blocks|key|1378756|text|因为NSURL可以代表比本地文件系统更多的文件系统，所以我不认为有一种通用的方法可以可靠地测试它们的存在。至少，Cocoa基金会不包含这样的功能(据我所知)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1378757|如果您只处理本地文件系统，我建议您为NSURL或NSFileManager创建一个带有urlExists:消息的类别。它会将NSURL转换为NSString+(标准化路径)，然后调用[NSFileManager+fileExistsAtPath:]消息。|offset|length|style|CODE|1378758|entityMap^0|0|I|5|O|D|17|A|1Q|5|1Y|8|2J|X|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]|$D|P|E|Q|F|G]|$D|R|E|S|F|G]|$D|T|E|U|F|G]|$D|V|E|W|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|I|$]]

Because NSURL can represents more that local file-systems, I don't think that there is a generic method that can test for their existence in a reliable way. At least, the Cocoa foundation does not contains such a function (as far as I know).

If you only deal with local file-systems, I suggest you to create a category for <code>NSURL</code> or for <code>NSFileManager</code>, with a <code>urlExists:</code> message. It would convert the <code>NSURL</code> to a <code>NSString</code> (normalized path) and then invoke the <code>[NSFileManager fileExistsAtPath:]</code> message.

blocks|key|1163707|text|NSURL具有以下方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1163708|-+(BOOL)checkResourceIsReachableAndReturnError:(NSError+**)error|code-block|syntax|javascript|1163709|它“返回文件URL指向的资源是否可以访问”。|1163710|NSURL+*theURL+=+[NSURL+fileURLWithPath:@"/Users/elisevanlooij/nonexistingfile.php"+
+++++++++++++++isDirectory:NO];
NSError+*err;
if+([theURL+checkResourceIsReachableAndReturnError:&err]+==+NO)
++++[[NSAlert+alertWithError:err]+runModal];|1163711|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

NSURL does have this method:

<pre><code>- (BOOL)checkResourceIsReachableAndReturnError:(NSError **)error
</code></pre>

Which "Returns whether the resource pointed to by a file URL can be reached."

<pre><code>NSURL *theURL = [NSURL fileURLWithPath:@"/Users/elisevanlooij/nonexistingfile.php" 
 isDirectory:NO];
NSError *err;
if ([theURL checkResourceIsReachableAndReturnError:&amp;err] == NO)
 [[NSAlert alertWithError:err] runModal];
</code></pre>

blocks|key|1163737|text|在iOS上我找不到其他方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1163738|NSURL+*storeURL+=+[[self+applicationDocumentsDirectory]+URLByAppendingPathComponent:@"file.type"];
if+([[NSFileManager+defaultManager]+fileExistsAtPath:[storeURL+path]])+{...}|code-block|syntax|javascript|1163739|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

On iOS I couldn't find any other way...

<pre><code>NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"file.type"];
if ([[NSFileManager defaultManager] fileExistsAtPath:[storeURL path]]) {...}
</code></pre>

blocks|key|1378865|text|在Swift中，您可以使用checkResourceIsReachable()方法，不幸的是，该方法要么返回true+(如果文件可访问)，要么抛出错误(解释无法访问的原因)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1378866|要获取布尔值true/false，请使用以下语法：|1378867|let+exists+=+(try?+inputFile.checkResourceIsReachable())+??+false|code-block|syntax|javascript|1378868|如果您想要记录此错误：|1378869|let+exists:+Bool
do+{
++exists+=+try+inputFile.checkResourceIsReachable()
}+catch+{
++exists+=+false
++print(error.localizedDescription)
}|1378870|请记住，这是一个昂贵的操作，并且它可能会立即过期(如果某个其他进程正在删除或卸载一个磁盘文件，而您正在检查它是否存在)。|1378871|一般来说，首选的方法不是检查文件是否存在，而是简单地尝试读取或写入文件，并在失败后处理任何错误。|1378872|entityMap^0|D|Q|1I|4|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|16|8|@]|D|@]|E|$]]|$1|U|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|V|$]]

In Swift you can use the <code>checkResourceIsReachable()</code> method, which unfortunately will either return <code>true</code> (if the file is reachable) or throw an error (explaining why it cannot be reached).

To get a bool true/false value instead, use this syntax:

<pre><code>let exists = (try? inputFile.checkResourceIsReachable()) ?? false
</code></pre>

If you'd like to log the error:

<pre><code>let exists: Bool
do {
 exists = try inputFile.checkResourceIsReachable()
} catch {
 exists = false
 print(error.localizedDescription)
}
</code></pre>

Keep in mind this is an expensive operation and it could be out of date immediately after (if some other process is deleting a or unmounts a disk file while you're checking if it exists).

In general the preferred approach is not to check wether a file exists, instead simply attempt to read or write to a file and handle any error afterwards if it fails.

Snow Leopard introduced many new methods to use NSURL objects to refer to files, not pathnames or Core Services' FSRefs.

However, there's one task I can't find a URL-based method for: Testing whether a file exists. I'm looking for a URL-based version of <a href="http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSFileManager_Class/Reference/Reference.html#//apple_ref/occ/instm/NSFileManager/fileExistsAtPath:" rel="noreferrer">-[NSFileManager <code>fileExistsAtPath:</code>]</a>. Like that method, it should return <code>YES</code> if the URL describes anything, whether it's a regular file, a directory, or anything else.

I could attempt to look up various <a href="http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html#//apple_ref/occ/instm/NSURL/resourceValuesForKeys:error:" rel="noreferrer">resource values</a>, but none of them are explicitly guaranteed to not exist if the file doesn't, and some of them (e.g., <a href="http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html#//apple_ref/doc/c_ref/NSURLEffectiveIconKey" rel="noreferrer">NSURLEffectiveIconKey</a>) could be costly if it does.

I could just use NSFileManager's <code>fileExistsAtPath:</code>, but if there's a more modern method, I'd prefer to use that.

Is there a simple method or function in Cocoa, CF, or Core Services that's guaranteed/documented to tell me whether a given file (or file-reference) URL refers to a file-system object that exists?

Testing file existence using NSURL

文件系统

Swift

雪豹引入了许多新的方法来使用NSURL对象来引用文件，而不是路径名或核心服务的FSRefs。但是，有一项任务我找不到基于URL的方法:测试文件是否存在。我在找一个基于网址的版本。与该方法类似，如果URL描述了任何内容，无论是常规文件、目录还是其他内容，它都应该返回YES。我可以尝试查找各种，但如果文件不存在，它们中的任...

问使用NSURL测试文件是否存在
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