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diff --git a/widget/cocoa/docs/macos-apis.md b/widget/cocoa/docs/macos-apis.md new file mode 100644 index 0000000000..8e54a2c1bf --- /dev/null +++ b/widget/cocoa/docs/macos-apis.md @@ -0,0 +1,188 @@ +# Using macOS APIs + +With each new macOS release, new APIs are added. Due to the wide range of platforms that Firefox runs on, +and due to the [wide range of SDKs that we support building with](./sdks.html#supported-sdks), +using macOS APIs in Firefox requires some extra care. + +## Availability of APIs, and runtime checks + +First of all, if you use an API that is supported by all versions of macOS that Firefox runs on, +i.e. 10.9 and above, then you don't need to worry about anything: +The API declaration will be present in any of the supported SDKs, and you don't need any runtime checks. + +If you want to use a macOS API that was added after 10.9, then you have to have a runtime check. +This requirement is completely independent of what SDK is being used for building. + +The runtime check [should have the following form](https://developer.apple.com/documentation/macos_release_notes/macos_mojave_10_14_release_notes/appkit_release_notes_for_macos_10_14?language=objc#3014609) +(replace `10.14` with the appropriate version): + +```objc++ +if (@available(macOS 10.14, *)) { + // Code for macOS 10.14 or later +} else { + // Code for versions earlier than 10.14. +} +``` + +`@available` guards can be used in Objective-C(++) code. +(In C++ code, you can use [these `nsCocoaFeatures` methods](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/nsCocoaFeatures.h#21-27) instead.) + +For each API, the API declarations in the SDK headers are annotated with `API_AVAILABLE` macros. +For example, the definition of the `NSVisualEffectMaterial` enum looks like this: + +```objc++ +typedef NS_ENUM(NSInteger, NSVisualEffectMaterial) { + NSVisualEffectMaterialTitlebar = 3, + NSVisualEffectMaterialSelection = 4, + NSVisualEffectMaterialMenu API_AVAILABLE(macos(10.11)) = 5, + // [...] + NSVisualEffectMaterialSheet API_AVAILABLE(macos(10.14)) = 11, + // [...] +} API_AVAILABLE(macos(10.10)); +``` + +The compiler understands these annotations and makes sure that you wrap all uses of the annotated APIs +in appropriate `@available` runtime checks. + +### Frameworks + +In some rare cases, you need functionality from frameworks that are not available on all supported macOS versions. +Examples of this are `Metal.framework` (added in 10.11) and `MediaPlayer.framework` (added in 10.12.2). + +In that case, you can either `dlopen` your framework at runtime ([like we do for MediaPlayer](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/MediaPlayerWrapper.mm#21-27)), +or you can [use `-weak_framework`](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-107026) +[like we do for Metal](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/toolkit/library/moz.build#301-304): + +```python +if CONFIG['OS_ARCH'] == 'Darwin': + OS_LIBS += [ + # Link to Metal as required by the Metal gfx-hal backend + '-weak_framework Metal', + ] +``` + +## Using new APIs with old SDKs + +If you want to use an API that was introduced after 10.12, you now have one extra thing to worry about. +In addition to the runtime check [described in the previous section](#using-macos-apis), you also +have to jump through extra hoops in order to allow the build to succeed, because +[our build target for Firefox has to remain at 10.12 in order for Firefox to run on macOS versions all the way down to macOS 10.12](./sdks.html#supported-sdks). + +In order to make the compiler accept your code, you will need to copy some amount of the API declaration +into your own code. Copy it from the newest recent SDK you can get your hands on. +The exact procedure varies based on the type of API (enum, objc class, method, etc.), +but the general approach looks like this: + +```objc++ +#if !defined(MAC_OS_VERSION_12_0) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_VERSION_12_0 +@interface NSScreen (NSScreen12_0) +// https://developer.apple.com/documentation/appkit/nsscreen/3882821-safeareainsets?language=objc&changes=latest_major +@property(readonly) NSEdgeInsets safeAreaInsets; +@end +#endif +``` + +See the [Supporting Multiple SDKs](./sdks.html#supporting-multiple-sdks) docs for more information on the `MAC_OS_X_VERSION_MAX_ALLOWED` macro. + +Keep these three things in mind: + + - Copy only what you need. + - Wrap your declaration in `MAC_OS_X_VERSION_MAX_ALLOWED` checks so that, if an SDK is used that + already contains these declarations, your declaration does not conflict with the declaration in the SDK. + - Include the `API_AVAILABLE` annotations so that the compiler can protect you from accidentally + calling the API on unsupported macOS versions. + +Our current code does not always follow the `API_AVAILABLE` advice, but it should. + +### Enum types and C structs + +If you need a new enum type or C struct, copy the entire type declaration and wrap it in the appropriate ifdefs. Example: + +```objc++ +#if !defined(MAC_OS_X_VERSION_10_12_2) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_12_2 +typedef NS_ENUM(NSUInteger, MPNowPlayingPlaybackState) { + MPNowPlayingPlaybackStateUnknown = 0, + MPNowPlayingPlaybackStatePlaying, + MPNowPlayingPlaybackStatePaused, + MPNowPlayingPlaybackStateStopped, + MPNowPlayingPlaybackStateInterrupted +} MP_API(ios(11.0), tvos(11.0), macos(10.12.2), watchos(5.0)); +#endif +``` +### New enum values for existing enum type + +If the enum type itself already exists, but gained a new value, define the value in an unnamed enum: + +```objc++ +#if !defined(MAC_OS_X_VERSION_10_12) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_12 +enum { NSVisualEffectMaterialSelection = 4 }; +#endif +``` + +(This is an example of an interesting case: `NSVisualEffectMaterialSelection` is available starting with +macOS 10.10, but it's only defined in SDKs starting with the 10.12 SDK.) + +### Objective-C classes + +For a new Objective-C class, copy the entire `@interface` declaration and wrap it in the appropriate ifdefs. + +I haven't personally tested this. If this does not compile (or maybe link?), you can use the following workaround: + + - Define your methods and properties as a category on `NSObject`. + - Look up the class at runtime using `NSClassFromString()`. + - If you need to create a subclass, do it at runtime using `objc_allocateClassPair` and `class_addMethod`. + [Here's an example of that.](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/VibrancyManager.mm#44-60) + +### Objective-C properties and methods on an existing class + +If an Objective-C class that already exists gains a new method or property, you can "add" it to the +existing class declaration with the help of a [category](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html): + +```objc++ +@interface ExistingClass (YourMadeUpCategoryName) +// methods and properties here +@end +``` + +### Functions + +With free-standing functions I'm not entirely sure what to do. +In theory, copying the declarations from the new SDK headers should work. Example: + +```objc++ +extern "C" { + __attribute__((warn_unused_result)) bool +SecTrustEvaluateWithError(SecTrustRef trust, CFErrorRef _Nullable * _Nullable CF_RETURNS_RETAINED error) + API_AVAILABLE(macos(10.14), ios(12.0), tvos(12.0), watchos(5.0)); + + __nullable +CFDataRef SecCertificateCopyNormalizedSubjectSequence(SecCertificateRef certificate) + __OSX_AVAILABLE_STARTING(__MAC_10_12_4, __IPHONE_10_3); +} +``` + +I'm not sure what the linker or the dynamic linker do when the symbol is not available. +Does this require [`__attribute__((weak_import))` annotations](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-107262-CJBJAEID)? + +And maybe this is where .tbd files in the SDK come in? So that the linker knows which symbols to allow? +So then that part cannot be worked around by copying code from headers. + +Anyway, what always works is the pure runtime approach: + + 1. Define types for the functions you need, but not the functions themselves. + 2. At runtime, look up the functions using `dlsym`. + +## Notes on Rust + +If you call macOS APIs from Rust code, you're kind of on your own. Apple does not provide any Rust +"headers", so there isn't really an SDK to speak of. So you have to supply your own API declarations +anyway, regardless of what SDK is being used for building. + +In a way, you're side-stepping some of the build time trouble. You don't need to worry about any +`#ifdefs` because there are no system headers you could conflict with. + +On the other hand, you still need to worry about API availability at runtime. +And in Rust, there are no [availability attributes](https://clang.llvm.org/docs/AttributeReference.html#availability) +on your API declarations, and there are no +[`@available` runtime check helpers](https://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available), +and the compiler cannot warn you if you call APIs outside of availability checks. |