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CAAtom refers to a string for identification in QuartzCore. As of 3.1, there are over 300 internal atoms.

Converting atoms to and from strings

Signature NSString* CAAtomGetString(uint32_t atomicValue);
Available in 2.0 –
Signature uint32_t CAInternAtom(NSString* atommicString);
Available in 2.0 –

There are only two exported C functions for CAAtom, CAAtomGetString() and CAInternAtom(), which convert a CAAtom between an integer and its corresponding string value. Most internal code in QuartzCore is identified using atoms. For example, a portion of disassembled code may read:

-[CAGradientLayer setLocations:]:
+00000 0008d9cc             stmdb             sp!,{r7,lr}
+00004 0008d9d0             add               r7,sp,#0x0
+00008 0008d9d4             mov               r1,#0xad
+0000c 0008d9d8             bl                CALayer_setter_kCAValueCopiedPointer
+00010 0008d9dc             ldmia             sp!,{r7,pc}                       ; return

Here, the magic number 0xAD is in fact an atom, which is unsurprisingly the value "locations":

cy# CAAtomGetString(0xad)

The CAAtom of 0 always returns <nil>, and that of an undefined atom is <unknown>. Custom atom values are always larger than the built-in ones.

External links

  • script to dump internal CAAtom's from the QuartzCore binary.