Question

我有一个块来下载转换图像到UIImage。这是我的代码

-(UIImage *)GetProfileImage
{
NSString *strimgBaseUrl=[[[NSBundle mainBundle] infoDictionary] objectForKey:@"BaseImageURL"];
NSString *strFilePath=[dm.dictUserProfile valueForKey:@"ImagePath"];
NSString *strImgURL=[NSString stringWithFormat:@"%@%@",strimgBaseUrl,strFilePath];
__block UIImage *imgProf=nil;

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSURL *imageURL = [NSURL URLWithString:strImgURL];
    NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
    UIImage *image = [UIImage imageWithData:imageData];
    dispatch_async(dispatch_get_main_queue(), ^{

        imgProf= image;
        if(image!=nil){
            imgProf=image;

        }


    });
});



  return imgProf;
}

但是这个块永远不会执行。它会转到return __block UIImage *imgProf=nil;为什么会这样？请帮帮我。

由于

Answer 1

代码工作正常。这是dispatch_async的目的。

形成文档

@function dispatch_async

调用dispatch_async（）总是在提交块后立即返回，并且永远不会等待块被   调用

这与dispatch_sync相反。

您可以像这样更改GetProfileImage

- (void)setProfileImageForImageView:(UIImageView *)imageView {
    NSString *strimgBaseUrl=[[[NSBundle mainBundle] infoDictionary] objectForKey:@"BaseImageURL"];
    NSString *strFilePath=[dm.dictUserProfile valueForKey:@"ImagePath"];
    NSString *strImgURL=[NSString stringWithFormat:@"%@%@",strimgBaseUrl,strFilePath];
    __block UIImage *imgProf=nil;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSURL *imageURL = [NSURL URLWithString:strImgURL];
        NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
        UIImage *image = [UIImage imageWithData:imageData];
        dispatch_async(dispatch_get_main_queue(), ^{

            imgProf= image;
            if(image!=nil){
                // imgProf=image;
                imageView.image = image;
            }


        });
    });
}

Answer 2

改为使用完成块，而不是从函数返回：

-(void)GetProfileImageWithCompletion:(void (^)(UIImage *profileImage))completionHandler
{
NSString *strimgBaseUrl=[[[NSBundle mainBundle] infoDictionary] objectForKey:@"BaseImageURL"];
NSString *strFilePath=[dm.dictUserProfile valueForKey:@"ImagePath"];
NSString *strImgURL=[NSString stringWithFormat:@"%@%@",strimgBaseUrl,strFilePath];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSURL *imageURL = [NSURL URLWithString:strImgURL];
    NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
    UIImage *image = [UIImage imageWithData:imageData];
    dispatch_async(dispatch_get_main_queue(), ^{

        if(image!=nil && completionHandler){
            completionHandler(image);
        }
    });
});
}

并像这样使用：

[self GetProfileImageWithCompletion:^(UIImage *profileImage)
   {
      //if image exists assign to image view
   }
];

dispatch_async（dispatch_get_global_queue（DISPATCH_QUEUE_PRIORITY_DEFAULT，0）块不执行

2 个答案: