Add ScopedObjCClassSwizzler in base/mac, absorbs objc_method_swizzle and ScopedClassSwizzler

base/mac/scoped_objc_class_swizzler.mm

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "base/mac/scoped_objc_class_swizzler.h"
+
+#include <string.h>
+
+#include "base/logging.h"
+
+namespace base {
+namespace mac {
+
+ScopedObjCClassSwizzler::ScopedObjCClassSwizzler(Class target,
+                                                 Class source,
+                                                 SEL selector) {
+  old_selector_impl_ = class_getInstanceMethod(target, selector);
+  new_selector_impl_ = class_getInstanceMethod(source, selector);
+  if (!old_selector_impl_ && !new_selector_impl_) {
+    // Try class methods.
+    old_selector_impl_ = class_getClassMethod(target, selector);
+    new_selector_impl_ = class_getClassMethod(source, selector);
+  }
+
+  DCHECK(old_selector_impl_);
+  DCHECK(new_selector_impl_);
+  if (!old_selector_impl_ || !new_selector_impl_)
+    return;
+
+  // The argument and return types must match exactly.
+  const char* old_types = method_getTypeEncoding(old_selector_impl_);
+  const char* new_types = method_getTypeEncoding(new_selector_impl_);
+  DCHECK(old_types);
+  DCHECK(new_types);
+  DCHECK_EQ(0, strcmp(old_types, new_types));
+  if (!old_types || !new_types || strcmp(old_types, new_types)) {
+    old_selector_impl_ = new_selector_impl_ = NULL;
+    return;
+  }
+
+  method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
+}
+
+ScopedObjCClassSwizzler::~ScopedObjCClassSwizzler() {
+  if (old_selector_impl_ && new_selector_impl_)
+    method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
+}
+
+IMP ScopedObjCClassSwizzler::GetOriginalImplementation() {
+  // Note that while the swizzle is in effect the "new" method is actually
+  // pointing to the original implementation, since they have been swapped.
+  return method_getImplementation(new_selector_impl_);
+}
+
+}  // namespace mac
+}  // namespace base