SpiderMonkey学习笔记（4）－－实现max()函数

本文主要是介绍SpiderMonkey学习笔记（4）－－实现max()函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

上一篇文章中的JsEngine2对SpiderMonkey稍加扩展，使得JS代码里可以使用alert()函数，但是并没有仔细解释如何使用JSNative。这一篇文章我们看看如何给JS提供一个max()函数，经一步研究JSNative。

1）添加JsEngine3类

添加一个新的C++类，命名为JsEngine3，并让它继承JsEngine2：

// JsEngine3.h
#include "JsEngine2.h"class JsEngine3 : public JsEngine2 {
public:JsEngine3();virtual ~JsEngine3();
};

2）添加C++版max()函数

代码如下所示，比较简单，就不多解释了：

// JsEngine3.cpp
#include "JsEngine3.h"static double max(double a, double b) {return a > b ? a : b;
}

3）添加JSNative版max()函数

static JSBool myjs_max(JSContext *cx, unsigned argc, jsval *vp) {double a, b;if (!JS_ConvertArguments(cx, argc, JS_ARGV(cx, vp), "dd", &a, &b))return JS_FALSE;double c = max(a, b);JS_SET_RVAL(cx, vp, DOUBLE_TO_JSVAL(c));return JS_TRUE;
}

让我们重点研究一下上面这个函数的实现，关于 JSNative的更多解释可以看这里。首先定义了两个double型临时变量a和b，接着调用JS_ConvertArguments()从vp中提取出a和b。JS_ConvertArguments()函数的使用方式有点像printf，它的第四个参数是一个字符串，后面是VARGs。这里我们传给它一个“dd”，代表两个double，分别赋给a和b。下面是从jsapi.h里面摘出来的JS_ConvertArguments()函数的声明和注释：

/** Format is a string of the following characters (spaces are insignificant),* specifying the tabulated type conversions:**   b      JSBool          Boolean*   c      uint16_t/jschar ECMA uint16_t, Unicode char*   i      int32_t         ECMA int32_t*   u      uint32_t        ECMA uint32_t*   j      int32_t         Rounded int32_t (coordinate)*   d      double          IEEE double*   I      double          Integral IEEE double*   S      JSString *      Unicode string, accessed by a JSString pointer*   W      jschar *        Unicode character vector, 0-terminated (W for wide)*   o      JSObject *      Object reference*   f      JSFunction *    Function private*   v      jsval           Argument value (no conversion)*   *      N/A             Skip this argument (no vararg)*   /      N/A             End of required arguments** The variable argument list after format must consist of &b, &c, &s, e.g.,* where those variables have the types given above.  For the pointer types* char *, JSString *, and JSObject *, the pointed-at memory returned belongs* to the JS runtime, not to the calling native code.  The runtime promises* to keep this memory valid so long as argv refers to allocated stack space* (so long as the native function is active).** Fewer arguments than format specifies may be passed only if there is a /* in format after the last required argument specifier and argc is at least* the number of required arguments.  More arguments than format specifies* may be passed without error; it is up to the caller to deal with trailing* unconverted arguments.*/
extern JS_PUBLIC_API(JSBool)
JS_ConvertArguments(JSContext *cx, unsigned argc, jsval *argv, const char *format, ...);

拿到参数a和b以后，调用C++版max()函数求出二者中较大的那个，赋值给临时变量c。然后使用JS_SET_VAL宏设置JS函数的返回值。DOUBLE_TO_JSVAL函数可以把double值转换为jsval。jsval的详细说明可以 看这里。最后返回JS_TRUE告诉SpiderMonkey函数调用成功。

4）注册myjs_max

static JSFunctionSpec myjs_global_functions[] = {JS_FS("max", myjs_max, 2, 0),JS_FS_END
};JsEngine3::JsEngine3() {if (!JS_DefineFunctions(cx, global, myjs_global_functions)) {return;}
}

JS_FS宏可以产生一个 JSFunctionSpec，JS_FS_END宏产生一个类似NULL的JSFunctionSpec。最后调用 JS_DefineFunctions函数注册max函数。完整的JsEngine3.cpp代码如下：

#include "JsEngine3.h"static double max(double a, double b) {return a > b ? a : b;
}static JSBool myjs_max(JSContext *cx, unsigned argc, jsval *vp) {double a, b;if (!JS_ConvertArguments(cx, argc, JS_ARGV(cx, vp), "dd", &a, &b))return JS_FALSE;double c = max(a, b);JS_SET_RVAL(cx, vp, DOUBLE_TO_JSVAL(c));return JS_TRUE;
}static JSFunctionSpec myjs_global_functions[] = {JS_FS("max", myjs_max, 2, 0),JS_FS_END
};JsEngine3::JsEngine3() {if (!JS_DefineFunctions(cx, global, myjs_global_functions)) {return;}
}JsEngine3::~JsEngine3() {}

5）测试max函数

修改ViewController.mm测试一下：

- (void)viewDidLoad
{[super viewDidLoad];// Do any additional setup after loading the view, typically from a nib.JsEngine3 engine;engine.execJS("alert(max(3.1415, 4));");
}

运行模拟器，输出中打印出了4！

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