通過runtime替換系統類實作的代碼（從github開源庫fdstackview中摘錄）

其中部分代碼為彙編；由此可見oc的runtime的靈活性和能力。此代碼僅供參考

// ----------------------------------------------------

// Runtime injection start.

// Assemble codes below are based on:

// https://github.com/0xced/NSUUID/blob/master/NSUUID.m

#pragma mark - Runtime Injection

__asm(

      ".section        __DATA,__objc_classrefs,regular,no_dead_strip\n"

#if    TARGET_RT_64_BIT

      ".align          3\n"

      "L_OBJC_CLASS_UIStackView:\n"

      ".quad           _OBJC_CLASS_$_UIStackView\n"

#else

      ".align          2\n"

      "_OBJC_CLASS_UIStackView:\n"

      ".long           _OBJC_CLASS_$_UIStackView\n"

#endif

      ".weak_reference _OBJC_CLASS_$_UIStackView\n"

      );

// Constructors are called after all classes have been loaded.

__attribute__((constructor)) static void FDStackViewPatchEntry(void) {

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        @autoreleasepool {

            // >= iOS9.

            if (objc_getClass("UIStackView")) {

                return;

            }

            Class *stackViewClassLocation = NULL;

#if TARGET_CPU_ARM

            __asm("movw %0, :lower16:(_OBJC_CLASS_UIStackView-(LPC0+4))\n"

                  "movt %0, :upper16:(_OBJC_CLASS_UIStackView-(LPC0+4))\n"

                  "LPC0: add %0, pc" : "=r"(stackViewClassLocation));

#elif TARGET_CPU_ARM64

            __asm("adrp %0, L_OBJC_CLASS_UIStackView@PAGE\n"

                  "add  %0, %0, L_OBJC_CLASS_UIStackView@PAGEOFF" : "=r"(stackViewClassLocation));

#elif TARGET_CPU_X86_64

            __asm("leaq L_OBJC_CLASS_UIStackView(%%rip), %0" : "=r"(stackViewClassLocation));

#elif TARGET_CPU_X86

            void *pc = NULL;

            __asm("calll L0\n"

                  "L0: popl %0\n"

                  "leal _OBJC_CLASS_UIStackView-L0(%0), %1" : "=r"(pc), "=r"(stackViewClassLocation));

#error Unsupported CPU

            if (stackViewClassLocation && !*stackViewClassLocation) {

                Class class = objc_allocateClassPair(FDStackView.class, "UIStackView", 0);

                if (class) {

                    objc_registerClassPair(class);

                    *stackViewClassLocation = class;

                }

        }

    });

}