Swift Lazy计算属性的线程安全分析

一、代码示例

//
//  ViewController.swift
//  LazyTest
//
//  Created by lilun.ios on 2021/7/30.
//

import UIKit

class ViewController: UIViewController {

    lazy var profileImageView: UIImageView = {

        let imageView = UIImageView(image: #imageLiteral(resourceName: "page1"))
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageView.contentMode = .scaleAspectFit
        return imageView

    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        self.view.addSubview(self.profileImageView)
    }

}

profileImageView 变量是不是线程安全的呢?

二、代码分析

lazy修饰的变量是在第一次访问的时候初始化的,如果多线程访问这个Lazy变量,将会导致不可预知的结果
有可能不同线程生成不同的lazy变量
有可能一个线程初始化还没完成,另外一个线程就返回了这个变量
反汇编这个变量getter的实现,判断这个变量是不是为空没有加锁,因此多线程访问下可能出现问题

int _$s8LazyTest14ViewControllerC012profileImageC0So07UIImageC0Cvg() {
    var_58 = r13;
    memset(&var_8, 0x0, 0x8);
    memset(&var_28, 0x0, 0x8);
    memset(&var_48, 0x0, 0x8);
    swift_beginAccess(var_58 + *direct field offset for LazyTest.ViewController.($__lazy_storage_$_profileImageView in _45D895C0736D9EBC78CE118C3607CFFF) : __C.UIImageView?, &var_20, 0x20, 0x0);
    rsi = *(var_58 + rdx);
    var_78 = rsi;
    [rsi retain];
    swift_endAccess(&var_20);
    if (var_78 != 0x0) {
            var_90 = var_78;
    }
    else {
            rax = closure #1 ();
            var_98 = rax;
            [rax retain];
            swift_beginAccess(var_58 + *direct field offset for LazyTest.ViewController.($__lazy_storage_$_profileImageView in _45D895C0736D9EBC78CE118C3607CFFF) : __C.UIImageView?, &var_40, 0x21, 0x0, &var_40);
            rdx = *(var_58 + rsi);
            *(var_58 + rsi) = var_98;
            [rdx release];
            swift_endAccess(&var_40);
            var_90 = var_98;
    }
    rax = var_90;
    return rax;
}