The document discusses escaping part of the Mac App Sandbox by using File Presenters. File Presenters allow sandboxed apps to work with temporary files on disk in a multi-threaded environment. The author implements a File Presenter class to enable "Related Items" functionality for a sandboxed app, allowing it to process temporary files related to files selected by the user. Setting up the Xcode target correctly registers the app's support for File Presenters and temporary related items.

1. Escaping)the
Mac$App$Sandbox
(or$at$least$a$small$part$of$it)

2. Ma#$Welch
Developer(working(on(iOS,(Mac,(and(node(
apps(by(night,(and(on(the(Force.com(
pla=orm(by(day.
ma#@welcher.net
@_Ma#Welch_
h#p://ma#wel.ch

3. Bearings

4. What%is%the%Mac%App%Sandbox?

5. App#Sandbox!is!an!access!control!technology!provided!in!OS!X,!
enforced!at!the!kernel!level.!Its!strategy!is!twofold:
• App$Sandbox$enables$you$to$describe$how$your$app$interacts$
with$the$system.$The$system$then$grants$your$app$the$access$it$
needs$to$get$its$job$done,$and$no$more.
• App$Sandbox$allows$the$user$to$transparently$grant$your$app$
addi=onal$access$by$way$of$Open$and$Save$dialogs,$drag$and$
drop,$and$other$familiar$user$interac=ons.
(From&the&Apple&"App&Sandbox&Design&Guide")

6. Sandbox(Filesystem(Limita2ons
Specifically,+sandboxing+limits+an+app+to+only+those+files+or+
directories+explicitly+opened+by+the+user.
For$most$cases$this$is$OK.$(In$fact,$Deckset$here$regularly$asks$
permission$to$access$a$photo$I$want$to$use$that's$on$my$filesystem.)

7. Temporary)Files
The$problem$is$that$temporary$files$are$
supposed$to$be$invisible$to$the$end$user.
What%kind%of%experience%would%this%be?
Now$mul(ply$this$bad$experience$by$10.$
Or$100.$Or$however$many$hundreds$of$
photos$(in$the$case$of$Bearings$app)$that$
need$to$be$processed.

8. Workaround:*File*Presenters
File%Presenters?

9. Yes,%File%Presenters.
• Around(since(10.7,(File(Presenters((and(their("parents"(File(
Coordinators)(have(made(working(with(the(file(system(in(a(mulDE
threaded,(mulDEprocessing(environment(easier(and(safer.

10. Yes,%File%Presenters.
• Around(since(10.7,(File(Presenters((and(their("parents"(File(
Coordinators)(have(made(working(with(the(file(system(in(a(mulDE
threaded,(mulDEprocessing(environment(easier(and(safer.

11. Yes,%File%Presenters.
• Around(since(10.7,(File(Presenters((and(their("parents"(File(
Coordinators)(have(made(working(with(the(file(system(in(a(mulDE
threaded,(mulDEprocessing(environment(easier(and(safer.

12. Yes,%File%Presenters.
• Around(since(10.7,(File(Presenters((and(their("parents"(File(
Coordinators)(have(made(working(with(the(file(system(in(a(mulDE
threaded,(mulDEprocessing(environment(easier(and(safer.

13. Yes,%File%Presenters.
• Around(since(10.7,(File(Presenters((and(their("parents"(File(
Coordinators)(have(made(working(with(the(file(system(in(a(mulDE
threaded,(mulDEprocessing(environment(easier(and(safer.

14. Implemen'ng)File)Presenters)for)Related)
Items
Apple%has%repurposed%File%Presenters%to%enable%"Related%Items"%in%
sandboxed%apps.
Related'Items
• Must&have&the&same&name&(minus&extension)&as&the&item&to&
which&they&are&related
• Must&have&a&known9ahead9of9;me&extension

15. Implemen'ng)File)Presenters)for)Related)
Items
Create&a&class&that&conforms&the&the&NSFilePresenter&protocol.&
Implement&these&methods:
1. primaryPresentedItemURL#–#The#URL#for#the#original#file
2. primaryItemURL#–#the#URL#for#the#temporary#file#(the#same#as#
above,#but#with#different#extension)
3. presentedItemOperationQueue#–#the#queue#on#which#the#
app#will#perform#file#presentaGon#tasks

16. WRFilePresenter
// WRFilePresenter.m
// Bearings
//
// Created by Matt Welch on 3/17/14.
//
#import "WRFilePresenter.h"
@implementation WRFilePresenter
{
NSOperationQueue* queue;
NSURL* pFileURL;
NSURL* tFileURL;
}
- (id) init {
self = [super init];
if (self) {
queue = [NSOperationQueue new];
[NSFileCoordinator addFilePresenter:self];
}
return self;
}
- (NSURL*) primaryPresentedItemURL {
return pFileURL;
}
- (NSURL *) presentedItemURL {
return tFileURL;
}
- (NSOperationQueue*) presentedItemOperationQueue {
return queue;
}
-(void) setURLs:(NSURL*)url {
pFileURL=url;
NSString *fURLS=[url absoluteString];
NSString *fURLSt=[NSString stringWithFormat:@"%@%@",fURLS,@"_temp_file_extension" ];
NSURL *surl = [NSURL URLWithString:fURLSt];
tFileURL=surl;
}
@end

17. Implemen'ng)File)Presenters
Given&a&file&with&a&url&of&realFileURL,&implement&a&presenter&for&
it:
WRFilePresenter *filePresenter=[[WRFilePresenter alloc] init];
[filePresenter setURLs:realFileURL];
And$we're$all$set$(at$least$as$far$as$code$is$concerned).

18. XCode&Target&Setup
In#the#Project#Navigator,#under#the#"Info"#tab,#there#is#a#
"Documents#Type"#sec=on.
• Set%"Extension"%to%be%the%known%temporary%file%extension.
• Add%NSIsRelatedItemType%of%type%Boolean%to%"Addi<onal%
document%type%proper<es"%and%set%it%to%YES

19. XCode&Target&Setup

20. Demo

21. Further'Informa.on
More%in(depth%informa0on%can%be%found%at:
h5p://ma5wel.ch/temporary(files(in(sandboxed(mac(apps/
This%presenta,on%is%on%github:
h2ps://github.com/ma2welch/mac_sandbox_tempfiles_deckset

22. A"ribu'ons
• happiness)from)a)sandbox)1)h2ps://www.flickr.com/photos/
celinesphotographer/326629023/
• App)Sandbox)Design)Guide)1)h2ps://developer.apple.com/
library/mac/documentaIon/Security/Conceptual/
AppSandboxDesignGuide/AboutAppSandbox/
AboutAppSandbox.html
• Paperama)1)h2ps://www.flickr.com/photos/kasaa/3103799093/