U3D开发中关于脚本方面的限制-有关IOS反射和JIT的支持问题

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U3D开发中关于脚本方面的限制-有关IOS反射和JIT的支持问题

U3D文档中说明了,反射在IOS是支持的,除了system.reflection.emit空间内的,其它都支持。JIT是不支持的

本质上来说即是:只要不在运行时动态生成代码的行为都支持,reflection.emit下的功能可以动态的生成代码(生成程序集,生成类,生成函数,生成类型等,是真正的生成代码),JIT也能动态的生成代码(如C#生成的IL在各平台上运行时可以由JIT编译成汇编语言)。

其它的并不会生成代码的反射功能是可以用的,如 type.gettype, getfield, Activator.CreateInstance   

文档如下:

Scripting restrictions

 

We strive to provide a common scripting API and experience across all platforms Unity supports. However, some platforms have inherent restrictions. To help you understand these restrictions and support cross-platform code, the following table describes which restrictions apply to each platform and scripting backend:

PlatformScripting BackendRestrictions
StandaloneMonoNone
WebGLIL2CPPAhead-of-time compile, No threads
iOSIL2CPPAhead-of-time compile
AndroidMonoNone
AndroidIL2CPPAhead-of-time compile
Samsung TVMonoAhead-of-time compile
TizenMonoAhead-of-time compile
XBox OneMonoAhead-of-time compile
XBox OneIL2CPPAhead-of-time compile
WiiUMonoAhead-of-time compile
PS VitaMonoAhead-of-time compile
PS VitaIL2CPPAhead-of-time compile
PS4MonoAhead-of-time compile
PS4IL2CPPAhead-of-time compile
Windows Store.NETUses .NET Core class libraries subset
Windows StoreIL2CPPAhead-of-time compile

Ahead-of-time compile

Some platforms do not allow runtime code generation. Therefore, any managed code which depends upon just-in-time (JIT) compilation on the target device will fail. Instead, we need to compile all of the managed code ahead-of-time (AOT). Often, this distinction doesn’t matter, but in a few specific cases, AOT platforms require additional consideration.

System.Reflection.Emit

An AOT platform cannot implement any of the methods in the System.Reflection.Emit namespace. Note that the rest of System.Reflection is acceptable, as long as the compiler can infer that the code used via reflection needs to exist at runtime.

Serialization

AOT platforms may encounter issues with serialization and deserlization due to the use of reflection. If a type or method is only used via reflection as part of serialization or deserialization, the AOT compiler cannot detect that code needs to be generated for the type or method.

Generic virtual methods

Generic methods require the compiler to do some additional work to expand the code written by the developer to the code actually executed on the device. For example, we need different code for List with an int or a double. In the presence of virtual methods, where behavior is determined at runtime rather than compile time, the compiler can easily require runtime code generation in places where it is not entirely obvious from the source code.

Suppose we have the following code, which works exactly as expected on a JIT platform (it prints “Message value: Zero” to the console once):

using UnityEngine;
using System;

public class AOTProblemExample : MonoBehaviour, IReceiver {
    public enum AnyEnum {
        Zero,
        One,
    }

    void Start() {
        // Subtle trigger: The type of manager *must* be
        // IManager, not Manager, to trigger the AOT problem.
        IManager manager = new Manager();
        manager.SendMessage(this, AnyEnum.Zero);
    }

    public void OnMessage<T>(T value) {
        Debug.LogFormat("Message value: {0}", value);
    }
}

public class Manager : IManager {
    public void SendMessage<T>(IReceiver target, T value) {
        target.OnMessage(value);
    }
}

public interface IReceiver {
    void OnMessage<T>(T value);
}

public interface IManager {
    void SendMessage<T>(IReceiver target, T value);
}

When this code is executed on an AOT platform with the IL2CPP scripting backend, this exception occurs:

ExecutionEngineException: Attempting to call method 'AOTProblemExample::OnMessage<AOTProblemExample+AnyEnum>' for which no ahead of time (AOT) code was generated.
  at Manager.SendMessage[T] (IReceiver target, .T value) [0x00000] in <filename unknown>:0 
  at AOTProblemExample.Start () [0x00000] in <filename unknown>:0 

Likewise, the Mono scripting backend provides this similar exception:

  ExecutionEngineException: Attempting to JIT compile method 'Manager:SendMessage<AOTProblemExample/AnyEnum> (IReceiver,AOTProblemExample/AnyEnum)' while running with --aot-only.
  at AOTProblemExample.Start () [0x00000] in <filename unknown>:0 

The AOT compiler does not realize that it should generate code for the generic method OnMessage with a T of AnyEnum, so it blissfully continues, skipping this method. When that method is called, and the runtime can’t find the proper code to execute, it gives up with this error message.

To work around an AOT issue like this, we can often force the compiler to generate the proper code for us. If we add a method like this to the AOTProblemExample class:

public void UsedOnlyForAOTCodeGeneration() {
    // IL2CPP needs only this line.
    OnMessage(AnyEnum.Zero);

    // Mono also needs this line. Note that we are
    // calling directly on the Manager, not the IManager interface.
    new Manager().SendMessage(null, AnyEnum.Zero);

    // Include an exception so we can be sure to know if this method is ever called.
    throw new InvalidOperationException("This method is used for AOT code generation only. Do not call it at runtime.");
}

When the compiler encounters the explicit call to OnMessage with a T of AnyEnum, it generates the proper code for the runtime to execute. The method UsedOnlyForAOTCodeGeneration does not ever need to be called; it just needs to exist for the compiler to see it.

No threads

Some platforms do not support the use of threads, so any managed code that uses the System.Threading namespace will fail at runtime. Also, some parts of the .NET class libraries implicitly depend upon threads. An often-used example is the System.Timers.Timer class, which depends on support for threads.

posted on 2018-09-09 23:04 时空观察者9号 阅读() 评论() 编辑 收藏

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