的响应性，但我是一个完全的新手线程和已经变得困惑。

在启动时显示一个警告:

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    if (buttonIndex == 0)
    {
        NSString *user = [[alertView textFieldAtIndex:0] text];
        NSString *pass = [[alertView textFieldAtIndex:1] text];
        [self loginToServerWithUsername:user andPassword:pass];
    }
}

在loginToServerWithUsername:方法中，应用程序调用一个方法:

[self checkFiles:sessionID];

这可能需要几秒钟，所以我尝试在后台执行:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                        [self checkFiles:sessionID];
                    });

checkFiles方法:

fileList = [[NSMutableString alloc] init];
NSArray *directoryPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [directoryPaths objectAtIndex:0];
NSString *downloadsFolderString = [documentsDirectory stringByAppendingPathComponent:DOWNLOADS_FOLDER];
NSError *error = nil;
NSString* file;
NSDirectoryEnumerator* enumerator = [[NSFileManager defaultManager] enumeratorAtPath:downloadsFolderString];
while (file = [enumerator nextObject])
{
    BOOL isDirectory = NO;        
    [[NSFileManager defaultManager] fileExistsAtPath: [NSString stringWithFormat:@"%@/%@",downloadsFolderString,file]
                                         isDirectory: &isDirectory];
            if (!isDirectory)
            {
                [fileList appendString:[NSString stringWithFormat:@"%@|", file]];
            }
}
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"dd/MM/yyyy HH:mm"];
NSString *timeOpened = [formatter stringFromDate:[NSDate date]];
NSString *post = [NSString stringWithFormat:@"sessionID=%@&fileList=%@&dateTime=%@&userID=%@", sessionID, fileList, timeOpened, userID];
NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
NSString *postLength = [NSString stringWithFormat:@"%d", [postData length]];
NSString *comparisonURLString = SERVER_COMPARE_URL_STRING;
NSURL *comparisonURL = [NSURL URLWithString:comparisonURLString];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:comparisonURL];
[request setHTTPMethod:@"POST"];
[request addValue:postLength forHTTPHeaderField:@"Content-Length"];
[request setHTTPBody:postData];
NSHTTPURLResponse *urlResponse = nil;
error = nil;
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error];
if (responseData)
{
    NSString *requiredFilesList = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
    NSArray *lines = [requiredFilesList componentsSeparatedByString: @"n"];        
    if (lines.count > 2)
    {
        dispatch_async(dispatch_get_main_queue(), ^(void){
            NSRange theRange;
            theRange.location = 2;
            theRange.length = [lines count] -3;
            numberOfFilesToBeDownloaded = theRange.length;
            if (numberOfFilesToBeDownloaded <= 0)
            {
                _jobStatusLabel.text = @"Documents up to date"; // is this the issue?
            }            
            if (numberOfFilesToBeDownloaded > 0)
            {
                NSArray *subArray = [lines subarrayWithRange:theRange];
                [self animateProgressBar];
                if (![eraseDevice isEqualToString:@"true"])
                {
                [self getFiles:subArray];
                }
            }
        });
    }
}
else
{
    NSLog(@"no response data from check files");
}

然而，直到checkFiles方法完成后，alertView才被解除。谁能告诉我如何有警报解散，而checkFiles在后台运行?

Answer 1

UI操作应该在主线程中完成。

例如:

_jobStatusLabel.text = @"Documents up to date"; // is this the issue?

应为

dispatch_async(dispatch_get_main_queue(), ^(void){
    _jobStatusLabel.text = @"Documents up to date"; // is this the issue?
});

Answer 2

显然你正在从后台线程更新UI元素。这将导致不可预测的行为。一个更好的方法是这样做-

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
     [self checkFiles:sessionID];
     dispatch_async(dispatch_get_main_queue(),^{
         //breakup your checkFiles method to background tasks & UI tasks. Put UI updating 
         //code here which will get executed in the main thread.
     });
});

这是编码的GCD模式。这里是异步线程同步执行代码。也就是说，checkFiles完成后，下一个代码(这里是一个异步线程)被执行。

你需要打破checkFiles方法。在后台线程中执行HTTP部分，并将解析后的responseData传递给主UI线程。之后更新UI。