Handling availability

[nvzqz]

• Handle class availability.

  Currently, unavailable OBJC_CLASS_$_* symbols cause a load-time linker error.

  This can be checked by either:

  • Weak linking symbols.

  • Getting the class via objc_getClass.

• Handle method availability.

  Currently, unavailable methods cause an exception.

  This can be checked by either:

  • Checking respondsToSelector: (high runtime cost each time).

  • Checking the OS version against the availability (one time cost).