TweetFollow Us on Twitter

The Ins and Outs of Drag and Drop

Volume Number: 18 (2002)
Issue Number: 11
Column Tag: Mac OS X

The Ins and Outs of Drag and Drop

by Andrew C. Stone

The most copied feature of the Mac OS X interface is the ubiquitous drag and drop. When NeXTStep advanced the techniques pioneered at Xerox's Palo Alto Research Center in the late '80's, the way in which people interacted with software was changed forever. The ability to move data and objects seamlessly between windows and applications without any additional steps is the hallmark of a native OS X application. This article will explore some more advanced techniques and some of the issues you might encounter when preparing your interface for drag and drop, as covered in other MacTech articles, such as http://www.stone.com/The_Cocoa_Files/What_a_Drag_.html. We'll cover making an entire window a receptacle for drag and drop, using central control to reduce code, dealing with temporary subviews such as field editors, and auto-swapping Tab views based on the type of data being dropped.

Many of Stone Design's applications fit into the category of "just drag and drop and you're done," such as PStill, GIFfun, PackUpAndGo, DOCtor and SliceAndDice. Taking PStill as an example, the user just drags a file onto the PStill window or application tile in the Dock or the Finder to convert the file to or redistill it as PDF:



Drag files onto Dock or Window

The strategy I like to employ is to make the entire window a valid drag target by subclassing NSWindow or NSPanel, and forwarding the actual methods to the window's delegate:

@interface NSObject(implement_this_in_delegate)
- (void)registerTypesForPanel:(NSPanel *)panel;
@end
@interface SDDragInPanel : NSPanel
{}
@end
@implementation SDDragInPanel
- (void)awakeFromNib
{
   [[self delegate] registerTypesForPanel:self];
}
- (unsigned int) draggingEntered:sender
{
    return [[self delegate] draggingEntered:sender];
}
- (unsigned int) draggingUpdated:sender
{
    return [[self delegate] draggingUpdated:sender];
}
- (BOOL) prepareForDragOperation:sender
{
        return [[self delegate] prepareForDragOperation:sender];
}
- (BOOL) performDragOperation:(id <NSDraggingInfo>)sender
{
    return [[self delegate] performDragOperation:sender];
}
@end

Typical code for delegate would be:

// Dragging stuff:
- (NSArray *)acceptableDragTypes{
    return [NSArray arrayWithObjects:NSFilenamesPboardType,nil];
}
- (void)registerTypesForPanel:(NSPanel *)panel;
{
   [panel registerForDraggedTypes:[self acceptableDragTypes]];
}
- (unsigned int)draggingEnteredOrUpdated:(id <NSDraggingInfo>)sender {
    // we want to ignore drags originating in our own window:
    if ([sender draggingSource] == dragWellView) return NSDragOperationNone;
    else {
        unsigned int sourceMask = [sender draggingSourceOperationMask];
        NSPasteboard *pboard = [sender draggingPasteboard];
        NSString *type = [pboard availableTypeFromArray:[self acceptableDragTypes]];
        if (type) return sourceMask;
        return NSDragOperationNone;
    }
}
- (unsigned int)draggingEntered:(id <NSDraggingInfo>)sender
{
    return [self draggingEnteredOrUpdated:sender];
}
- (unsigned int)draggingUpdated:(id <NSDraggingInfo>)sender
{
    return [self draggingEnteredOrUpdated:sender];
}
- (BOOL)performDragOperation:(id <NSDraggingInfo>)sender
{
    NSPasteboard *pboard = [sender draggingPasteboard];
    NSString *type = [pboard availableTypeFromArray:[self acceptableDragTypes]];
    BOOL loaded = NO;
    id ts = nil;
    if (type) {
        if ([type isEqualToString:NSFilenamesPboardType]) {
            NSArray *files = [pboard propertyListForType:NSFilenamesPboardType];
            unsigned i = [files count];
            while (i-- > 0) {
                NSString *f = [files objectAtIndex:i];
                if ([[self acceptableFileTypes] containsObject:[f pathExtension]] && 
                (ts = [SomeObject objectWithContentsOfFile:f])!=nil) {
                    loaded = YES;
          break;
      }
            }
        }
     }
    return loaded;
}
- (BOOL)prepareForDragOperation:sender
{
    return YES;
}
Be sure to register the view for the accepted types. A good place to do this is in 
-(void)awakeFromNib. This method is called on any object instantiated in a NIB (NeXT Interface 
Builder) file that has an implementation of awakeFromNib after all the objects are created and linked 
up, but before the window appears on screen. By implementing a single method acceptableDragTypes that 
returns which types you actually accept, you can avoid out-of-synch code when you add more types to 
open later:

   [panel registerForDraggedTypes:[self acceptableDragTypes]];

So we are done, right? Not quite, because of the way NSTextFields work. When you click or tab into a text field, a shared NSTextView is inserted into the view hierarchy. When the user drags a file over any part of the window that doesn't have an active textfield, the draggingEntered works as planned. But when you pass over the active text field, the NSTextView's drag validation methods come into play. The solution is to subclass NSTextView to also forward the methods to your window's delegate - or just to the window, since the window will forward on to the delegate:

#import <Cocoa/Cocoa.h>
@interface DragTextView : NSTextView
{
}
@end
#import "DragTextView.h"
@implementation DragTextView
// override drag stuff...
- (id)initWithFrame:(NSRect)r
{
    [super initWithFrame:r];
    [self registerForDraggedTypes:[[[self window] delegate] acceptableDragTypes]];
   // this is so TAB and RETURN end editing
   // instead of being inserted into the field:
    [self setFieldEditor:YES];  
    return self;
}
// note we just pass it up to the window:
- (unsigned int) draggingEntered:sender
{
    return [[self window] draggingEntered:sender];
}
... etc, just passing on the method to the window
@end

Now we have our custom text view, but how do we make sure our text view is used in place of the standard text view? We can't set z in Interface Builder, but we can code it. If a window's delegate implements a method called -windowWillReturnFieldEditor:(NSWindow *)sender toObject:(id)client, the Appkit code will call this method and use the text view it returns if non-nil, otherwise it uses a standard text view set in field editor mode.)

// add textView as  an iVar to the NSWindowController subclass which controls the window
- (id)windowWillReturnFieldEditor:(NSWindow *)sender toObject:(id)client {
    if (sender == [self window]) {
        if (!textView) textView = [[DragTextView alloc]initWithFrame:[myField bounds]];
        return textView;
    }
    return nil;
}

At this point, our interface is ready to accept the correct files and data at any location in the window.

There's one final issue: what if a user can drag a file out of your interface (for example, in PStill, you can drag the distilled PDF file out of the "drag out well") and that file type can also be dragged in to the application (for example, PStill accepts PDF files as input). A user might start a drag out of the application, change her mind, and drop the file back onto the application window. In this case, the application should probably not process the file. Therefore, the window delegate should check the draggingSource to make sure it's not a component of the window itself. This is why we have this line in the draggingEnteredOrUpdated code above:

    if ([sender draggingSource] == dragWellView) return NSDragOperationNone;

Autoswapping Tab Views

The concept of filtering the dragging methods through the window's delegate can be very useful when your window contains an NSTabView with different acceptable types in each view. In Create(R), for example, there is a resources library which can accept art, images, effects, blends, patterns and pages:


Create(R) lets you store many different types of resources - and it will swap to the correct tab view as necessary

Each of these tabviews has an NSScrollView, which contains an NSMatrix. When a user 
drags in a certain type that is not correct for the current view, but is acceptable in another one of 
the tab views, the tab view should automatically switch to the other view so that the drag can drop 
successfully in the right place. We do this by first checking if we can deal with it - and if not, 
we'll ask the window controller (which keeps track of the other views) to check the other resource 
managers. Note we also don't want to accept drags that start from this particular resource's matrix:

- (unsigned int)draggingEntered:(id <NSDraggingInfo>)sender
{
   return [self draggingEnteredOrUpdated:sender checkOthers:YES];
}
- (unsigned int)draggingUpdated:(id <NSDraggingInfo>)sender
{
    return [self draggingEnteredOrUpdated:sender checkOthers:YES];
}
- (unsigned int)draggingEnteredOrUpdated:(id <NSDraggingInfo>)sender 
checkOthers:(BOOL)checkOthers
{
    if ([sender draggingSource] == dragMatrix) return NSDragOperationNone;
    else {
        NSPasteboard *pboard = [sender draggingPasteboard];
        NSString *type = [pboard availableTypeFromArray:[self acceptableDraggedTypes]];
        if (type) {
            unsigned int sourceMask = [sender draggingSourceOperationMask];
            if ([type isEqualToString:NSFilenamesPboardType]) {
                NSArray *filenames = [pboard propertyListForType:NSFilenamesPboardType];
                if ([filenames count] == 1) {
                    NSString *filename = [filenames objectAtIndex:0];
                    if ([[[self resourceClass] fileTypes] containsObject:
         [filename pathExtension]])
                     return sourceMask;
                }
            } else return sourceMask;
        }
    }
    if (checkOthers) return [_controller draggingEnteredOrUpdated:sender];
    else return NSDragOperationNone;
}

The _controller's implementation might look something like this:

- (unsigned int)draggingEnteredOrUpdated:(id <NSDraggingInfo>)sender
{
    int i, c = [_resourceSources count];
    unsigned int returnValue;
    for (i = 0; i < c; i++) {
        ResourceSource *res = [_resourceSources objectAtIndex:i];
   if (res == _currentSource) continue;   // already checked!
        if ((returnValue = [res draggingEnteredOrUpdated:sender checkOthers:NO]) 
        != NSDragOperationNone) {
            [self showResourceSourceNamed:[res resourceSourceName]];
       return returnValue;
        }
    }
    return NSDragOperationNone;
}
- (void)showResourceSourceNamed:(NSString *)name
{
    [tabView selectTabViewItemAtIndex:[tabView indexOfTabViewItemWithIdentifier:name]];
}

Because the matrix may not fill the scroll view entirely, we'll also have to subclass the scroll view to forward draggingEntered methods to the matrix. To the end user, the entire scroll view is seen as the target, not just the matrix!

- (unsigned int)draggingEntered:(id <NSDraggingInfo>)sender
{
    return [[self documentView]draggingEntered:sender];
}

... etc. for all the other methods.

Application Tile Drag Support

You only have to perform a few tasks to add support for drag and drop to your Application icon and its Dock tile. First, you'll need to alert the system of the valid file types handled by your application. Then, you'll implement a method in the Application's delegate subclass which calls the actual method to deal with that file type.

First, add information about which files can be opened by your application in Project Builder's application Target Inspector, in the "Document Types" pane:


Be sure to add the file types that your application can open in Project Builder

Second, set your application's delegate. You can do this programmatically with NSApplication's setDelegate:. Or, you can use Interface Builder: (a) instantiate an object of your delegate class in your main NIB file, and (b) connect the File's Owner instance variable "delegate" to this new object.

Third, implement a single method in your delegate's class:

- (BOOL)application:(NSApplication *)sender openFile:(NSString *)path
{
   MyDocument *doc = [[NSDocumentController sharedDocumentController] 
   openDocumentWithContentsOfFile:path display:YES];
   return doc;
}

Now, not only will the dock tile accept drag and drop, but Finder will display your application as a choice for opening that kind of document.

Conclusion

If you want your application to really sing, be sure users can take full advantage of drag and drop everywhere! Not only will it make program interaction easier and more fun, it makes demoing the app more spectacular!


Andrew Stone is founder, janitor and chief computer scientist at Stone Design, www.stone.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Chromium 45.0.2454.85 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 45.0.2454.85: Note: Does not contain the "... Read more
OmniFocus 2.2.5 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
iFFmpeg 5.7.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
VOX 2.6 - Music player that supports man...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
Box Sync 4.0.6567 - Online synchronizati...
Box Sync gives you a hard-drive in the Cloud for online storage. Note: You must first sign up to use Box. What if the files you need are on your laptop -- but you're on the road with your iPhone? No... Read more
Carbon Copy Cloner 4.1.4 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
OmniGraffle Pro 6.3.1 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
Monosnap 3.1.2 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs! Capture: Capture full screen, just part of the screen, or a selected window Make your crop area pixel perfect with... Read more
Alfred 2.7.2 - Quick launcher for apps a...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
Microsoft Remote Desktop 8.0.19 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more

Goat Simulator MMO Simulator (Games)
Goat Simulator MMO Simulator 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** Coffee Stain Studios brings next-gen... | Read more »
Worms™ 4 (Games)
Worms™ 4 1.02 Device: iOS Universal Category: Games Price: $4.99, Version: 1.02 (iTunes) Description: The latest instalment in the worldwide mega hit franchise! Coming soon to iPhone, iPad and iPod touch. When the guys and girls at... | Read more »
The Deer God (Games)
The Deer God 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: 30% off launch sale!!! “It can be a struggle, but it's all worth it when you're shooting fire out your antlers.” Kotaku “The... | Read more »
AppSpy's Patreon campaign kicks off
Occasionally you'll see us use AppSpy's videos here on 148Apps to support an article we've written. That's because we're part of Steel Media, and AppSpy is Steel's video arm, so we're all part of one happy family. [Read more] | Read more »
We're Sorry to Report that Moonrise...
Moonrise is a very promising-looking, Pokemon-esque monster collecting and battling game that we were really looking forward to reviewing, but unfortunately it looks like that's never going to happen. [Read more] | Read more »
The Latest Update for The Sims FreePlay...
Commerce has gotten a little more active with the newest update for The Sims FreePlay, making Sunset Mall more of a hangout than ever before. [Read more] | Read more »
This Week at 148Apps: August 24-28, 2015
The Apps of August With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out... | Read more »
NASCAR in Real Racing 3? Sure, Why Not?
I have to give Firemonkeys credit - it's very cool of them to add NASCAR to Real Racing 3 via an update rather than making a separate game for it. But that's a different discussion for another time; for now let's sit back and enjoy driving in... | Read more »
The nuyu is an Inexpensive Activity Moni...
Today, Health o Meter nuyu has announced a series of health and fitness-related products, including the aforementioned activity monitor along with a wireless scale. All at a decent pricepoint, no less. [Read more] | Read more »
The Makers of Overkill are Trying Someth...
Craneballs, the studio responsible for the Overkill series, is taking a little break from all that violence (a little break) to bring us Cube Worm - a 3D take on one of the most classic PC/calculator games in existence. [Read more] | Read more »

Price Scanner via MacPrices.net

Will You Buy An iPad Pro? – The ‘Book Mystiqu...
It looks like we may not have to wait much longer to see what finally materializes as a new, larger-panel iPad (Pro/Plus?) Usually reliable Apple product prognosticator KGI Securities analyst Ming-... Read more
eFileCabinet Announces SMB Document Managemen...
Electronic document management (EDM) eFileCabinet, Inc., a hosted solutions provider for small to medium businesses, has announced that its SecureDrawer and eFileCabinet Online services will be... Read more
WaterField Designs Unveils American-Made, All...
San Francisco’s WaterField Designs today unveiled their all-leather Cozmo 2.0 — an elegant attach laptop bag with carefully-designed features to suit any business environment. The Cozmo 2.0 is... Read more
Apple’s 2015 Back to School promotion: Free B...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
128GB MacBook Airs on sale for $100 off MSRP,...
B&H Photo has 11″ & 13″ MacBook Airs with 128GB SSDs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799.99, $100 off MSRP... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Apple and Cisco Partner to Deliver Fast-Lane...
Apple and Cisco have announced a partnership to create a “fast lane” for iOS business users by optimizing Cisco networks for iOS devices and apps. The alliance integrates iPhone with Cisco enterprise... Read more
Apple offering refurbished 2015 13-inch Retin...
The Apple Store is offering Apple Certified Refurbished 2015 13″ Retina MacBook Pros for up to $270 (15%) off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple refurbished 2015 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2015 11″ and 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-year warranty is included with... Read more

Jobs Board

Simply Mac *Apple* Specialist- Repair Techn...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
Simply Mac- *Apple* Specialist- Store Manag...
Simply Mac is the largest premier retailer for Apple products and solutions. We're looking for dedicated individuals with a passion to simplify and enhance the Read more
*Apple* Evangelist - JAMF Software (United S...
The Apple Evangelist is responsible for building and cultivating strategic relationships with Apple 's small and mid-market business development field teams. This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.