UIActivityIndicatorView不旋转

Question

在向UIScrollView添加UIActivityIndicatorView时，我遇到了这个问题；一切正常，除非滚动UIScrollView，否则它不会开始旋转。

有人能帮我解决这个问题吗？

谢谢。

下面是一些代码：

    UIActivityIndicatorView *loaderActivity = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    loaderActivity.center = CGPointMake(87/2,y+56/2);
    loaderActivity.tag=tag;
    [mainScrollView addSubview:loaderActivity];
    [loaderActivity startAnimating];
    [loaderActivity release];

Answer 1

您需要在活动指示器上调用startAnimating以使其具有动画效果。或者，在界面构建器中，你可以勾选“动画”复选框。

Answer 2

只有在滚动视图中滚动时才会显示动画，这说明您对startAnimating的调用是在后台线程中发生的。UIKit调用应该在主线程中进行。

您可以通过添加如下代码来验证它是否在后台线程中发生：

if ([NSThread isMainThread]) {
    NSLog(@"Running on main thread.");
} else {
    NSLog(@"Running on background thread.");
}

您需要确保问题中显示的所有代码都在主线程上运行。为此，您可以将代码更改为如下所示：

// this code would be wherever your existing code was
[self performSelectorOnMainThread:@selector(addActivityIndicatorToView:) withObject:mainScrollView waitUntilDone:YES];

// this would be a new method in the same class that your existing code is in
- (void) addActivityIndicatorToView:(UIView*) view {
    UIActivityIndicatorView *loaderActivity = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    loaderActivity.center = CGPointMake(87/2,y+56/2);
    loaderActivity.tag=tag;
    [view addSubview:loaderActivity];
    [loaderActivity startAnimating];
    [loaderActivity release];
}

Answer 3

activityIndicator = [[UIActivityIndicatorView alloc] 
initWithFrame:CGRectMake(87/2,y+56/2);
[activityIndicator setActivityIndicatorViewStyle:UIActivityIndicatorViewStyleWhite];
activityIndicator.tag=tag;
[mainScrollView addSubview:loaderActivity];
[activityIndicator startAnimating];
[activityIndicator release];