通过runtime替换系统类实现的代码（从github开源库fdstackview中摘录）

其中部分代码为汇编；由此可见oc的runtime的灵活性和能力。此代码仅供参考

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

// 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));
#else
#error Unsupported CPU
#endif
            
            if (stackViewClassLocation && !*stackViewClassLocation) {
                Class class = objc_allocateClassPair(FDStackView.class, "UIStackView", 0);
                if (class) {
                    objc_registerClassPair(class);
                    *stackViewClassLocation = class;
                }
            }
        }
    });
}