TweetFollow Us on Twitter

What a Drag Volume Number: 16 (2000)
Issue Number: 12
Column Tag: Mac OS X

What a Drag!

By Andrew C. Stone

Creating drag wells in Cocoa

The richness of the user interface in Macintosh OS X is due, in part, to the abundance of elements that allow dragging and dropping of data within and between applications. For example, Cocoa uses drag and drop for applying color swatches to text selections and objects. Stone Design's flagship application, Create[TM], makes heavy use of the drag and drop metaphor. A library of resources allows users to drag in components, pages, effects, blends, images, and patterns. The user can quickly save a selection of graphics to one of many image formats with Create's "Image Well". This article will cover the basics of creating a custom control which allows a user to drag in a file, and then allows the user to drag out the file.


Create's image well lets you drag out data in various image formats.

First, a bit of article administrivia. Mac OS X has been a fast moving target with the various developer releases, Public Beta, and soon OS X GM (for Golden Master which always reminds me of that Eddy Murphy movie about G)! Because of the changes introduced with each release and the several month lag time between penning articles and when MacTech hits the stands, there are times when what I've written doesn't jive with the current reality. To address this, I've added a section to the Stone Design website for MacTech article updates and errata. So if you get confused, check out www.stone.com/dev/MacTech_Errata/ to see if there are updates, and if not, email me so I can post fixes.

It's actually quite simple to create new types of user interface controls using Cocoa because of the well designed hierarchy of classes that comprise the Application framework. Your first job is to find an existing class that your control can inherit from, which will reduce the amount of code that you have to write. I keep /Developer/Documentation/Cocoa/cocoa.html open so I can quickly navigate to the AppKit or Foundation API's. But an even easier way to look at the object hierarchy is to use InterfaceBuilder. First launch ProjectBuilder and create a new Cocoa Application project named ImageWellTester. Click on the "Resources" triangle in the Files and Groups outline view, and double-click the MainMenu.nib. This will launch InterfaceBuilder.

Click on the "Classes" tab of the document window, and then explore the NSObject outline view:


Interface Builder lets you explore the hierarchy of objects offered by Cocoa.

Our drag well is definitely a subclass of NSView, but we can gain further functionality by subclassing NSControl, which gives us the target/action support. Select NSControl in the Classes pane. If you hold down the CONTROL key while clicking on NSControl, a context-sensitive menu appears. Do this and select "Subclass". A new class which inherits from NSControl is created and named, by default, MyControl. Rename the class to DragWell.

Why write code if you can have it written for you? You can use InterfaceBuilder to produce stub files where all you have to do is fill in the functionality in the stubbed out methods. Click on the "outlet" icon to add instance variables to the class, and the "+" icon to add actions to the class. For now, we don't have any to add. Be sure DragWell is selected, and choose select "Create Files..." from the Classes menu (or again using the context menu via CONTROL key). They will be named DragWell.m and DragWell.h - leave the "Insert into Project Builder" selected and click OK.

Drag out a "Custom View" from the IB palette window, second tab, onto to the main window. Choose Tools->Inspector->Custom Class popup button, and click on "DragWell". The custom view should now say "DragWell". Save the file.

Go back to ProjectBuilder, select the newly added DragWell.h and DragWell.m and you'll see the stub:

#import <Cocoa/Cocoa.h>

@interface DragWell : NSControl
{
}

@end
...
#import "DragWell.h"

@implementation DragWell

@end

Every custom view must define its own drawing method, drawRect:, where the actual drawing of the view takes place. We'll want a simple, depressed bezeled look, with an icon to drag in the center. And our control will adhere to two protocols - NSDraggingDestination and NSDraggingSource, to allow both dragging out of info and accepting dragged in info. A complete list of the methods required by the protocols are found in:

/System/Library/Frameworks/AppKit.framework/Headers/NSDragging.h 

Here's the 'well' commented code for the DragWell - type it in (or download it from www.stone.com/dev/MacTech/Jan-2001/). You can copy and paste it as a template for use in other objects, modifying where needed to copy other data types to the drag pasteboard as needed.

+++++++++++ DragWell.h ++++++++++++++++

#import <Cocoa/Cocoa.h>

@interface DragWell : NSControl
{
    NSImage *image;      // the image displayed and dragged by the user
    NSString *file;      // the file path that the user will drag out
    unsigned int last;   // for optimizing the draggingUpdated: method
}

// if you want to programmatically set a file in the well:
- (void)setFile:(NSString *)newFile;

@end

+++++++++++ DragWell.m ++++++++++++++++

#import "DragWell.h"

/* Example source by Andrew Stone andrew@stone.com www.stone.com for MacTech */

@implementation DragWell

// when our DragWell is reconstituted from the nib file, awakeFromNib gets called

- (void) awakeFromNib
{
     // this is where you register for various types of drag pasteboards:
     [self registerForDraggedTypes:[NSArray arrayWithObject:NSFilenamesPboardType]];
}

// Never add an ivar which allocates memory without releasing it in dealloc:
// Don't Litter!

- (void)dealloc {
    [file release];
    [image release];
    [super dealloc];
}

// Every custom view must implement drawRect:
// We'll draw our depressed bezeled rect
// and if there is a file, we'll draw an image for dragging out

- (void)drawRect:(NSRect)rects
{
    // one simple call draws the depressed bezel:
    NSDrawGrayBezel([self bounds] , [self bounds]);

    // if you like the pinstripes, then include these two lines:
    [[NSColor windowBackgroundColor] set];
    NSRectFill(NSInsetRect([self bounds],3.0,3.0));

    // find the center of the view and draw the image over:
    if (image != nil) {
        NSPoint p;
        NSSize sz = [image size];
        p.x = ([self bounds].size.width - sz.width)/2.;
        p.y = ([self bounds].size.height - sz.height)/2.;
        [image compositeToPoint:p operation:NSCompositeSourceOver];
    }
}

// When we set the file, we also set the image which we get from the NSWorkspace:

- (void)setFile:(NSString *)newFile {

    // it's good practice to not do any work if nothing would change:
    if (![newFile isEqualToString:file]) {
    
        // live clean and let your works be seen:
        [file release];
        [image release];
        
        // copy the newFile into our instance variable file:
        file = [newFile copyWithZone:[self zone]];
        
        // check if the file is nil or empty string:
        if (!file || [file isEqualToString:@""]) {
            image = nil;
        } else {
            // we have a file, now let's find the image for that file:
            image = [[[NSWorkspace sharedWorkspace] iconForFile:file]retain];
        }
        
        // we'll need to redraw next event 
        // this is the approved Cocoa way of setting ourselves "dirty"
        [self setNeedsDisplayInRect:[self bounds]];
    }
}

// here we copy the filename to the pasteboard
// I like to factor this out because it may vary from object to object:

- (BOOL)copyDataTo:pboard
{
    if (file != nil && ![file isEqualToString:@""]) {
        [pboard declareTypes:[NSArray arrayWithObject:NSFilenamesPboardType] owner:self];
        [pboard setPropertyList:[NSArray arrayWithObject:file] forType:NSFilenamesPboardType];
        return YES;
    } else return NO;
}

// DRAGGING SOURCE PROTOCOL METHODS
// To add drag away functionality to a control, implement these:

// draggingSourceOperationMaskForLocal lets you control the behavior of what should
// happen with the data on the pasteboard: copy, link or any
// the isLocal flag tells you whether the object querying is from within your app
// or from another application running on the system

- (unsigned int)draggingSourceOperationMaskForLocal:(BOOL)isLocal
{
        if (isLocal) return NSDragOperationCopy;
        return NSDragOperationCopy|NSDragOperationGeneric|NSDragOperationLink;
}

// The simple dragImage:at:offset:event:pasteboard:source:slideback: method
// is all we do to initiate and run the actual drag sequence
// But we only do this if we have an image and we successfully write our data
// to the pasteboard in copyDataTo: method

- (void)mouseDown:(NSEvent *)e
{
        NSPoint location;
        NSSize size;
        NSPasteboard *pb = [NSPasteboard pasteboardWithName:(NSString *) NSDragPboard];

        if (image && [self copyDataTo:pb]) {
                size = [image size];
    location.x = ([self bounds].size.width - size.width)/2;
    location.y = ([self bounds].size.height - size.height)/2;

                [self dragImage:image at:location offset:NSZeroSize event:(NSEvent *)e
pasteboard:pb source:self slideBack:YES];
        }
}

// DRAGGING DESTINATION PROTOCOL METHODS
// To add drag acceptance functionality to a control, implement these methods:

// this is called when the drag enters our view
// by returning NSDragOperationCopy
- (unsigned int) draggingEntered:sender
{
    NSPasteboard   *pboard;

     last      = NSDragOperationNone;
     pboard    = [sender draggingPasteboard];

// we don't acept drags if we are the provider!!
     if ([sender draggingSource] == self) return NSDragOperationNone;

     if ([[pboard types] containsObject:NSFilenamesPboardType]) {
         if (image == nil) {
            image = [[[NSWorkspace sharedWorkspace] iconForFile:[[pboard
propertyListForType:NSFilenamesPboardType]objectAtIndex:0]]retain];   
             [self setNeedsDisplayInRect:[self bounds]];
          }
   // we'll copy or link depending on the intent of the dragging source:
   last = [sender draggingSourceOperationMask]; 
     }
     return last;
}

// instead of constantly rechecking the pasteboard as the mouse moves inside the view
// we'll simply return the cached value that we set in 'last' in draggingEntered:

- (unsigned int) draggingUpdated:sender
{
     return last;
}

// Because we're providing feedback by setting the file right when the user enters
// we'll need to undo that work if the user does not let go of the drag inside and exits instead:

- (void) draggingExited:sender
{
    // the user has exited -> clean up:     
    if ([sender draggingSource] != self)  {
        if (file == nil) {
            // then unset the file image we set in mouseEntered as feedback...
            [image release];
            image = nil;
            [self setNeedsDisplayInRect:[self bounds]];
        } 
        last = NSDragOperationNone;
    }
}

// any dragging clean up might be done here
// don't forget to return YES!

- (BOOL) prepareForDragOperation:sender
{
     return YES;
}

// Actually do the work in this method if it's not too time consuming
// Otherwise, you may consider returning YES, and doing the work
// in concludeDragOperation to prevent the drag from sliding back
// because it "timed out"

- (BOOL) performDragOperation:(id <NSDraggingInfo>)sender
{
    NSPasteboard   *pboard;
    pboard    = [sender draggingPasteboard];
    [self setFile:[[pboard propertyListForType:NSFilenamesPboardType]objectAtIndex:0]];
    return YES;
}

- (void)concludeDragOperation:(id <NSDraggingInfo>)sender
{
    // we already did the work in draggingEntered
    // You might notify some other object that the file is here
}

// this is good if you want to be able to drag out data even if the window is not
// front most, the first click will do more than just bring the window to front -
// It will also allow the drag to begin on that first mouse down

- (BOOL)acceptsFirstMouse:(NSEvent *)theEvent {
    return YES;
}

@end

Conclusion

Adding drag and drop support to controls and views is very easy in Cocoa. Because ease of use and intuitive behavior is the keystone of Mac OS X, be sure to make your applications conform to this model.


Andrew Stone <andrew@stone.com> washes windows at Stone Design Corp <http://www.stone.com/> and divides his time between writing applications for Mac OS X and raising farm animals, including children.

 
AAPL
$119.00
Apple Inc.
+1.40
MSFT
$47.75
Microsoft Corpora
+0.28
GOOG
$540.37
Google Inc.
-0.71

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 7.2.0.412 - Voice-over-internet ph...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
OneNote 15.4 - Free digital notebook fro...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more

Latest Forum Discussions

See All

Lucha Amigos (Games)
Lucha Amigos 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Forget Ninja Turtles, and meet Wrestlers Turtles! Crazier, Spicier and…Bouncier! Sling carapaces of 7 Luchadores to knock all... | Read more »
Raby (Games)
Raby 1.0.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.3 (iTunes) Description: ***WARNING - Raby runs on: iPhone 5, iPhone 5C, iPhone 5S, iPhone 6, iPhone 6 Plus, iPad Mini Retina, iPad Mini 3, iPad 4, iPad Air,... | Read more »
Oddworld: Stranger's Wrath (Games)
Oddworld: Stranger's Wrath 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: ** PLEASE NOTE: Oddworld Stranger's Wrath requires at least an iPhone 4S, iPad 2, iPad Mini or iPod Touch 5th gen... | Read more »
Bounce On Back (Games)
Bounce On Back 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Dwelp (Games)
Dwelp 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: === 50% off for a limited time, to celebrate release === Dwelp is an elegant little puzzler with a brand new game mechanic. To complete a... | Read more »
Make Way for Fat Chicken, from the Maker...
Make Way for Fat Chicken, from the Makers of Scrap Squad Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Relevant Games has announced they will be releasing their reverse tower defense game, | Read more »
Tripnary Review
Tripnary Review By Jennifer Allen on November 26th, 2014 Our Rating: :: TRAVEL BUCKET LISTiPhone App - Designed for the iPhone, compatible with the iPad Want to create a travel bucket list? Tripnary is a fun way to do exactly that... | Read more »
Ossian Studios’ RPG, The Shadow Sun, is...
Ossian Studios’ RPG, The Shadow Sun, is Now Available for $4.99 Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Mmmm, Tasty – Having the Angry Birds for...
The very first Angry Birds debuted on iOS back in 2009. When you sit back and tally up the number of Angry Birds games out there and the impact they’ve had on pop culture as a whole, you just need to ask yourself: “How would the birds taste... | Read more »
Rescue Quest Review
Rescue Quest Review By Jennifer Allen on November 26th, 2014 Our Rating: :: PATH BASED MATCH-3Universal App - Designed for iPhone and iPad Guide a wizard to safety by matching gems. Rescue Quest might not be an entirely original... | Read more »

Price Scanner via MacPrices.net

Black Friday: 15% off iTunes Gift Cards
Staples is offering 15% off $50 and $100 iTunes Gift Cards on their online store as part of their Black Friday sale. Click here for more information. Shipping is free. Best Buy is offering $100... Read more
BEVL Releases Dock Tailored for iPhone 6 and...
Seattle based BEVL has released their first product: an iPhone dock that is divergent in build quality, rock-solid function and visual simplicity to complement the iPhone. BEVL is now accepting... Read more
Black Friday: $150 off 13-inch Retina MacBook...
 Best Buy has 13-inch 2.6GHz Retina MacBook Pros on sale for $150 off MSRP on their online store as part of their Black Friday sale. Choose free shipping or free local store pickup (if available).... Read more
Black Friday: $300 off 15-inch Retina MacBook...
 B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for $300 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina... Read more
Black Friday: Up to $140 off MacBook Airs, fr...
 B&H Photo has 2014 MacBook Airs on sale for up to $140 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 $100... Read more
Black Friday: 13-inch 2.5GHz MacBook Pro on s...
 Best Buy has the 13″ 2.5GHz MacBook Pro on sale for $899.99 on their online store as part of their Black Friday sale. Choose free shipping or free instant local store pickup (if available). Their... Read more
Black Friday: 21-inch 1.4GHz iMac on sale for...
 Best Buy has the 21″ 1.4GHz iMac on sale for $899.99 on their online store as part of their Black Friday sale. Their price is $200 off MSRP. Choose free shipping or free local store pick up. Price... Read more
Black Friday iPad Air 2 sale prices, $100 off...
 Best Buy has iPad Air 2s on sale for $100 off MSRP on their online store for Black Friday. Choose free shipping or free local store pickup (if available). Sale prices available for online orders... Read more
2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has the new 1.4GHz Mac mini on sale for $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new model. Adorama... Read more
Early Black Friday pricing on 27-inch 5K iMac...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.