SwiftObject 杂记

一、前言

　　看了一段时间的Swift，慢慢转变了一些对Swift的看法。

　　Swift作为苹果新晋的开发语言、具有模板编程、函数编程、协议多继承、vTable静态绑定、值引用类型区分、Option类型等动态语法的多种特性。

　　Swift作为一门跨平台的语言，非常强调性能，静态绑定是Swift跟OC语言的动态派发迥异的区别。Objetive-C中的Runtime优秀的设计让OC这门语言有着优秀的动态特性，

　　　　Swift的语言设计中一部分为了和OC桥接，一部分也参考了OC的设计让Swift具有一些动态特性，比如自省。

　　Swift强调安全，Optional代替OC中的空指针，同时优化了性能。Optional调用链遇到中间的Optional不会继续执行。同时值类型的语义的大量使用可以规避线程安全带来的问题。

二、本次文章主要探究Swift编译运行机制

　　1）编写一个控制台程序，将可执行文件拖入Hopper进行反编译

　　注：下一部分代码借鉴 https://mp.weixin.qq.com/s/zIkB9KnAt1YPWGOOwyqY3Q 《Swift 对象内存模型探究（一）》

//
//  main.swift
//  TestSwift2
//
//  Created by lunli on 2018/10/9.
//  Copyright  2018年 lunli. All rights reserved.
//

import Foundation
import ObjectiveC

class People{
    var xxxxx = 1
}

class Driver:People{
    var yyyyy = 2
}

class SellerPeople{
    var zzzzz = 3
}

struct Person {
    var name: String
    var age: Int
}

class ViewController {
    var x = 123456
    var person: Person? {
        didSet{

            print("Called after setting the new value")
            if let name = person?.name {
                print("New name is \(name) and old name is \(String(describing: oldValue?.name))")
            }
        }
        willSet(myNewValue) {

            print("Called before setting the new value")
            if let newName = myNewValue?.name {
                print("New name is \(newName)")
            }
        }
    }

}

var p = Driver()
print(type(of: p))
print(String.init(format: "%s", object_getClassName(p)))

var v = ViewController()
v.person = Person(name: "1234", age: 123)

class Human : ViewController {
    var age: Int?
    var name: String?
    var nicknames: [String] = [String]()

    //返回指向 Human 实例头部的指针
    func headPointerOfClass() -> UnsafeMutablePointer<Int8> {
        let opaquePointer = Unmanaged.passUnretained(self as AnyObject).toOpaque()
        let mutableTypedPointer = opaquePointer.bindMemory(to: Int8.self, capacity: MemoryLayout<Human>.stride)
        return UnsafeMutablePointer<Int8>(mutableTypedPointer)
    }
}

let human = Human()
let arrFormJson = ["goudan","zhaosi", "wangwu"]

//拿到指向 human 堆内存的 void * 指针
let humanRawPtr = UnsafeMutableRawPointer(human.headPointerOfClass())

//nicknames 数组在内存中偏移 64byte 的位置(16 + 16 + 32)
let humanNickNamesPtr =  humanRawPtr.advanced(by: 64).assumingMemoryBound(to: Array<String>.self)
//human.nicknames
//[]
let test = humanRawPtr.advanced(by: 0).assumingMemoryBound(to: AnyObject.self)
let test2 = humanRawPtr.advanced(by: 8).assumingMemoryBound(to: Int64.self)
print(test)

humanNickNamesPtr.initialize(to: arrFormJson)
print(human.nicknames)          //["goudan","zhaosi", "wangwu"]

print((human as AnyObject).isKind(of: ViewController.self))

　　拖入Hopper之后，可以看到

　　我们可以清晰看到SellerPeople的类的定义，和其MetaClass的定义；其定义跟现有的OC Class类型定义比较类似，一个指针指向父类，一个指向元类，Cache未知、一个vtable表示方法调用列表、data表示类中的变量数据

　　上面的截图可以看到SellerPeople中ivar的定义和名字方法的定义、通过这些信息可以完全实现一个对象类型的检测、属性列表、方法列表等。

在DEBUG模式生成的可执行文件下面，我们用命令行对其中的符号进行恢复，可以清晰看到类名、变量名。

LUNLI-MC1:Swift 二进制分析 lunli$ nm TestSwift2|xcrun swift-demangle|grep "ViewController"
0000000100002120 t TestSwift2.ViewController.person.didset : TestSwift2.Person?
0000000100002ae0 T TestSwift2.ViewController.person.getter : TestSwift2.Person?
0000000100002d70 T TestSwift2.ViewController.person.materializeForSet : TestSwift2.Person?
0000000100002d40 t closure #1 : () in TestSwift2.ViewController.person.materializeForSet : TestSwift2.Person?
00000001004f3cd8 S direct field offset for TestSwift2.ViewController.person : TestSwift2.Person?
0000000100002110 T variable initialization expression of TestSwift2.ViewController.person : TestSwift2.Person?
0000000100002b60 T TestSwift2.ViewController.person.setter : TestSwift2.Person?
00000001000026f0 t TestSwift2.ViewController.person.willset : TestSwift2.Person?
0000000100002e10 T TestSwift2.ViewController.__allocating_init() -> TestSwift2.ViewController
0000000100002f60 T TestSwift2.ViewController.init() -> TestSwift2.ViewController
0000000100511110 s reflection metadata field descriptor TestSwift2.ViewController
0000000100589e38 b lazy cache variable for type metadata for TestSwift2.ViewController
0000000100001cd0 T type metadata accessor for TestSwift2.ViewController
0000000100584d60 d full type metadata for TestSwift2.ViewController
0000000100584d38 D metaclass for TestSwift2.ViewController
00000001004f3cf0 S nominal type descriptor for TestSwift2.ViewController
0000000100584d70 D type metadata for TestSwift2.ViewController
0000000100003030 T TestSwift2.ViewController.__deallocating_deinit
0000000100002fd0 T TestSwift2.ViewController.deinit
0000000100589ad8 S TestSwift2.v : TestSwift2.ViewController
0000000100507a0e s _symbolic ____ 10TestSwift214ViewControllerC

　　

　　2）既然能看到任意swift定义的class结构，那么 swift是否存在一个共同的父类结构？

　　搜索之后看到一个答案：

　　

　　　　原文：https://stackoverflow.com/questions/24137368/why-is-there-no-universal-base-class-in-swift

　　我写了一点代码来模拟这个特性，一段OC和Swift混编的代码

　　

import Foundation

class People{
    var xxxxx = 1
}

class Driver:People{
    var yyyyy = 2
}

class SellerPeople{
    var zzzzz = 3
}

struct Person {
    var name: String
    var age: Int
}

class ViewController1 : NSObject {

    var obj = SellerPeople()
    @objc var str = "Hello World"
    var x = 123456
    var person: Person? {
        didSet{

            print("Called after setting the new value")
            if let name = person?.name {
                print("New name is \(name) and old name is \(String(describing: oldValue?.name))")
            }
        }
        willSet(myNewValue) {

            print("Called before setting the new value")
            if let newName = myNewValue?.name {
                print("New name is \(newName)")
            }
        }
    }

    @objc func getSwiftObj() -> AnyObject {
        return obj
    }

}

class Human : ViewController1 {
    var age: Int?
    var name: String?
    var nicknames: [String] = [String]()

    //返回指向 Human 实例头部的指针
    func headPointerOfClass() -> UnsafeMutablePointer<Int8> {
        let opaquePointer = Unmanaged.passUnretained(self as AnyObject).toOpaque()
        let mutableTypedPointer = opaquePointer.bindMemory(to: Int8.self, capacity: MemoryLayout<Human>.stride)
        return UnsafeMutablePointer<Int8>(mutableTypedPointer)
    }
}

　　　　注意getSwiftObject方法返回的是一个Swift对象，我们将这个对象拿到OC中用调试器进行调试

可以看到调试器中打印出来，一个纯Swift没有指定父类的类默认继承的对象是SwiftObject，参考这个类的实现如下

https://github.com/apple/swift/blob/f4db1dd7a4abba2685247e1a7415d4fcb91f640d/stdlib/public/runtime/SwiftObject.h

//===--- SwiftObject.h - Native Swift Object root class ---------*- C++ -*-===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This implements the Objective-C root class that provides basic `id`-
// compatibility and `NSObject` protocol conformance for pure Swift classes.
//
//===----------------------------------------------------------------------===//

#ifndef SWIFT_RUNTIME_SWIFTOBJECT_H
#define SWIFT_RUNTIME_SWIFTOBJECT_H

#include "swift/Runtime/Config.h"
#include <cstdint>
#include <utility>
#include "swift/Runtime/HeapObject.h"
#if SWIFT_OBJC_INTEROP
#include "llvm/Support/Compiler.h"
#include <objc/NSObject.h>
#endif

#if SWIFT_OBJC_INTEROP

#if SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
// Source code: "SwiftObject"
// Real class name: mangled "Swift._SwiftObject"
#define SwiftObject _TtCs12_SwiftObject
#else
// Pre-stable ABI uses un-mangled name for SwiftObject
#endif

#if __has_attribute(objc_root_class)
__attribute__((__objc_root_class__))
#endif
SWIFT_RUNTIME_EXPORT @interface SwiftObject<NSObject> {
 @private
  Class isa;
  SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS;
}

- (BOOL)isEqual:(id)object;
- (NSUInteger)hash;

- (Class)superclass;
- (Class)class;
- (instancetype)self;
- (struct _NSZone *)zone;

- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

- (BOOL)isProxy;

+ (BOOL)isSubclassOfClass:(Class)aClass;
- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;

- (BOOL)respondsToSelector:(SEL)aSelector;
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;

- (instancetype)retain;
- (oneway void)release;
- (instancetype)autorelease;
- (NSUInteger)retainCount;

- (NSString *)description;
- (NSString *)debugDescription;
@end

namespace swift {

NSString *getDescription(OpaqueValue *value, const Metadata *type);

}

#endif

#endif

　　这个类包含一个实例变量isa，这个Class是一个指针，指向的是OC Runtime中类定义的结构；同时这个类实现了NSObject的协议，猜测这样的设计是为了跟OC进行桥接。

3）Swift的对象的内存布局

　　这里的内容主要来自第一篇提到的文章，一个Swift引用指向的内存区域，

　　　　第一个指针应该是isa，

　　　　第二个8字节（32位机器为4字节）为引用计数，

　　　　第三块区域是vtable指针，

　　　　接下来的是继承的变量

　　　　接下来是这个类实现的变量

　　可见这里的对象内存布局和C++有些类似，但是增加了一个isa的东西，让这个对象具有了一些动态特征。方法执行采用vtable寻址的方式，快速高效。