依次打开几个UIDocument并等待它们完成 [英] Opening Several UIDocuments in Succession and Waiting for Them to Finish

问题描述

我无法理解UIDocuments异步打开的方式.我有一系列用户创建的UIDocuments.当用户按下按钮时，我需要打开一些文档，从文档中拉一些字符串，然后将这些字符串连接到一个新文件中.

I'm having trouble getting my head around the way UIDocuments open asynchronously. I have a series of UIDocuments that the user has created. When the user presses a button I need to open some of the documents, pull some strings from them, and concatenate the strings into a new file.

我有一个数组，用于跟踪需要打开哪些文档.我的想法是我应该遍历数组，打开每个文档，并将其内容附加到NSMutableString.问题在于openWithCompletionHandler是异步进行的，因此循环在文件被打开(并再次关闭)之前前进并完成，返回一个空字符串.

I have an array that keeps track of which documents need to be opened. My thought was that I should loop through the array, open each document, and append its contents to an NSMutableString. The problem is that openWithCompletionHandler does its work asynchronously, so the loop marches ahead and finishes, returning an empty string, before the documents have been opened (and closed again).

这里有一些代码:

__block NSMutableString *combinedString;

for (entryClass *entry in documentList)
{
    MyDocumentClass *document = [[MyDocumentClass alloc] initWithFileURL:[entry fileURL]];

    if ([document documentState] & UIDocumentStateClosed) {
        [document openWithCompletionHandler:^(BOOL success) {

            if (success) {
                [combinedString appendString:[document documentBody]];
            }

            [document closeWithCompletionHandler:nil];
        }];
    }
}

NSLog(@"Combined String: %@", combinedString);

自然会发生什么情况，因为文档在后台打开时循环结束，所以combinedString返回为空.我可以将文档处理移至它自己的返回字符串的方法，但是我认为在读取文档之前，这仍然会返回.我大概必须设置一个进度指示器并让用户等待-可能没关系.

Naturally what happens is that combinedString comes back empty because the loop finishes while the documents are opening in the background. I could move the document handling to its own method that returns a string but I think this will still return before the document is read. I'll presumably have to put up a progress indicator and make user wait--that's probably okay.

推荐答案

您可以对调度组执行此操作.请参见低级并发API .

You can do this with dispatch groups. See the sections "Groups" and "Using dispatch_group_t with Existing API" at Low-Level Concurrency APIs.

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