在Swift中使用JavaScript的方法和技巧

2015-01-27    分類:iOS開發、WEB開發、程式設計開發、首頁精華0人評論發表於2015-01-27

本文作者Nate Cook是一位獨立的Web及移動應用開發者,是繼Mattt大神之後NSHipster的主要維護者,也是非常知名活躍的Swift博主,並且還是支援自動生成Swift線上文件的SwiftDoc.org網站創造者。在本文中,他介紹了在Swift中使用JavaScript的方法和技巧,對於iOS和Web應用工程師有著非常實用的價值,以下為譯文:

在RedMonk釋出的2015年1月程式語言排行榜中,Swift採納率排名迅速飆升,從剛剛面世時的68位躍至22位,Objective-C仍然穩居TOP
10,而JavaScript則憑藉著其在iOS平臺上原生體驗優勢成為了年度最火熱的程式語言。

而早在2013年蘋果釋出的OS X Mavericks和iOS 7兩大系統中便均已加入了JavaScriptCore框架,能夠讓開發者輕鬆、快捷、安全地使用JavaScript語言編寫應用。不論叫好叫罵,JavaScript霸主地位已成事實。開發者們趨之若鶩,JS工具資源層出不窮,用於OS
X和iOS系統等高速虛擬機器也蓬勃發展起來。

JSContext/JSValue

JSContext即JavaScript程式碼的執行環境。一個Context就是一個JavaScript程式碼執行的環境,也叫作用域。當在瀏覽器中執行JavaScript程式碼時,JSContext就相當於一個視窗,能輕鬆執行建立變數、運算乃至定義函式等的JavaScript程式碼:

//Objective-C
JSContext *context = [[JSContext alloc] init];
[context evaluateScript:@"var num = 5 + 5"];
[context evaluateScript:@"var names = ['Grace', 'Ada', 'Margaret']"];
[context evaluateScript:@"var triple = function(value) { return value * 3 }"];
JSValue *tripleNum = [context evaluateScript:@"triple(num)"];
//Swift
let context = JSContext()
context.evaluateScript("var num = 5 + 5")
context.evaluateScript("var names = ['Grace', 'Ada', 'Margaret']")
context.evaluateScript("var triple = function(value) { return value * 3 }")
let tripleNum: JSValue = context.evaluateScript("triple(num)")

像JavaScript這類動態語言需要一個動態型別(Dynamic Type), 所以正如程式碼最後一行所示,JSContext裡不同的值均封裝在JSValue物件中,包括字串、數值、陣列、函式等,甚至還有Error以及null和undefined。

JSValue包含了一系列用於獲取Underlying Value的方法,如下表所示:

JavaScript Type
JSValue method
Objective-C Type
Swift Type
string toString NSString String!
boolean toBool BOOL Bool
number toNumbertoDoubletoInt32

toUInt32

NSNumberdoubleint32_t

uint32_t

NSNumber!DoubleInt32

UInt32

Date toDate NSDate NSDate!
Array toArray NSArray [AnyObject]!
Object toDictionary NSDictionary [NSObject : AnyObject]!
Object toObjecttoObjectOfClass: custom type custom type

想要檢索上述示例中的tripleNum值,只需使用相應的方法即可:

//Objective-C
NSLog(@"Tripled: %d", [tripleNum toInt32]);
// Tripled: 30
//Swift
println("Tripled: \(tripleNum.toInt32())")
// Tripled: 30

下標值(Subscripting Values)

通過在JSContext和JSValue例項中使用下標符號可以輕鬆獲取上下文環境中已存在的值。其中,JSContext放入物件和陣列的只能是字串下標,而JSValue則可以是字串或整數下標。

//Objective-C
JSValue *names = context[@"names"];
JSValue *initialName = names[0];
NSLog(@"The first name: %@", [initialName toString]);
// The first name: Grace
//Swift
let names = context.objectForKeyedSubscript("names")
let initialName = names.objectAtIndexedSubscript(0)
println("The first name: \(initialName.toString())")
// The first name: Grace

而Swift語言畢竟才誕生不久,所以並不能像Objective-C那樣自如地運用下標符號,目前,Swift的方法僅能實現objectAtKeyedSubscript()和objectAtIndexedSubscript()等下標。

函式呼叫(Calling Functions)

我們可以將Foundation類作為引數,從Objective-C/Swift程式碼上直接呼叫封裝在JSValue的JavaScript函式。這裡,JavaScriptCore再次發揮了銜接作用。

//Objective-C
JSValue *tripleFunction = context[@"triple"];
JSValue *result = [tripleFunction callWithArguments:@[@5] ];
NSLog(@"Five tripled: %d", [result toInt32]);
//Swift
let tripleFunction = context.objectForKeyedSubscript("triple")
let result = tripleFunction.callWithArguments([5])
println("Five tripled: \(result.toInt32())")

異常處理(Exception Handling)

JSContext還有一個獨門絕技,就是通過設定上下文環境中exceptionHandler的屬性,可以檢查和記錄語法、型別以及出現的執行時錯誤。exceptionHandler是一個回撥處理程式,主要接收JSContext的reference,進行異常情況處理。

//Objective-C
context.exceptionHandler = ^(JSContext *context, JSValue *exception) {
   NSLog(@"JS Error: %@", exception);
};
[context evaluateScript:@"function multiply(value1, value2) { return value1 * value2 "];
// JS Error: SyntaxError: Unexpected end of script
//Swift
context.exceptionHandler = { context, exception in
    println("JS Error: \(exception)")
}
context.evaluateScript("function multiply(value1, value2) { return value1 * value2 ")
// JS Error: SyntaxError: Unexpected end of script

JavaScript函式呼叫

瞭解了從JavaScript環境中獲取不同值以及呼叫函式的方法,那麼反過來,如何在JavaScript環境中獲取Objective-C或者Swift定義的自定義物件和方法呢?要從JSContext中獲取本地客戶端程式碼,主要有兩種途徑,分別為Blocks和JSExport協議。

  • Blocks (塊)

在JSContext中,如果Objective-C程式碼塊賦值為一個識別符號,JavaScriptCore就會自動將其封裝在JavaScript函式中,因而在JavaScript上使用Foundation和Cocoa類就更方便些——這再次驗證了JavaScriptCore強大的銜接作用。現在CFStringTransform也能在JavaScript上使用了,如下所示:

//Objective-C
context[@"simplifyString"] = ^(NSString *input) {
   NSMutableString *mutableString = [input mutableCopy];
   CFStringTransform((__bridge CFMutableStringRef)mutableString, NULL, kCFStringTransformToLatin, NO);
   CFStringTransform((__bridge CFMutableStringRef)mutableString, NULL, kCFStringTransformStripCombiningMarks, NO);
   return mutableString;
};
NSLog(@"%@", [context evaluateScript:@"simplifyString('안녕하새요!')"]);
//Swift
let simplifyString: @objc_block String -> String = { input in
    var mutableString = NSMutableString(string: input) as CFMutableStringRef
    CFStringTransform(mutableString, nil, kCFStringTransformToLatin, Boolean(0))
    CFStringTransform(mutableString, nil, kCFStringTransformStripCombiningMarks, Boolean(0))
    return mutableString
}
context.setObject(unsafeBitCast(simplifyString, AnyObject.self), forKeyedSubscript: "simplifyString")

println(context.evaluateScript("simplifyString('안녕하새요!')"))
// annyeonghasaeyo!

需要注意的是,Swift的speedbump只適用於Objective-C block,對Swift閉包無用。要在一個JSContext裡使用閉包,有兩個步驟:一是用@objc_block來宣告,二是將Swift的knuckle-whitening unsafeBitCast()函式轉換為 AnyObject。

  • 記憶體管理(Memory Management)

程式碼塊可以捕獲變數引用,而JSContext所有變數的強引用都保留在JSContext中,所以要注意避免迴圈強引用問題。另外,也不要在程式碼塊中捕獲JSContext或任何JSValues,建議使用[JSContext currentContext]來獲取當前的Context物件,根據具體需求將值當做引數傳入block中。

  • JSExport協議

藉助JSExport協議也可以在JavaScript上使用自定義物件。在JSExport協議中宣告的例項方法、類方法,不論屬性,都能自動與JavaScrip互動。文章稍後將介紹具體的實踐過程。

JavaScriptCore實踐

我們可以通過一些例子更好地瞭解上述技巧的使用方法。先定義一個遵循JSExport子協議PersonJSExport的Person model,再用JavaScript在JSON中建立和填入例項。有整個JVM,還要NSJSONSerialization幹什麼?

  • PersonJSExports和Person

Person類執行的PersonJSExports協議具體規定了可用的JavaScript屬性。,在建立時,類方法必不可少,因為JavaScriptCore並不適用於初始化轉換,我們不能像對待原生的JavaScript型別那樣使用var person = new Person()。

//Objective-C
// in Person.h -----------------
@class Person;
@protocol PersonJSExports <JSExport>
    @property (nonatomic, copy) NSString *firstName;
    @property (nonatomic, copy) NSString *lastName;
    @property NSInteger ageToday;
    - (NSString *)getFullName;
    // create and return a new Person instance with `firstName` and `lastName`
    + (instancetype)createWithFirstName:(NSString *)firstName lastName:(NSString *)lastName;
@end
@interface Person : NSObject <PersonJSExports>
    @property (nonatomic, copy) NSString *firstName;
    @property (nonatomic, copy) NSString *lastName;
    @property NSInteger ageToday;
@end
// in Person.m -----------------
@implementation Person
- (NSString *)getFullName {
    return [NSString stringWithFormat:@"%@ %@", self.firstName, self.lastName];
}
+ (instancetype) createWithFirstName:(NSString *)firstName lastName:(NSString *)lastName {
    Person *person = [[Person alloc] init];
    person.firstName = firstName;
    person.lastName = lastName;
    return person;
}
@end
//Swift
// Custom protocol must be declared with `@objc`
@objc protocol PersonJSExports : JSExport {
    var firstName: String { get set }
    var lastName: String { get set }
    var birthYear: NSNumber? { get set }
    func getFullName() -> String
    /// create and return a new Person instance with `firstName` and `lastName`
    class func createWithFirstName(firstName: String, lastName: String) -> Person
}
// Custom class must inherit from `NSObject`
@objc class Person : NSObject, PersonJSExports {
    // properties must be declared as `dynamic`
    dynamic var firstName: String
    dynamic var lastName: String
    dynamic var birthYear: NSNumber?
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
    class func createWithFirstName(firstName: String, lastName: String) -> Person {
        return Person(firstName: firstName, lastName: lastName)
    }
    func getFullName() -> String {
        return "\(firstName) \(lastName)"
    }
}
  • 配置JSContext

建立Person類之後,需要先將其匯出到JavaScript環境中去,同時還需匯入Mustache JS庫,以便對Person物件應用模板。

//Objective-C
// export Person class
context[@"Person"] = [Person class];
// load Mustache.js
NSString *mustacheJSString = [NSString stringWithContentsOfFile:... encoding:NSUTF8StringEncoding error:nil];
[context evaluateScript:mustacheJSString];
//Swift
// export Person class
context.setObject(Person.self, forKeyedSubscript: "Person")
// load Mustache.js
if let mustacheJSString = String(contentsOfFile:..., encoding:NSUTF8StringEncoding, error:nil) {
    context.evaluateScript(mustacheJSString)
}
  • JavaScript資料&處理

以下簡單列出一個JSON範例,以及用JSON來建立新Person例項。

注意:JavaScriptCore實現了Objective-C/Swift的方法名和JavaScript程式碼互動。因為JavaScript沒有命名好的引數,任何額外的引數名稱都採取駝峰命名法(Camel-Case),並附加到函式名稱上。在此示例中,Objective-C的方法createWithFirstName:lastName:在JavaScript中則變成了createWithFirstNameLastName()。

//JSON
[
    { "first": "Grace",     "last": "Hopper",   "year": 1906 },
    { "first": "Ada",       "last": "Lovelace", "year": 1815 },
    { "first": "Margaret",  "last": "Hamilton", "year": 1936 }
]
//JavaScript
var loadPeopleFromJSON = function(jsonString) {
    var data = JSON.parse(jsonString);
    var people = [];
    for (i = 0; i < data.length; i++) {
        var person = Person.createWithFirstNameLastName(data[i].first, data[i].last);
        person.birthYear = data[i].year;
        people.push(person);
    }
    return people;
}
  • 動手一試

現在你只需載入JSON資料,並在JSContext中呼叫,將其解析到Person物件陣列中,再用Mustache模板渲染即可:

//Objective-C
// get JSON string
NSString *peopleJSON = [NSString stringWithContentsOfFile:... encoding:NSUTF8StringEncoding error:nil];
// get load function
JSValue *load = context[@"loadPeopleFromJSON"];
// call with JSON and convert to an NSArray
JSValue *loadResult = [load callWithArguments:@[peopleJSON]];
NSArray *people = [loadResult toArray];
// get rendering function and create template
JSValue *mustacheRender = context[@"Mustache"][@"render"];
NSString *template = @"{{getFullName}}, born {{birthYear}}";
// loop through people and render Person object as string
for (Person *person in people) {
   NSLog(@"%@", [mustacheRender callWithArguments:@[template, person]]);
}
// Output:
// Grace Hopper, born 1906
// Ada Lovelace, born 1815
// Margaret Hamilton, born 1936
//Swift
// get JSON string
if let peopleJSON = NSString(contentsOfFile:..., encoding: NSUTF8StringEncoding, error: nil) {
    // get load function
    let load = context.objectForKeyedSubscript("loadPeopleFromJSON")
    // call with JSON and convert to an array of `Person`
    if let people = load.callWithArguments([peopleJSON]).toArray() as? [Person] {

        // get rendering function and create template
        let mustacheRender = context.objectForKeyedSubscript("Mustache").objectForKeyedSubscript("render")
        let template = "{{getFullName}}, born {{birthYear}}"

        // loop through people and render Person object as string
        for person in people {
            println(mustacheRender.callWithArguments([template, person]))
        }
    }
}
// Output:
// Grace Hopper, born 1906
// Ada Lovelace, born 1815
// Margaret Hamilton, born 1936

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