在深入研究 NSPasteboard 并调试应用程序后，我意识到 Photos.app 在剪贴板中传递了“承诺的文件”，并在苹果邮件列表中找到了这个线程的一些答案：http://prod.lists.apple.com/archives/cocoa-dev/2015/Apr/msg00448.html
以下是我最终解决问题的方法，在处理将文件拖放到文档中的类中。该类是一个视图控制器，处理常规的拖放方法，因为它在响应链中。
一个方便的方法检测拖动发送者是否有任何与文件相关的内容：
- (BOOL)hasFileURLOrPromisedFileURLWithDraggingInfo:(id <NSDraggingInfo>)sender
{
NSArray *relevantTypes = @[@"com.apple.pasteboard.promised-file-url", @"public.file-url"];
for(NSPasteboardItem *item in [[sender draggingPasteboard] pasteboardItems])
{
if ([item availableTypeFromArray:relevantTypes] != nil)
{
return YES;
}
}
return NO;
}
如果不是 "承诺的文件"，我还有一种提取URL的方法：
- (NSURL *)fileURLWithDraggingInfo:(id <NSDraggingInfo>)sender
{
NSPasteboard *pasteboard = [sender draggingPasteboard];
NSDictionary *options = [NSDictionary dictionaryWithObject:@YES forKey:NSPasteboardURLReadingFileURLsOnlyKey];
NSArray *results = [pasteboard readObjectsForClasses:[NSArray arrayWithObject:[NSURL class]] options:options];
return [results lastObject];
}
这里最后介绍一下处理拖放事件的方法。这不是完全精确的代码，因为我简化了内部的拖放处理，并将其转换为方便的方法，以便隐藏应用程序特定部分。我还有一个专门处理文件系统事件的类FileSystemEventCenter，留给读者作为练习。此外，在这里呈现的情况下，我只处理一个文件的拖动。您需要根据自己的情况调整这些部分。
- (NSDragOperation)draggingEntered:(id <NSDraggingInfo>)sender
{
if ([self hasFileURLOrPromisedFileURLWithDraggingInfo:sender])
{
[self updateAppearanceWithDraggingInfo:sender];
return NSDragOperationCopy;
}
else
{
return NSDragOperationNone;
}
}
- (NSDragOperation)draggingUpdated:(id <NSDraggingInfo>)sender
{
return [self draggingEntered:sender];
}
- (void)draggingExited:(id <NSDraggingInfo>)sender
{
[self updateAppearanceWithDraggingInfo:nil];
}
- (void)draggingEnded:(id <NSDraggingInfo>)sender
{
[self updateAppearanceWithDraggingInfo:nil];
}
- (BOOL)prepareForDragOperation:(id <NSDraggingInfo>)sender
{
return [self hasFileURLOrPromisedFileURLWithDraggingInfo:sender];
}
- (BOOL)performDragOperation:(id <NSDraggingInfo>)sender
{
// promised URL
NSPasteboard *pasteboard = [sender draggingPasteboard];
if ([[pasteboard types] containsObject:NSFilesPromisePboardType])
{
// promised files have to be created in a specific directory
NSString *tempPath = [NSTemporaryDirectory() stringByAppendingPathComponent:[[NSUUID UUID] UUIDString]];
if ([[NSFileManager defaultManager] createDirectoryAtPath:tempPath withIntermediateDirectories:NO attributes:nil error:NULL] == NO)
{
return NO;
}
// the files will be created later: we keep an eye on that using filesystem events
// `FileSystemEventCenter` is a wrapper around FSEvent
NSArray *filenames = [sender namesOfPromisedFilesDroppedAtDestination:[NSURL fileURLWithPath:tempPath]];
DLog(@"file names: %@", filenames);
if (filenames.count > 0)
{
self.promisedFileNames = filenames;
self.directoryForPromisedFiles = tempPath.stringByStandardizingPath;
self.targetForPromisedFiles = [self dropTargetForDraggingInfo:sender];
[[FileSystemEventCenter defaultCenter] addObserver:self selector:@selector(promisedFilesUpdated:) path:tempPath];
return YES;
}
else
{
return NO;
}
}
// URL already here
NSURL *fileURL = [self fileURLWithDraggingInfo:sender];
if (fileURL)
{
[self insertURL:fileURL target:[self dropTargetForDraggingInfo:sender]];
return YES;
}
else
{
return NO;
}
}
- (void)promisedFilesUpdated:(FDFileSystemEvent *)event
{
dispatch_async(dispatch_get_main_queue(),^
{
if (self.directoryForPromisedFiles == nil)
{
return;
}
NSString *eventPath = event.path.stringByStandardizingPath;
if ([eventPath hasSuffix:self.directoryForPromisedFiles] == NO)
{
[[FileSystemEventCenter defaultCenter] removeObserver:self path:self.directoryForPromisedFiles];
self.directoryForPromisedFiles = nil;
self.promisedFileNames = nil;
self.targetForPromisedFiles = nil;
return;
}
for (NSString *fileName in self.promisedFileNames)
{
NSURL *fileURL = [NSURL fileURLWithPath:[self.directoryForPromisedFiles stringByAppendingPathComponent:fileName]];
if ([[NSFileManager defaultManager] fileExistsAtPath:fileURL.path])
{
[self insertURL:fileURL target:[self dropTargetForDraggingInfo:sender]];
[[FileSystemEventCenter defaultCenter] removeObserver:self path:self.directoryForPromisedFiles];
self.directoryForPromisedFiles = nil;
self.promisedFileNames = nil;
self.targetForPromisedFiles = nil;
return;
}
}
});
}
苹果在10.12版本中使用NSFilePromiseReceiver使这个过程变得更加简单了。虽然仍然是一个有点复杂的过程，但已经简化了一些。
这是我处理它的方法。实际上，我将其拆分为扩展，但我已经为这个示例进行了简化。
override func performDragOperation(_ sender: NSDraggingInfo) -> Bool {
let pasteboard: NSPasteboard = sender.draggingPasteboard()
guard let filePromises = pasteboard.readObjects(forClasses: [NSFilePromiseReceiver.self], options: nil) as? [NSFilePromiseReceiver] else {
return
}
var images = [NSImage]()
var errors = [Error]()
let filePromiseGroup = DispatchGroup()
let operationQueue = OperationQueue()
let newTempDirectory: URL
do {
let newTempDirectory = (NSTemporaryDirectory() + (UUID().uuidString) + "/") as String
let newTempDirectoryURL = URL(fileURLWithPath: newTempDirectory, isDirectory: true)
try FileManager.default.createDirectory(at: newTempDirectoryURL, withIntermediateDirectories: true, attributes: nil)
}
catch {
return
}
filePromises.forEach({ filePromiseReceiver in
filePromiseGroup.enter()
filePromiseReceiver.receivePromisedFiles(atDestination: newTempDirectory,
options: [:],
operationQueue: operationQueue,
reader: { (url, error) in
if let error = error {
errors.append(error)
}
else if let image = NSImage(contentsOf: url) {
images.append(image)
}
else {
errors.append(PasteboardError.noLoadableImagesFound)
}
filePromiseGroup.leave()
})
})
filePromiseGroup.notify(queue: DispatchQueue.main,
execute: {
// All done, check your images and errors array
})
}
[Error Domain=NSCocoaErrorDomain Code=3072 "The operation was cancelled."] 错误。 - Alex*** Assertion failure in -[NSFilePromiseReceiver receivePromisedFilesAtDestination:options:operationQueue:reader:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/AppKit/AppKit-1561.20.106/AppKit.subproj/NSFilePromiseReceiver.m:329 - Mark Bridges
hasFileURLOrPromisedFileURLWithDraggingInfo似乎已被弃用，但我找不到解决方案... Promises API在我看来非常古老且非常不稳定，所以我目前的“解决方案”是放弃对它的支持... - Sam