Related
I have bundled a framework/SDK from Philips Hue written in Objective-C into my Swift project. The code of the SDK is well in-line documented. Thus, I have fired up appledoc and created a custom docset: appledoc --project-name HueSDK --project-company Philips --install-docset ..
The target was to implement it directly into Xcode's Documentation window:
I have realized that this is not working anymore in Xcode 10, whereas reports show that it was working for older Xcode versions (<8). This is quite a pity as Xcode very often directs the user to the Documentation window, e.g. clicking on a link within the Quick Help Inspector:
Do you know any possibility how to integrate custom Docsets into Xcode's Documentation window?
I have an iOS app which I need to build a supporting app for macOS. I was hoping to share code between them, specially the DataModel.
As the iOS app was using Pods, there was already a workspace.
I pulled in the macOS project into the workspace, and I can see all its files with no problem. But I don't seem to be able to figure out how to 'use' the class files from the iOS project in the macOS project. Would it have anything to do with Modules?
Or should I just make a new Target that is a macOS application?
I'm using Xcode 8.3.3 at the moment, and the projects are in Swift.
Select the source files you want to use and tick the "Target Membership" for your macOS target in the Inspector:
Update:
To use common code in different projects, make an extra project for your common code which builds as a Framework and include this framework in the iOS and in the macOS.
This tutorial might be helpful.
When using an embedded framework (dyld) in Xcode 6.0.1 with deployment target less that iOS 8 I get:
Build is successful
Runtime library loading error
Error:
dyld: Library not loaded: #rpath/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
Referenced from: /private/var/mobile/Containers/Bundle/Application/DC65ACA9-98E5-46CD-95F8-829D3416F6C0/musiXmatch.app/musiXmatch
Reason: image not found
(lldb)
For some time I was thinking that this is my problem as well, but for normal apps (non-iOS-8-extension) you just need to change one build setting in your casual Xcode 6 iOS Universal Framework target (set Mach-O Type to Static Library):
There should be no problem with iTunes Connect and iOS 7 after that :)
So,
after digging around I came out with the solution
Supposed to have yours MyEmbeddedFramework.framework to add to the app, do this
Remove MyEmbeddedFramework.framework in the tab General > Embedded Binaries
Remove the Build Phases > Copy Phase "Frameworks" if you have MyEmbeddedFramework.framework there.
Clean Build Folder
Move the MyEmbeddedFramework.framework in the void Embedded Frameworks section.
You will see now that a new Build Phase > Embedded Frameworks is created by XCode6 (not you, it is done automatically)
Now if you have 5, it should run without erros.
So to recap, to make it works you should see MyEmbeddedFramework.framework in
A) General > Embedded Binaries
B) Build Phase > Embedded Frameworks
It worked fine on iPhone5/iOS8 not on iPhone4S/iOS7 where I get:
dyld: Library not loaded: #rpath/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
Referenced from: /var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/musiXmatch
Reason: no suitable image found. Did find:
/private/var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/Frameworks/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2: incompatible cpu-subtype: 0x0000000B in /private/var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/Frameworks/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
The problem was in the EmbeddedFramework. I had to
1) Set Architecture to default
2) Set Valid Architectures to: armv7, armv7s and armv64 (as Apple suggests armv64 is needed to have Embedded Frameworks working).
Then I was able to run the app with an embedded framework on
iPhone5S/iPhone5C iOS8
iPhone5S/iPhone5C iOS7
iPod 5th gen / iOS7
iPhone4S / iOS7
iPhone4 / iOS7
Anyways when submitting to iTunesConnect I get some errors for the Minimum Required Version:
The MinimumOSVersion of framework "..." is invalid. The minimum value is iOS 8.0;
Invalid Architecture: Apps that include and app extension and a framework must support arm64;
As of right now there is no way to use an embedded framework to share code between an app and widget and have it run on iOS 8 as well as iOS 7 & previous.
Here's some more reading on that http://atomicbird.com/blog/ios-app-extension-tips
Frameworks vs. iOS 7
If you are sharing code between an app and an extension, one nice way to do so is to create your own embedded framework to hold the code. On iOS 8 it'll load dynamically for both cases, so you're set.
If you still support iOS 7 (or earlier), it's not so clear cut. Embedded frameworks don't work there. The App Extension Programming Guide breezily notes that you can use dlopen to deal with this. With that approach you write code to load the framework dynamically at run time rather than rely on iOS loading it for you, if you've verified that the code is running on a version of iOS that supports doing so.
But how do you use that code on iOS 7? You don't. If your shared code is in an embedded framework, there's no way to execute it on iOS 7. It's just unavailable.
The dlopen approach might be handy if you only need the shared code on iOS 8. If you need it on iOS 7, you'll need to include it in the app target. And once you do that, you have no need of the framework. You could still use a framework for the app extension, but doing so is not actually useful. You'd be doing the work of creating the framework but not getting any benefit from it. Just include the shared code in both targets.
And from Apple's extension guide https://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/ExtensibilityPG.pdf
If you link to an embedded framework from your containing app, you can still deploy it to versions of iOS older than 8.0, even though embedded frameworks are not available in those versions.
Fixed the error in xcode 6.1.1
using vim or vi open the project.pbxproj file.
At the end of the file (search for 8.1) , there would be Begin XCBuildConfiguration section
Look for your framework.
In out case even though the deployment target was set to 7.1 via Xcode -> general in target settings, the entry in the file had 8.1 for both debug & release
Here's the old file section looks like:
CURRENT_PROJECT_VERSION = 1;
DEFINES_MODULE = YES;
DYLIB_COMPATIBILITY_VERSION = 1;
DYLIB_CURRENT_VERSION = 1;
DYLIB_INSTALL_NAME_BASE = "#rpath";
GCC_PREPROCESSOR_DEFINITIONS = (
"DEBUG=1",
"$(inherited)",
);
INFOPLIST_FILE = ENFramework/Info.plist;
INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
IPHONEOS_DEPLOYMENT_TARGET = 8.1;
LD_RUNPATH_SEARCH_PATHS = "$(inherited) #executable_path/Frameworks #loader_path/Frameworks";
PRODUCT_NAME = "$(TARGET_NAME)";
SKIP_INSTALL = YES;
VERSIONING_SYSTEM = "apple-generic";
VERSION_INFO_PREFIX = "";
New section looks like :
CURRENT_PROJECT_VERSION = 1;
DEFINES_MODULE = YES;
DYLIB_COMPATIBILITY_VERSION = 1;
DYLIB_CURRENT_VERSION = 1;
DYLIB_INSTALL_NAME_BASE = "#rpath";
GCC_PREPROCESSOR_DEFINITIONS = (
"DEBUG=1",
"$(inherited)",
);
INFOPLIST_FILE = ENFramework/Info.plist;
INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
IPHONEOS_DEPLOYMENT_TARGET = 7.1;
LD_RUNPATH_SEARCH_PATHS = "$(inherited) #executable_path/Frameworks #loader_path/Frameworks";
PRODUCT_NAME = "$(TARGET_NAME)";
SKIP_INSTALL = YES;
VERSIONING_SYSTEM = "apple-generic";
VERSION_INFO_PREFIX = "";
Now we don't get error just a warning (but works on iOS 7.1 device) :
ld: warning: embedded dylibs/frameworks only run on iOS 8 or later
This looks like an xcode bug which incorrectly sets different ios targets and then causes error.
Going deeper on Apple Documentation I found out about dlopen command, which is used to make the linking of the libraries on some conditions, depending on system versions and libraries supported.
dlopen example of use:
Is the function 'dlopen()' private API?
So let's look at the solution provided by Apple Docs:
Deploying a Containing App to Older Versions of iOS
If you link to an embedded framework from your containing app, you can
still deploy it to versions of iOS older than 8.0, even though
embedded frameworks are not available in those versions.
The mechanism that lets you do this is the dlopen command, which you use to conditionally link and load a framework bundle. You employ this command as an alternative to the build-time linking you can specify in the Xcode General or Build Phases target editor. The main idea is to link embedded frameworks into your containing app only when running in iOS 8.0 or newer.
You must use Objective-C, not Swift, in your code statements that conditionally load a framework bundle. The rest of your app can be written in either language, and the embedded framework itself can likewise be written in either language.
After calling dlopen, access the embedded framework classes using the following type of statement:
MyLoadedClass *loadedClass = [[NSClassFromString (#"MyClass") alloc] init];
IMPORTANT
If your containing app target links to an embedded framework, it must
include the arm64 architecture or it will be rejected by the App
Store.
To set up an app extension Xcode project to take advantage of conditional linking
For each of your contained app extensions, set the deployment target
to be iOS 8.0 or later, as usual. Do this in the “Deployment info”
section of the General tab in the Xcode target editor.
For your containing app, set the deployment target to be the oldest
version of iOS that you want to support.
In your containing app, conditionalize calls to the dlopen command
within a runtime check for the iOS version by using the
systemVersion method. Call the dlopen command only if your
containing app is running in iOS 8.0 or later. Be sure to use
Objective-C, not Swift, when making this call.
Certain iOS APIs use embedded frameworks via the dlopen command. You must conditionalize your use of these APIs just as you do when calling dlopen directly. These APIs are from the CFBundleRef opaque type:
CFBundleGetFunctionPointerForName
CFBundleGetFunctionPointersforNames
And from the NSBundle class:
load
loadAndReturnError:
classNamed:
In a containing app you are deploying to versions of iOS older than
8.0, call these APIs only within a runtime check that ensures you are running in iOS 8.0 or newer, and call these APIs using Objective-C.
We tried running the latest code on the following configurations:
iOS 8+ — iPhone 5s
iOS 7.1.2 — iPhone 4
iOS 6.1.3 — iPad 4
The App is working fine on all the three devices but the warning is present in the Xcode while compiling .
"embedded dylibs/frameworks only run on iOS 8 or later”
Also I tried to Archive the App in order to submit it to the app store it went on fine.
Also, found out a link where in an apple developer stated this to be a bug
https://devforums.apple.com/message/999579#999579
Just for the record... I had this issue when changing a project from iOS8 to iOS7 deployment type.
The app used cocoapods and no custom embedded frameworks.
I had to change the main project two targets
Application
Application-Test
Changing Mach-O Type to static (from above answer).
Then on the cocoapods project. Under each sub pod project changing the Mach-O type to static, leaving the main pod Project Mach-O setting to blank.
I set the Mach-O Type to EXECUTABLE and it worked for me. Setting it to Static, Dynamic or Bundle created other errors when I ran it.
Target > "Your App" > Build Settings > Linking > Mach-O Type > Executable
I solve this problem following way :
Use same deployment target in both target "Embedded Framework" and "main App" target.
So, temporary, i said no to dynamic library, while many devices on iOS 7. How i solved my problem. I was need lib for transferring model between app and extension. So, i put my model to JSON string into shared container. And it works like a charm.
When you use dynamic library on ios you must code signed the library. In the Xcode 6, you should select the "Code Sign On Copy". And with the Xcode5, you should sign the library by your self with run script. like :
LOCATION="${BUILT_PRODUCTS_DIR}"/"${FRAMEWORKS_FOLDER_PATH}"
IDENTITY="iPhone Developer: xxxxx"
codesign --verbose --force --sign "$IDENTITY" "$LOCATION/BeeFramework.framework/BeeFramework"
Remove the use frameworks! from your PodFile if you want the Framework to work in iOS 7.0. i.e. run pod deintegrate command, modify your PodFile and then rerun the pod install command
Also after this, I had to add all the .h file's of the Framework in the Bridging file, that fixed the issue. Also remove the import TestLibrary from the swift files
I had a bug when updating to xcode 7.3. And I had solution for me.
- Change targets in pods project -> 7.0
- Hope it useful!
I was running into an issue where i needed to include some libraries as embedded frameworks otherwise i received the above error and when I did just that, I received errors when submitting to the app store.
My solution was to use Pods and make sure you uncomment the "use_frameworks!" line.
iOS Dynamic framework pre iOS v8
Dynamic framework aka Embedded framework is supported from iOS v8, but you can try to change Mach-O type[About] to Dynamic Library (which is set by default for supported target)
Dynamic linking error[About]
dyld: Library not loaded: #rpath/<some_path>
It is an dynamic linker error which links binary in load or runtime
[Vocabulary]
When using an embedded framework (dyld) in Xcode 6.0.1 with deployment target less that iOS 8 I get:
Build is successful
Runtime library loading error
Error:
dyld: Library not loaded: #rpath/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
Referenced from: /private/var/mobile/Containers/Bundle/Application/DC65ACA9-98E5-46CD-95F8-829D3416F6C0/musiXmatch.app/musiXmatch
Reason: image not found
(lldb)
For some time I was thinking that this is my problem as well, but for normal apps (non-iOS-8-extension) you just need to change one build setting in your casual Xcode 6 iOS Universal Framework target (set Mach-O Type to Static Library):
There should be no problem with iTunes Connect and iOS 7 after that :)
So,
after digging around I came out with the solution
Supposed to have yours MyEmbeddedFramework.framework to add to the app, do this
Remove MyEmbeddedFramework.framework in the tab General > Embedded Binaries
Remove the Build Phases > Copy Phase "Frameworks" if you have MyEmbeddedFramework.framework there.
Clean Build Folder
Move the MyEmbeddedFramework.framework in the void Embedded Frameworks section.
You will see now that a new Build Phase > Embedded Frameworks is created by XCode6 (not you, it is done automatically)
Now if you have 5, it should run without erros.
So to recap, to make it works you should see MyEmbeddedFramework.framework in
A) General > Embedded Binaries
B) Build Phase > Embedded Frameworks
It worked fine on iPhone5/iOS8 not on iPhone4S/iOS7 where I get:
dyld: Library not loaded: #rpath/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
Referenced from: /var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/musiXmatch
Reason: no suitable image found. Did find:
/private/var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/Frameworks/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2: incompatible cpu-subtype: 0x0000000B in /private/var/mobile/Applications/739D9C44-3B91-4D4F-805B-83BE66C9CBCA/musiXmatch.app/Frameworks/ObjectiveLyricsTouch2.framework/ObjectiveLyricsTouch2
The problem was in the EmbeddedFramework. I had to
1) Set Architecture to default
2) Set Valid Architectures to: armv7, armv7s and armv64 (as Apple suggests armv64 is needed to have Embedded Frameworks working).
Then I was able to run the app with an embedded framework on
iPhone5S/iPhone5C iOS8
iPhone5S/iPhone5C iOS7
iPod 5th gen / iOS7
iPhone4S / iOS7
iPhone4 / iOS7
Anyways when submitting to iTunesConnect I get some errors for the Minimum Required Version:
The MinimumOSVersion of framework "..." is invalid. The minimum value is iOS 8.0;
Invalid Architecture: Apps that include and app extension and a framework must support arm64;
As of right now there is no way to use an embedded framework to share code between an app and widget and have it run on iOS 8 as well as iOS 7 & previous.
Here's some more reading on that http://atomicbird.com/blog/ios-app-extension-tips
Frameworks vs. iOS 7
If you are sharing code between an app and an extension, one nice way to do so is to create your own embedded framework to hold the code. On iOS 8 it'll load dynamically for both cases, so you're set.
If you still support iOS 7 (or earlier), it's not so clear cut. Embedded frameworks don't work there. The App Extension Programming Guide breezily notes that you can use dlopen to deal with this. With that approach you write code to load the framework dynamically at run time rather than rely on iOS loading it for you, if you've verified that the code is running on a version of iOS that supports doing so.
But how do you use that code on iOS 7? You don't. If your shared code is in an embedded framework, there's no way to execute it on iOS 7. It's just unavailable.
The dlopen approach might be handy if you only need the shared code on iOS 8. If you need it on iOS 7, you'll need to include it in the app target. And once you do that, you have no need of the framework. You could still use a framework for the app extension, but doing so is not actually useful. You'd be doing the work of creating the framework but not getting any benefit from it. Just include the shared code in both targets.
And from Apple's extension guide https://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/ExtensibilityPG.pdf
If you link to an embedded framework from your containing app, you can still deploy it to versions of iOS older than 8.0, even though embedded frameworks are not available in those versions.
Fixed the error in xcode 6.1.1
using vim or vi open the project.pbxproj file.
At the end of the file (search for 8.1) , there would be Begin XCBuildConfiguration section
Look for your framework.
In out case even though the deployment target was set to 7.1 via Xcode -> general in target settings, the entry in the file had 8.1 for both debug & release
Here's the old file section looks like:
CURRENT_PROJECT_VERSION = 1;
DEFINES_MODULE = YES;
DYLIB_COMPATIBILITY_VERSION = 1;
DYLIB_CURRENT_VERSION = 1;
DYLIB_INSTALL_NAME_BASE = "#rpath";
GCC_PREPROCESSOR_DEFINITIONS = (
"DEBUG=1",
"$(inherited)",
);
INFOPLIST_FILE = ENFramework/Info.plist;
INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
IPHONEOS_DEPLOYMENT_TARGET = 8.1;
LD_RUNPATH_SEARCH_PATHS = "$(inherited) #executable_path/Frameworks #loader_path/Frameworks";
PRODUCT_NAME = "$(TARGET_NAME)";
SKIP_INSTALL = YES;
VERSIONING_SYSTEM = "apple-generic";
VERSION_INFO_PREFIX = "";
New section looks like :
CURRENT_PROJECT_VERSION = 1;
DEFINES_MODULE = YES;
DYLIB_COMPATIBILITY_VERSION = 1;
DYLIB_CURRENT_VERSION = 1;
DYLIB_INSTALL_NAME_BASE = "#rpath";
GCC_PREPROCESSOR_DEFINITIONS = (
"DEBUG=1",
"$(inherited)",
);
INFOPLIST_FILE = ENFramework/Info.plist;
INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
IPHONEOS_DEPLOYMENT_TARGET = 7.1;
LD_RUNPATH_SEARCH_PATHS = "$(inherited) #executable_path/Frameworks #loader_path/Frameworks";
PRODUCT_NAME = "$(TARGET_NAME)";
SKIP_INSTALL = YES;
VERSIONING_SYSTEM = "apple-generic";
VERSION_INFO_PREFIX = "";
Now we don't get error just a warning (but works on iOS 7.1 device) :
ld: warning: embedded dylibs/frameworks only run on iOS 8 or later
This looks like an xcode bug which incorrectly sets different ios targets and then causes error.
Going deeper on Apple Documentation I found out about dlopen command, which is used to make the linking of the libraries on some conditions, depending on system versions and libraries supported.
dlopen example of use:
Is the function 'dlopen()' private API?
So let's look at the solution provided by Apple Docs:
Deploying a Containing App to Older Versions of iOS
If you link to an embedded framework from your containing app, you can
still deploy it to versions of iOS older than 8.0, even though
embedded frameworks are not available in those versions.
The mechanism that lets you do this is the dlopen command, which you use to conditionally link and load a framework bundle. You employ this command as an alternative to the build-time linking you can specify in the Xcode General or Build Phases target editor. The main idea is to link embedded frameworks into your containing app only when running in iOS 8.0 or newer.
You must use Objective-C, not Swift, in your code statements that conditionally load a framework bundle. The rest of your app can be written in either language, and the embedded framework itself can likewise be written in either language.
After calling dlopen, access the embedded framework classes using the following type of statement:
MyLoadedClass *loadedClass = [[NSClassFromString (#"MyClass") alloc] init];
IMPORTANT
If your containing app target links to an embedded framework, it must
include the arm64 architecture or it will be rejected by the App
Store.
To set up an app extension Xcode project to take advantage of conditional linking
For each of your contained app extensions, set the deployment target
to be iOS 8.0 or later, as usual. Do this in the “Deployment info”
section of the General tab in the Xcode target editor.
For your containing app, set the deployment target to be the oldest
version of iOS that you want to support.
In your containing app, conditionalize calls to the dlopen command
within a runtime check for the iOS version by using the
systemVersion method. Call the dlopen command only if your
containing app is running in iOS 8.0 or later. Be sure to use
Objective-C, not Swift, when making this call.
Certain iOS APIs use embedded frameworks via the dlopen command. You must conditionalize your use of these APIs just as you do when calling dlopen directly. These APIs are from the CFBundleRef opaque type:
CFBundleGetFunctionPointerForName
CFBundleGetFunctionPointersforNames
And from the NSBundle class:
load
loadAndReturnError:
classNamed:
In a containing app you are deploying to versions of iOS older than
8.0, call these APIs only within a runtime check that ensures you are running in iOS 8.0 or newer, and call these APIs using Objective-C.
We tried running the latest code on the following configurations:
iOS 8+ — iPhone 5s
iOS 7.1.2 — iPhone 4
iOS 6.1.3 — iPad 4
The App is working fine on all the three devices but the warning is present in the Xcode while compiling .
"embedded dylibs/frameworks only run on iOS 8 or later”
Also I tried to Archive the App in order to submit it to the app store it went on fine.
Also, found out a link where in an apple developer stated this to be a bug
https://devforums.apple.com/message/999579#999579
Just for the record... I had this issue when changing a project from iOS8 to iOS7 deployment type.
The app used cocoapods and no custom embedded frameworks.
I had to change the main project two targets
Application
Application-Test
Changing Mach-O Type to static (from above answer).
Then on the cocoapods project. Under each sub pod project changing the Mach-O type to static, leaving the main pod Project Mach-O setting to blank.
I set the Mach-O Type to EXECUTABLE and it worked for me. Setting it to Static, Dynamic or Bundle created other errors when I ran it.
Target > "Your App" > Build Settings > Linking > Mach-O Type > Executable
I solve this problem following way :
Use same deployment target in both target "Embedded Framework" and "main App" target.
So, temporary, i said no to dynamic library, while many devices on iOS 7. How i solved my problem. I was need lib for transferring model between app and extension. So, i put my model to JSON string into shared container. And it works like a charm.
When you use dynamic library on ios you must code signed the library. In the Xcode 6, you should select the "Code Sign On Copy". And with the Xcode5, you should sign the library by your self with run script. like :
LOCATION="${BUILT_PRODUCTS_DIR}"/"${FRAMEWORKS_FOLDER_PATH}"
IDENTITY="iPhone Developer: xxxxx"
codesign --verbose --force --sign "$IDENTITY" "$LOCATION/BeeFramework.framework/BeeFramework"
Remove the use frameworks! from your PodFile if you want the Framework to work in iOS 7.0. i.e. run pod deintegrate command, modify your PodFile and then rerun the pod install command
Also after this, I had to add all the .h file's of the Framework in the Bridging file, that fixed the issue. Also remove the import TestLibrary from the swift files
I had a bug when updating to xcode 7.3. And I had solution for me.
- Change targets in pods project -> 7.0
- Hope it useful!
I was running into an issue where i needed to include some libraries as embedded frameworks otherwise i received the above error and when I did just that, I received errors when submitting to the app store.
My solution was to use Pods and make sure you uncomment the "use_frameworks!" line.
iOS Dynamic framework pre iOS v8
Dynamic framework aka Embedded framework is supported from iOS v8, but you can try to change Mach-O type[About] to Dynamic Library (which is set by default for supported target)
Dynamic linking error[About]
dyld: Library not loaded: #rpath/<some_path>
It is an dynamic linker error which links binary in load or runtime
[Vocabulary]
How can I obtain and use 10.6 as base SDK in Xcode 4.5.2?
I downloaded Xcode 4.5.2 from the Mac App Store (through a link on the Apple developer site).
Is there a way I can somehow download the 10.6 SDK separately and start using it as a base SDK?
Strangely enough, I can download the documentation for the 10.6 SDK through Xcode's Preferences window, but not the SDK itself. Any ideas?
No, you can't download the 10.6 SDK separately.
Is there a reason you can't use the 10.8 or 10.7 SDK?
Keep in mind that just because you build against the 10.8 SDK, that doesn't mean you can't also have that built application work on 10.8, 10.7 and 10.6. (This is often a cause of confusion among new developers). You control backwards-compatibility through the Deployment Target setting like shown in the image below.
By default, the deployment target is generally set to the same version of OS X as the SDK is, but changing it to 10.6, for example, should allow it to run on a machine with OS X 10.6. (Of course, you should really test to make sure that's the case).
Download xcode with MacOSX10.6.sdk. Now copy MacOSX10.6.sdk inside /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
Its also working fine with Xcode 5.0.