unity 异步加载gameobject

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attitude 2024-12-18 12:07:32

文章标签 System Threading Async 文章分类 游戏开发

Unity 使用Async、Await语法实现异步线程

我们都知道Unity是单线程，哪怕后面高版本渲染这块移动到了其他线程。

Async、Await是C# 5.0的新加的语法，是个非常优雅的语法糖。

我们看看异步函数的声明：

async void SyncFunctionTest()
{
}

很简洁明了，就一个async关键字。虽然被标记异步关键字，但是，还是在当前线程执行。最后我们答应下线程的ID，证明我之前的说法是正确的，代码是这样的：

void Start()
{
	Debug.Log(Thread.CurrentThread.ManagedThreadId);
	SyncFunctionTest();
}

async void SyncFunctionTest()
{
	Debug.Log(Thread.CurrentThread.ManagedThreadId);
}

我们看看控制台：

unity 异步加载gameobject_Async

好了，进入主题，实现一个线程的切换！那么在Unity中我们如何优化的切换一个线程呢？线程池？new Thread? 不不不！

我们看下代码：

using System.Collections;
using System.Collections.Generic;
using System.Threading;
using UnityEngine;

namespace EngineAsyncUtility
{
    public partial class AsyncUtility : MonoBehaviour
    {
        public static AsyncUtility Instance;

        private static int UnityThreadID { get; set; }
        public static bool InUnityThreadContext => Thread.CurrentThread.ManagedThreadId == UnityThreadID;
        public static SynchronizationContext UnitySyncContext { get; private set; }
        public static SynchronizationContext BackgroundSyncContext { get; private set; }

        [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
        public static void AsyncUtilityInitlize()
        {
            UnityThreadID = Thread.CurrentThread.ManagedThreadId;
            UnitySyncContext = SynchronizationContext.Current;
            BackgroundSyncContext = new SynchronizationContext();
            
            Instance = new GameObject("AsyncUtility").AddComponent<AsyncUtility>();
            UnityEngine.Object.DontDestroyOnLoad(Instance);
        }
    }
}

SynchronizationContext ? 好陌生，这个是个什么鬼？用个通俗的大白话来说就是一个线程间上下文调度对象。那么它是如何完成一个线程间的上下文调度？我们看看源码：

/// <summary>When overridden in a derived class, dispatches a synchronous message to a synchronization context.</summary>
    /// <param name="d">The <see cref="T:System.Threading.SendOrPostCallback" /> delegate to call.</param>
    /// <param name="state">The object passed to the delegate.</param>
    /// <exception cref="T:System.NotSupportedException">The method was called in a Windows Store app. The implementation of <see cref="T:System.Threading.SynchronizationContext" /> for Windows Store apps does not support the <see cref="M:System.Threading.SynchronizationContext.Send(System.Threading.SendOrPostCallback,System.Object)" /> method.</exception>
    [__DynamicallyInvokable]
    public virtual void Send(SendOrPostCallback d, object state)
    {
      d(state);
    }

    /// <summary>When overridden in a derived class, dispatches an asynchronous message to a synchronization context.</summary>
    /// <param name="d">The <see cref="T:System.Threading.SendOrPostCallback" /> delegate to call.</param>
    /// <param name="state">The object passed to the delegate.</param>
    [__DynamicallyInvokable]
    public virtual void Post(SendOrPostCallback d, object state)
    {
      ThreadPool.QueueUserWorkItem(new WaitCallback(d.Invoke), state);
    }

我只截取了部分代码，也是上下文调度的主要函数Send和Post，从明面上看，Send是在当前线程完成的调用，Post是从线程池取出来调用。

另外，最终被执行到Thread.cs中的GetExecutionContextReader函数中来切换上下文线程，(中间部分函数我没追踪到。)

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
    internal ExecutionContext.Reader GetExecutionContextReader()
    {
      return new ExecutionContext.Reader(this.m_ExecutionContext);
    }

其实最重要还是 send和post，你提交的方式决定了是否是在当前线程来提交。

AsyncUtility .cs大致就说到这里，那么我们如何使用Async和Await呢？

首先实现一个 SynchronizationContextAwaiter 有过Async和Await基本理解的都知道如果我们要自定义Async/Await，就要自行实现一个Awaiter并继承INotifyCompletion。

最后我实现的源码是这样的：

using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
using UnityEngine;

namespace EngineAsyncUtility.Awaiters
{
    public struct SynchronizationContextAwaiter : INotifyCompletion
    {
        private static readonly SendOrPostCallback _postCallback = state => ((Action)state)();
        private readonly SynchronizationContext _context;
        
        public SynchronizationContextAwaiter(SynchronizationContext context)
        {
            this._context = context;
        }

        public bool IsCompleted => _context == SynchronizationContext.Current;

        public void OnCompleted(Action continuation)
        {
            _context.Post(_postCallback, continuation);
        }

        public void GetResult() { }
    }
}

最后我们扩展下SynchronizationContext类：

namespace EngineAsyncUtility.Extensions
{
    public static class Extensions
    {
        public static SynchronizationContextAwaiter GetAwaiter(this SynchronizationContext scontext)
            => new SynchronizationContextAwaiter(scontext);
    }
}

OK！基本大致就是这样，非常简单！我们写个测试代码验证下结果：

void Start()
    {
        SyncContextTest();
    }
    async void SyncContextTest()
    {
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread true");
        await AsyncUtility.BackgroundSyncContext;
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread false");
        await AsyncUtility.UnitySyncContext; 
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread true");
    }

结果很明显了。

unity 异步加载gameobject_System_02

最后我们做个极端的测试，我们都知道，如果我们在其他线程操作UI，那么Unity就会给你throw一个error！我们试下是否真的切换到其他线程了！测试代码：

public Text t1;
    public Text t2;
    public Text t3;
    void Start()
    {
        SyncContextTest();
    }
    async void SyncContextTest()
    {
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread true");
        t1.text = "Unity Thread true";
        await AsyncUtility.BackgroundSyncContext;
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread false");
        try
        {
            t2.text = "Unity Thread false";
        }
        catch (Exception e)
        {
           Debug.LogError(e);
        }
        await AsyncUtility.UnitySyncContext; 
        Debug.Log(AsyncUtility.InUnityThreadContext);
        Debug.Log("Unity Thread true");
        t3.text = "Unity Thread false";
    }

unity 异步加载gameobject_Async_03