GCD12 :创建并发线程 和 调用后台线程

问题:

你想在程序中运行单独的任务时,拥有最大的控制权。例如,你想要根据用户要求,来运行一个长计算请求,同时,主线程 UI 可以自由的与用户交互和做别的事情。

讨论:

在程序中使用线程:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSString *fileToDownload = @"http://www.OReilly.com";
    [NSThread detachNewThreadSelector:@selector(downloadNewFile:) toTarget:self withObject:fileToDownload];
}
- (void)downloadNewFile:(id)paramObject{
    @autoreleasepool {
        NSString *fileURl = (NSString *)paramObject;
        NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:fileURl]];
        NSURLResponse *response = nil;
        NSError *error = nil;
        NSData *downloadData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
        if ([downloadData length] > 0) {
            NSLog(@"下载成功");
        }else{
            NSLog(@"下载失败");
        }
    }
}

在应用程序中,无论你使用多线程或不使用,至少有 1 个 线程被创建。该线程叫做”main UI 线程”,被附加到主事件处理循环中(main run loop)。

每一个线程都需要创建一个 autorelease pool,当做是该线程第一个被创建的对象。如果不这样做,当线程退出的时候,你分配在线程中的对象会发生内存泄露。

二  调用后台线程

问题:

你想知道一个最简单的方法,来创建一个线程,而不需要直接处理线程。

方案:

使用 NSObject 的实例方法 performSelectorInBackground:withObject:

例子:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    [self performSelectorInBackground:@selector(firstCounter) withObject:nil];
    [self performSelectorInBackground:@selector(secondCounter) withObject:nil];
    [self performSelectorInBackground:@selector(thirdCounter) withObject:nil];
    return YES;
}
- (void)firstCounter{
    @autoreleasepool{
        NSUInteger counter = 0;
        for (counter = 0;counter < 1000;counter++){
            NSLog(@"First Counter = %lu", (unsigned long)counter);
        }
    }
}
- (void) secondCounter{
    @autoreleasepool {
        NSUInteger counter = 0;
        for (counter = 0;counter < 1000;counter++){
            NSLog(@"Second Counter = %lu", (unsigned long)counter); }
    }
}
- (void) thirdCounter{
    @autoreleasepool {
        NSUInteger counter = 0;
        for (counter = 0;counter < 1000;counter++){
            NSLog(@"Third Counter = %lu", (unsigned long)counter); }
    }
}

输出为

GCD_Demo5[2507:351136] Third Counter = 0
GCD_Demo5[2507:351136] Third Counter = 1
GCD_Demo5[2507:351136] Third Counter = 2
GCD_Demo5[2507:351136] Third Counter = 3
GCD_Demo5[2507:351134] First Counter = 0
GCD_Demo5[2507:351136] Third Counter = 4
GCD_Demo5[2507:351136] Third Counter = 5

performSelectorInBackground:withObject:方法为我们在后台创建了一个线程。这等同于我们为 selectors 创建一个新的线程。最重要的事情是我们必须记住通过此方法为 selector 创 建的线程,selector 必须有一个 autorelease pool,就想其它线程一样,在引用计数内存环境中。