iOS版：dispatch_async和UIImageWriteToSavedPhotosAlbum [英] iOS: dispatch_async and UIImageWriteToSavedPhotosAlbum

问题描述

刚学如何分配任务的线程中，或异步分派。我明白了触及的任何操作视图，必须在主线程中完成的。什么： UIImageWriteToSavedPhotosAlbum ？ 我会假设这可能​​会在后台线程来完成，但是是我错了？

此外，如果它应该在后台线程完成，是否有这两个调用之间的差以下时，为一个保存一个UIImage和其他从视图可以节省一个UIImage？

  UIImageWriteToSavedPhotosAlbum（_someUIImage，无，无，无）;UIImageWriteToSavedPhotosAlbum（_imageView.image，无，无，无）;
 

这是我使用此设置在主线程，并在后台任务运行的HUD办法，那就是我的意图。

  [HUD_ code showMessage：@保存图像];
dispatch_queue_t concurrentQueue =
dispatch_get_global_queue（DISPATCH_QUEUE_PRIORITY_DEFAULT，0）;
dispatch_async（concurrentQueue，^ {
        UIImageWriteToSavedPhotosAlbum（someUIImage，无，无，无）;
            dispatch_async（dispatch_get_main_queue（），^ {
                [HUD_ code解雇];
         }）;
}）;
 

解决方案

<一个href=\"http://developer.apple.com/library/ios/#documentation/uikit/reference/UIKit_Framework/Introduction/Introduction.html#//apple_ref/doc/uid/TP40006955-CH1-SW2\"相对=nofollow> UIKit类都记录是从主线程可用的只有，除非另有说明。 （例如，<一个href=\"http://developer.apple.com/library/ios/#documentation/uikit/reference/UIFont_Class/Reference/Reference.html\"相对=nofollow> UIFont 被证明是线程安全的。）

有关于UIKit中的功能的线程安全性没有显式声明毯的（来自不同的班级），所以它不是安全的假设，他们一般是线程安全的。事实上，一些UIKit的功能，如<一个href=\"http://developer.apple.com/library/ios/#documentation/uikit/reference/UIKitFunctionReference/Reference/reference.html#//apple_ref/doc/uid/TP40006894\"相对=nofollow> UIGraphicsBeginImageContext ，明确记载是线程安全的，暗示的UIKit功能的不可以一般线程安全的。

由于 UIImageWriteToSavedPhotosAlbum 可发送异步完成的消息，你应该称呼它在主线程上，并使用其完成支持来执行 [HUD_ code驳回​​] 。

Just learning how to allocate tasks among threads, or dispatch asynchronously. I understand that any operation that "touches" a view must be done on the main thread. What about: UIImageWriteToSavedPhotosAlbum? I would assume this could be done on a background thread, but am I mistaken?

Also, if it should be done on a background thread, is there a difference between these two calls below, as one saves a UIImage and the other saves a UIImage from a view?

UIImageWriteToSavedPhotosAlbum(_someUIImage ,nil,nil,nil);

UIImageWriteToSavedPhotosAlbum(_imageView.image ,nil,nil,nil);

By the way I am using this setup to run an HUD in the main thread and to tasks in the background, that is my intention.

[HUD_code showMessage:@"saving image"];
dispatch_queue_t concurrentQueue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
        UIImageWriteToSavedPhotosAlbum(someUIImage ,nil,nil,nil);
            dispatch_async(dispatch_get_main_queue(), ^{
                [HUD_code dismiss];
         });
});

解决方案

UIKit classes are documented to be usable from the main thread only, except where documented otherwise. (For example, UIFont is documented to be thread-safe.)

There's no explicit blanket statement about the thread safety of UIKit functions (as distinct from classes), so it's not safe to assume they generally thread-safe. The fact that some UIKit functions, like UIGraphicsBeginImageContext, are explicitly documented to be thread-safe, implies that UIKit functions are not generally thread-safe.

Since UIImageWriteToSavedPhotosAlbum can send an asynchronous completion message, you should just call it on the main thread and use its completion support to perform your [HUD_code dismiss].

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