无法从NSCachesDirectory访问XML文件

Question

我在 NSCachesDirectory 中有一个xml文件，当我尝试访问该文件时，如果要从NSBundle访问该文件，则得到零值，而同一个文件则正常工作。屏幕截图和代码片段下方-

 // load the data associated to the trackers
- (bool) doLoadTrackersData {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *cacheDirectory = [paths objectAtIndex:0];
    NSString *bookCode = [[NSUserDefaults standardUserDefaults] valueForKey:@"bookCode"];
    NSURL *url = [[NSURL fileURLWithPath:cacheDirectory] URLByAppendingPathComponent:[NSString stringWithFormat:@"%@/%@/%@%s",bookCode,bookCode,bookCode,".xml"]];
    NSLog(@"The path for xml ImageTarget is  = %@",url.absoluteString);
    dataSetStonesAndChips = [self loadObjectTrackerDataSet:url.absoluteString];
    dataSetTarmac = [self loadObjectTrackerDataSet:@"Tarmac.xml"];
    if ((dataSetStonesAndChips == NULL) || (dataSetTarmac == NULL)) {
        NSLog(@"Failed to load datasets");
        return false;
    }
    if (! [self activateDataSet:dataSetStonesAndChips]) {
        NSLog(@"Failed to activate dataset");
        return false;
    }
    return true;
}

The path for xml ImageTarget is = file:///var/mobile/Containers/Data/Application/55240D59-8811-4A35-867F-D648197FB611/Library/Caches/AHW2/AHW2/AHW2.xml

本地访问xml文件的代码，并且可以正常工作-

// load the data associated to the trackers
- (bool) doLoadTrackersData {
    dataSetStonesAndChips = [self loadObjectTrackerDataSet:@"AHW2.xml"];
    dataSetTarmac = [self loadObjectTrackerDataSet:@"Tarmac.xml"];
    if ((dataSetStonesAndChips == NULL) || (dataSetTarmac == NULL)) {
        NSLog(@"Failed to load datasets");
        return false;
    }
    if (! [self activateDataSet:dataSetStonesAndChips]) {
        NSLog(@"Failed to activate dataset");
        return false;
    }
    return true;
}

loadObjectTrackerDataSet -

的方法

// Load the image tracker data set
- (Vuforia::DataSet *)loadObjectTrackerDataSet:(NSString*)dataFile
{
    NSLog(@"loadObjectTrackerDataSet (%@)", dataFile);
    Vuforia::DataSet * dataSet = NULL;

    // Get the Vuforia tracker manager image tracker
    Vuforia::TrackerManager& trackerManager = Vuforia::TrackerManager::getInstance();
    Vuforia::ObjectTracker* objectTracker = static_cast<Vuforia::ObjectTracker*>(trackerManager.getTracker(Vuforia::ObjectTracker::getClassType()));

    if (NULL == objectTracker) {
        NSLog(@"ERROR: failed to get the ObjectTracker from the tracker manager");
        return NULL;
    } else {
        dataSet = objectTracker->createDataSet();

        if (NULL != dataSet) {
            NSLog(@"INFO: successfully loaded data set");

            // Load the data set from the app's resources location
            if (!dataSet->load([dataFile cStringUsingEncoding:NSASCIIStringEncoding], Vuforia::STORAGE_APPRESOURCE)) {
                NSLog(@"ERROR: failed to load data set");
                objectTracker->destroyDataSet(dataSet);
                dataSet = NULL;
            }
        }
        else {
            NSLog(@"ERROR: failed to create data set");
        }
    }

    return dataSet;
}