修复我的网络活动指示器

Question

我的网络活动指示器有问题,有时它会在不应该显示时继续显示.

我为它编写了我自己的经理并将其换成了一个使用这样的NSAssert语句的人...

- (void)setNetworkActivityIndicatorVisible:(BOOL)setVisible {
    static NSInteger NumberOfCallsToSetVisible = 0;
    if (setVisible)
        NumberOfCallsToSetVisible++;
    else
        NumberOfCallsToSetVisible--;

    // The assertion helps to find programmer errors in activity indicator management.
    // Since a negative NumberOfCallsToSetVisible is not a fatal error,
    // it should probably be removed from production code.
    NSAssert(NumberOfCallsToSetVisible >= 0, @"Network Activity Indicator was asked to hide more often than shown");

    // Display the indicator as long as our static counter is > 0.
    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:(NumberOfCallsToSetVisible > 0)];
}

我在SO上发现了它,它立即指出我使用这个功能出了问题.

我的所有网络活动都是通过一个NSOperationQueue由单例类管理的单独运行.每个操作都是NSOperation的子类(实际上是TemplateOperation的子类,它是NSOperation的子类).

无论如何,所有的下载和上传工作正常,我这样做就像这样......

- (void)sendRequest:(NSURLRequest *)request
{
    NSError *error = nil;
    NSURLResponse *response = nil;

    [[NetworkManager sharedInstance] setNetworkActivityIndicatorVisible:YES];
    NSData *data = [NSURLConnection sendSynchronousRequest:request
                                         returningResponse:&response
                                                     error:&error];
    [[NetworkManager sharedInstance] setNetworkActivityIndicatorVisible:NO];

    // other stuff...

    [self processData:data];
}

重要的线路是在我NSURLConnection同步发送之前和之后.

在我发送请求之前,我将网络活动指示器设置为可见(使用我的经理类),然后立即将其设置为不可见.

除了NSAssert指出,某些地方这种情况没有发生.

难道从多个线程运行此函数可能会导致问题吗？我该怎么解决这个问题？

Answer 1

整数递增或递减不是线程安全的(据我所知),因此如果两个线程"同时"调用您的方法,则计数可能无法正确更新.

一种解决方案是在方法中添加一些同步指令(例如@synchronized).或者您使用原子递增/递减函数:

#include <libkern/OSAtomic.h>

- (void)setNetworkActivityIndicatorVisible:(BOOL)setVisible {
    static volatile int32_t NumberOfCallsToSetVisible = 0;
    int32_t newValue = OSAtomicAdd32((setVisible ? +1 : -1), &NumberOfCallsToSetVisible);

    NSAssert(newValue >= 0, @"Network Activity Indicator was asked to hide more often than shown");
    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:(newValue > 0)];
}