下载开始时，writeToFile卡住了

Question

当我使用Objective C函数writeToFile从url下载一些文件时，我的iPhone和iPhone模拟器卡住了，直到下载文件为止如何防止这种情况？

NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:url]];
if(data){
    NSString *saveStringPath = [NSString stringWithFormat:@"/%@/%@.mp3", stringPath, songNameField.text];
    BOOL status = [data writeToFile:saveStringPath atomically:NO];}