无法在mapview上显示UIActivityIndi​​cator

Question

在我的代码中，我初始化了一个UIActivityIndi​​cator，并将其添加为子视图，但它从未出现过。出了什么问题？

-(void) update {

    UIActivityIndicatorView *activityView=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    activityView.center=self.view.center;
    [self.view addSubview:activityView];
    [activityView startAnimating];

    /** Stuff to  access to server, this stuff works correctly.*/
    [activityView stopAnimating];
}

Answer 1

由于您starting和stoping活动与runloop相同，因此更新UI不会发生变化。您正在全力以赴server stuff在同一个synchronously上阻止runLoop并且没有给activity指标启动时间。您应该在下一个runLoop启动服务器内容，这样活动指标就会有时间开始使用performSelector，延迟时间为0.0，它会启动activity indicator而不是立即执行服务器内容runloop而不是停止activityInddicator

-(void) update {

    UIActivityIndicatorView *activityView=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    activityView.center=self.view.center;
    [self.view addSubview:activityView];
    [activityView startAnimating];
    //Use performSelector with 0.0 so activity indicator starts  
    [self performSelector:@selector(thisWillStartAtNextRunLoop) withObject:nil afterDelay:0.0];
}

-(void)thisWillStartAtNextRunLoop{

    /** Stuff to  access to server, this stuff works correctly.*/
    [activityView stopAnimating];
}

修改在main thread执行，因为所有的UI更新都必须在main thread上 试试

-(void) update {
  dispatch_async(dispatch_get_main_queue(), ^{

    UIActivityIndicatorView *activityView=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    activityView.center=self.view.center;
    [self.view addSubview:activityView];
    [activityView startAnimating];
 });
    /** Stuff to  access to server, this stuff works correctly.*/
}