Objective-C 内部机制：非脆弱实例变量

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// #import <objc/NSObject.h>
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@interface NSObject <NSObject> {
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  Class             isa;                // 0x00
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}
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@end
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// #import <AppKit/NSResponder.h>
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@interface NSResponder: NSObject {
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  id                _nextResponder;     // 0x04
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}
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@end
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// #import <AppKit/NSView.h>
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typedef struct __VFlags {
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  unsigned int flags;
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} _VFlags;
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@class _NSViewAuxiliary;
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@interface NSView: NSResponder {
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  /* All instance variables are private */
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  NSRect            _frame;             // 0x08
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  NSRect            _bounds;            // 0x18
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  NSView           *_superview;         // 0x28
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  NSArray          *_subviews;          // 0x2c
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  NSWindow         *_window;            // 0x30
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  id                _unused_was_gState; // 0x34
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  id                _frameMatrix;       // 0x38
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  CALayer          *_layer;             // 0x3c
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  id                _dragTypes;         // 0x40
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  _NSViewAuxiliary *_viewAuxiliary;     // 0x44
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  _VFlags           _vFlags;            // 0x48
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  struct __VFlags2 {
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    unsigned int flags;
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  }                 _vFlags2;           // 0x4c
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}
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@end

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struct NSViewHeapLayout {
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  Class             isa;                // 0x00
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  id                _nextResponder;     // 0x04
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  NSRect            _frame;             // 0x08
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  NSRect            _bounds;            // 0x18
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  NSView           *_superview;         // 0x28
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  NSArray          *_subviews;          // 0x2c
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  NSWindow         *_window;            // 0x30
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  id                _unused_was_gState; // 0x34
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  id                _frameMatrix;       // 0x38
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  CALayer          *_layer;             // 0x3c
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  id                _dragTypes;         // 0x40
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  _NSViewAuxiliary *_viewAuxiliary;     // 0x44
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  _VFlags           _vFlags;            // 0x48
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  struct __VFlags2  _vFlags2;           // 0x4c
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};

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NSView *superview = aView->_superview;

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NSView *superview = *(NSView **)((intptr_t)aView + 0x28);

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extern uint32_t OBJC_IVAR_$_NSView._superview;
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NSView *superview = *(NSView **)((intptr_t)aView + OBJC_IVAR_$_NSView._superview);

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static Class realizeClassWithoutSwift(Class cls, Class previously) {
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  // ...
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  // Reconcile instance variable offsets / layout.
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  // This may reallocate class_ro_t, updating our ro variable.
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  if (supercls && !isMeta) reconcileInstanceVariables(cls, supercls, ro);
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  // ...
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}

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static void reconcileInstanceVariables(Class cls, Class supercls, const class_ro_t*& ro) {
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  // ...
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  if (ro->instanceStart >= super_ro->instanceSize) {
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    // Superclass has not overgrown its space. We're done here.
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    return;
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  }
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  if (ro->instanceStart < super_ro->instanceSize) {
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    // Superclass has changed size. This class's ivars must move.
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    // Also slide layout bits in parallel.
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    // This code is incapable of compacting the subclass to
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    //   compensate for a superclass that shrunk, so don't do that.
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    class_ro_t *ro_w = make_ro_writeable(rw);
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    ro = rw->ro();
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    moveIvars(ro_w, super_ro->instanceSize);
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  }
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}

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static void moveIvars(class_ro_t *ro, uint32_t superSize) {
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  uint32_t diff = superSize - ro->instanceStart;
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  if (ro->ivars) {
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    // Find maximum alignment in this class's ivars
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    uint32_t maxAlignment = 1;
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    for (const auto& ivar : *ro->ivars) {
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      if (!ivar.offset) continue;  // anonymous bitfield
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      uint32_t alignment = ivar.alignment();
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      if (alignment > maxAlignment) maxAlignment = alignment;
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    }
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    // Compute a slide value that preserves that alignment
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    uint32_t alignMask = maxAlignment - 1;
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    diff = (diff + alignMask) & ~alignMask;
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    // Slide all of this class's ivars en masse
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    for (const auto& ivar : *ro->ivars) {
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      if (!ivar.offset) continue;  // anonymous bitfield
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      uint32_t oldOffset = (uint32_t)*ivar.offset;
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      uint32_t newOffset = oldOffset + diff;
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      *ivar.offset = newOffset;
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    }
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  }
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  *(uint32_t *)&ro->instanceStart += diff;
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  *(uint32_t *)&ro->instanceSize += diff;
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}

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上述 for 循环“滑动”了实例变量（ivar）的偏移量——从类起始位置向后移动，以容纳更大的基类，同时保持对齐。它所写入的 ivar 变量正是实例变量偏移量符号，例如 `OBJC_IVAR_$_NSView._superview`（如前文所述）。
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因此，通过增加一步间接访问来读写实例变量值，并承受微小的启动开销，Objective-C 运行时（runtime）得以优雅地消除一项重大的 ABI（应用程序二进制接口）兼容性问题，且仅带来极小的开销与复杂度。