ios - 在主线程上执行 dispatch_async 后 UI 没有立即更新？

Question

这是我的代码:

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    for (int i = 0; i < size; i++)
    {

        CGRect frame;
        frame.origin.x = self.scrollView.frame.size.width * i;
        frame.size = self.scrollView.frame.size;

        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@"%@%@x%@%@",
                                                        // some web service data

        UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageWithData:data]];

        UIView *subview = [[UIView alloc] initWithFrame:frame];

        imageView.frame = CGRectMake(self.view.frame.origin.x, self.view.frame.origin.y, self.view.bounds.size.width, self.view.bounds.size.height);
        [subview addSubview:imageView];
        [self.dataViewsArray addObject:subview];
        NSLog(@"dwonload bacgraound");

    }

    dispatch_async(dispatch_get_main_queue(), ^(void){
        [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
        NSLog(@"starting some UI updates");
        [self makeScrollPagingView];


    });
});

和 [self makeScrollPagingView] - 只是将 subview 添加到我的 ScrollView 中？
我的代码完成后有 10 秒的延迟。然后更新ui。请帮忙

Answer 1

更快的解决方案是不同时加载图像，而是并行加载:

for (int i = 0; i < size; i++) {
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@""]]];
        CGDataProviderRef provider = CGDataProviderCreateWithCFData((CFDataRef)data);
        CGImageRef imageRef = CGImageCreateWithPNGDataProvider(provider, NULL, false, kCGRenderingIntentDefault); // assuming PNG format, JPG is also available
        CGDataProviderRelease(provider);

        dispatch_async(dispatch_get_main_queue(), ^(void){
            UIImage *image = [UIImage imageWithCGImage:imageRef];
            CGImageRelease(imageRef);

            UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
            // Layout and position of just this UIImageView
        });
    });
}

但是，根据您的用途，这可能是以牺牲可读性/简单性为代价的过早优化。在主线程上创建 UIImage 可能就足够了。

for (int i = 0; i < size; i++) {
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@""]]];

        dispatch_async(dispatch_get_main_queue(), ^(void){
            UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageWithData:data]];
            // Layout and position of just this UIImageView
        });
    });
}

您是否布局代码不依赖于一次加载所有图像。就像 Sashcha 所说，永远不要在后台线程中编写与 UI 相关的代码。

一些 UIImageView 库或类别可能会简化您需要编写的代码。 AFNetworking是一个很好的开始，它有 UIImageView+AFNetworking你应该看看。