iOS下载图像时的活动指示器

Question

我要下载一组图像来创建动画。我如何使下载开始时显示活动指示器和结束时隐藏活动指示器。

self.img = [[UIImageView alloc] initWithFrame:CGRectMake(0, 65, self.view.frame.size.width, self.view.frame.size.width-70)];

        img.animationImages = [NSArray arrayWithObjects:
                                   [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
                                   [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
                                   [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
                                   [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],


        img.animationDuration = 20.0f;
        img.animationRepeatCount = 0;
        [img startAnimating];
        [self.view addSubview: img];

Answer 1

您的图像正在同步下载，这意味着您正在阻塞主线程（这很糟糕）。相反，在后台线程上运行下载，这将允许主线程以及应用程序的UI继续运行。使用您的示例代码，以下是在后台运行时的样子：

self.img = [[UIImageView alloc] initWithFrame:CGRectMake(0, 65, self.view.frame.size.width, self.view.frame.size.width-70)];
self.img.animationDuration = 20.0f;
self.img.animationRepeatCount = 0;
[self.img startAnimating];
[self.view addSubview:self.img];
// START ANIMATING YOUR ACTIVITY INDICATOR HERE
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
    NSArray *images = @[
        [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
        [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
        [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
        [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"URL"]]],
    ];
    dispatch_async(dispatch_get_main_queue(), ^(void) {
        // STOP ANIMATING YOUR ACTIVITY INDICATOR HERE
        [self.img setAnimationImages:images];
    });
});

Answer 2

在下载开始时编写以下代码：UIActivityIndicatorView*微调器=[[UIActivityIndicatorView alloc]initWithActivityIndirectorStyle:UIActivityIndecatorViewStyleGray]；spinner.frame=CGRectMake（0，0，320，50）；[spinner startAnimationing]；

下载结束时编写以下代码：[spinner stopAnimating]；