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cocos–将静态调用包装为实例调用

CocosWrapper

什么是CocosWrapper？

Wrapper是一个包装器，包装器用于消除平台差异化，消除版本差异化等等，其构想源自于GradleWrapper。

而CocosWrapper是为了实现从cocos使用ts调用原生类、方法的一个包装器，而原生区分了iOS和Android，Wrapper就是为了消除这样的平台差异。

与此同时，CocosWrapper也承担了别的功能，总结如下：

  • 消除平台差异化
  • 简化调用过程
  • 规范编码
  • 驱动开发人员代码编写风格上更”类型化”即更加”TypeScript”
  • 易于静态检查错误
  • 易于后期维护
  • 但不利于首次开发，第一个Wrapper编写需要具备一定的变成素养，否则容易遗漏

问题解析

常规开发下，cocos提供了jsb这个命名空间，全称是js-binding，这是一个c++编写的模块，用于将js与c++进行绑定，而实际上c++可以看作是每个平台下的原生代码。
jsb提供了js向原生调用的桥梁，而具体过程大概是这样的

                ios:oc++ -> oc/swift
js->jsb->c++->
                android:ndk -> java/kotlin

cocos中提供了这样一个方法

jsb.reflection.callStaticMethod (className: string, methodName: string, methodSignature: string, ...parameters:any): any

有且只有这么一个方法，这就至少有两个缺陷，只能调用静态方法，参数都是string，给所调方法的传惨也只能用string

那我们是不是能够想一个办法，对这个callStaticMethod进行一定程度的包装，对调用方屏蔽这几个缺陷，同时让调用方认为自己调的是一个常规的ts方法。

比如现在我们想要获取当前App的包版本，这个具有平台差异性的，iOS/Android获取自己的包版本方案是不一样的。

如果使用原始jsb直接获取版本号，如下例子：

  • iOS
VersionUtil.swift

class VersionUtil {
    static func currrentBundleVersion(): String {
        ………
    }
}
  • Android
package com.boostfield.mujica.utility

object VersionUtility {
    fun getBundleVersion(): String {
        ………
    }
}

而也给cocos调用带来了困难：

let version: string
if (isIOS) {
    // 需要告知类名、方法名
    version = jsb.reflection.callStaticMethod("VersionUtil", "currrentBundleVersion:")
} else if (isAndroid) {
    // 需要告知全限定名、方法名、方法签名
    version = jsb.reflection.callStaticMethod("com.boostfield.mujica.utility.VersionUtility", "currrentBundleVersion", "()Ljava/lang/String;")
}

这样的纯字符串操作，一方面很容易编码出错，其次难以维护，想要调参、修改也很困难。

技术方案解析

根据上面这张图我们看看什么是系统调用，系统调用其实是中断的一种，可以把它称为同步软中断，所谓中断就是发出一个信号让cpu停下正在执行的命令，转而跳转到某一个
别的地方，先干点别的事，再回来继续之前的命令。那么同理，函数调用实际上就是让cpu暂停当前的函数，跳转到被调用函数后再返回，而被调用函数是通过函数指针找到的，
指针存储着函数所在的首地址，我们通常使用object.method()这种写法来调用函数，其实在编译过程中把这个调用写成了寻址+中断，这是大家常见的做法，而这也是在编码
的时候就已经知道具体要调用的是什么类里的什么成员函数了，但如果在编码时不确定呢？

静态语言可以在运行时反射，动态语言也有类似机制，他们大都需要以字符串变量来决定所要用到的类和方法，而实际上cocos的jsb就是利用了这一机制。反射固然灵活，
可也增加了编码难度，对编码人员的业务素质要求更高，而且不利于检查排错，我们在js调用native过程中并不需要用到灵活性，反而是更希望将其固化下来，因此有了以下方案。

以VersionUtil为例：

按照常规方法，我们需要这么写

let version: string
if (isIOS) {
version = jsb.reflection.callStaticMethod("VersionUtil", let version: string
if (isIOS) {
    version = jsb.reflection.callStaticMethod("VersionUtil", "isLatest:")
} else if (isAndroid) {
    version = jsb.reflection.callStaticMethod("com.boostfield.mujica.utility.VersionUtil", "isLatest", "()Ljava/lang/String;")
}

而我们的目标是希望像常规调用那样

let versionUtilWrapper: VerstionUtilWrapper
if (isIOS) {
    versionUtilWrapper = new VersionUtilWrapperIos()
} else if (isAndroid) {
let versionUtilWrapper: VerstionUtilWrapper
if (isIOS) {
    versionUtilWrapper = new VersionUtilWrapperIos()
} else if (isAndroid) {
    versionUtilWrapper = new VersionUtilWrapperaAndroid()
}
versionUtilWrapper.isLatest()
versionUtilWrapper.xxxx()

1、制定接口规范

kotlin-multiplatform这个技术可以将kotlin所编写的纯逻辑代码编译成各平台的库，我们可以使用kotlin制定wrapper接口规范

// VersionUtilWrapper.kt

interface VersionUtilWrapper {
    /**
     * 检查此次启动程序是否为最新安装
     * @param null
     * @return {
     * isLatest: boolean
     * }
     */
    fun isLatest(parameters: MutableMap<Any, Any>?): MutableMap<Any, Any>
}

编译后产出物：

//iOS

__attribute__((swift_name("VersionUtilWrapper")))
@protocol SharedLibraryVersionUtilWrapper
@required
- (SharedLibraryMutableDictionary<id, id> *)isLatestParameters:(SharedLibraryMutableDictionary<id, id> * _Nullable)parameters __attribute__((swift_name("isLatest(parameters:)")));
@end;
//Android

@kotlin.js.ExperimentalJsExport public interface VersionUtilWrapper {
    public abstract fun isLatest(parameters: kotlin.collections.MutableMap<kotlin.Any, kotlin.Any>?): kotlin.collections.MutableMap<kotlin.Any, kotlin.Any>
}

2、按照规范实现接口

在原生项目中导入SharedLibrary后, 分别实现这个Wrapper

  • iOS
//VersionUtilWrapper.h
@interface VersionUtilWrapper : BaseWrapper <SharedLibraryVersionUtilWrapper>

@end

//VersionUtilWrapper.m
@implementation VersionUtilWrapper

- (nonnull SharedLibraryMutableDictionary<id,id> *)isLatestParameters:(SharedLibraryMutableDictionary<id,id> * _Nullable)parameters {
    SharedLibraryMutableDictionary *result = [[SharedLibraryMutableDictionary alloc] init];
    [result setValue:[VersionUtil isLatest] forKey:@"isLatest"];
    return result;
}
@end
  • Android
//VersionUtilWrapper.java

public Map<Object, Object> isLatest(@Nullable Map<Object, Object> map) {
    Map<Object, Object> result = new HashMap<>();
    result.put("isLatest",VersionUtil.INSTANCE.isLatest());
    return result;
}

3、cocos编写虚基类

//version_util_wrapper.ts

export default class VersionUtilWrapper extends BaseWrapper {
    protected className: string = 'VersionUtilWrapper'
    protected methodName = 'callMethod:'

    protected isLatestMethod: string = ''
    protected shouldClearCocosProjectManifestMethod: string = ''
    protected resetShouldClearCocosProjectManifestMethod: string = ''

    isLatest(): boolean {
        let body = new ParameterBody(
            this.isLatestMethod,
            {},
            '(Ljava/lang/String;)Ljava/lang/String;',
            'org.cocos2dx.javascript.wrapper4cocos.VersionUtilWrapper'
        )
        let result = this.callStaticMethod(body)
        return result['isLatest']
    }
}

这个虚基类就是我们之后将要使用到的VersionUtilWrapper，我们会构造一个该类的对象实例，而且会根据当前运行环境是iOS还是Android来构造不同实现的实例对象，
其实现的差异化就体现在：iOS/Android在反射时用到的类名和方法名有所不同

4、cocos编写各平台实现类

// version_util_wrapper_ios.ts

export default class VersionUtilWrapperIos extends VersionUtilWrapper {
    protected isLatestMethod: string = 'isLatestParameters:'
}
// version_util_wrapper_android.ts

export default class VersionUtilWrapperAndroid extends VersionUtilWrapper {
    protected isLatestMethod: string = 'isLatest'
    className = 'org/cocos2dx/javascript/wrapper4cocos/VersionUtilWrapper'
}

实际上在各平台实现类，其差异就是在于iOS/Android在反射时用到的类名和方法名的不同

总结

想要调用原生类和方法其实不止这么一种方式，还可以通过修改js-bridge来实现，其原理是在jsb层写c++代码调用原生并暴露接口，而在cocos层直接调用
jsb的接口，但是cocos creator是依赖于jsb的，每次修改jsb都要重新编译cocos creator，这位团队协作带来了困难和负担，我们希望尽可能的在代码层面
实现改动，而尽量不要修改编译工具链本身。

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