Accessing native iOS and Android APIs through JavaScript

In this article we are going through the basic concepts of how native APIs are accessed through JavaScript. Our focus is on how primitive types are mapped between JavaScript and the corresponding native platform. We then continue with explaining how complex objects are represented and accessed. At the end, we talk about TypeScript and the @nativescript/types add-on which gives you TypeScript definitions for the Android and iOS development platforms.

NativeScript lets you access all native APIs from the underlying platform. To achieve this behaviour, many things happen under the hood. One of them is marshalling - the conversion between JavaScript and Objective-C data types for iOS and Java data types for Android.

In this article, you will learn how to call native APIs from JavaScript with various data types parameters. For more information, see the platform-specific resources about data conversion in the iOS Runtime and Android Runtime sections.

Numeric Types

All native numeric types (e.g., char, short, int, double, float on iOS and byte, short, int, long, double, float on Android) are implicitly converted to JavaScript number and vice versa. For example, when you run the following code on iOS:

console.log(`pow(2.5, 3) = ${pow(2.5, 3)}`);

console.log(`pow(2.5, 3) = ${pow(2.5, 3)}`);

the iOS Runtime converts the JavaScript number literals to native doubles and passes them to the native pow(double x, double y) function. The returned native integer is automatically converted to a JavaScript number and passed to console.log(). The same is valid for Android:

Android

console.log(`min(3, 4) = ${java.lang.Math.min(3, 4)}`);

console.log(`min(3, 4) = ${java.lang.Math.min(3, 4)}`);

The native java.lang.Math.min() method expects two integers. The Android Runtime knows the signature of the function java.lang.Math.min() and translates the literals 3 and 4 to their representation in a Java integer data type. The returned integer is also automatically translated to a JavaScript number and passed to console.log().

String

JavaScript strings are implicitly marshalled to java.lang.String on Android and NSString on iOS and vice versa.

let button = new UIButton();
button.setTitleForState('Button title', UIControlStateNormal); // 'Button title' is converted to NSString
console.log(button.titleLabel.text); // The returned NSString is converted to JavaScript string

let button = new UIButton();
button.setTitleForState('Button title', UIControlStateNormal); // 'Button title' is converted to NSString
console.log(button.titleLabel.text); // The returned NSString is converted to JavaScript string

Android

const file = new java.io.File('myfile.txt'); // 'myfile.txt' is converted to java.lang.String

const file = new java.io.File('myfile.txt'); // 'myfile.txt' is converted to java.lang.String

The exception to this are the methods on NSString classes declared as returning instancetype - init methods and factory methods. This means that a call to NSString.stringWithString whose return type in Objective-C is instancetype will return a wrapper around a NSString instance rather than a JavaScript string.

Exception: Methods on NSString classes declared as returning instancetype (e.g., init methods and factory methods). For example, calls to NSString.stringWithString return instancetype results in Objective-C. In your NativeScript code, such calls will return a wrapper around a NSString instance instead of a JavaScript string.

Boolean

JavaScript boolean values are implicitly marshalled to boolean on Android and BOOL on iOS and vice versa.

let str = NSString.stringWithString('YES');
let isTrue = str.boolValue();

let str = NSString.stringWithString('YES');
let isTrue = str.boolValue();

Android

let str = new java.lang.String('Hello world!');
let result = str.endsWith('world!');
console.log(result); // true

let str = new java.lang.String('Hello world!');
let result = str.endsWith('world!');
console.log(result); // true

Array

JavaScript arrays map to specialized Java arrays on Android and NSArray on iOS.

// nsArray is not a JavaScript array but a JavaScript wrapper around a native NSArray
let nsArray = NSArray.arrayWithArray(['Four', 'Five', 'Two', 'Seven']);
let jsArray = ['One', 'Two', 'Three']; // pure JavaScript array
let firstCommon = nsArray.firstObjectCommonWithArray(jsArray);
console.log(firstCommon); // Two

// nsArray is not a JavaScript array but a JavaScript wrapper around a native NSArray
let nsArray = NSArray.arrayWithArray(['Four', 'Five', 'Two', 'Seven']);
let jsArray = ['One', 'Two', 'Three']; // pure JavaScript array
let firstCommon = nsArray.firstObjectCommonWithArray(jsArray);
console.log(firstCommon); // Two

Android

The following code snippet shows how to call a ns.example.Math.minElement(int[] array) from JavaScript:

let numbers = [3, 6, 19, -2, 7, 6];
let min = ns.example.Math.minElement(numbers); // -2

let numbers = [3, 6, 19, -2, 7, 6];
let min = ns.example.Math.minElement(numbers); // -2

Classes and Objects

All native classes are represented in the JavaScript world by a constructor function. Each static method on a native class becomes a function on its JavaScript constructor function and each instance method becomes a function on the JavaScript prototype. Although quite intuitive, instantiating objects and calling methods via JavaScript has some specifics (particularly on iOS) which are explained below.

Working With Classes and Objects on iOS

Here is an example of how an instance of the NSMutableArray class is made and consumed in JavaScript:

let array = new NSMutableArray();
array.addObject(new NSObject());

let array = new NSMutableArray();
array.addObject(new NSObject());

This snippet creates an instance of NSMutableArray and adds an object to it using the addObject(object) method. Here is what happens behind the curtains: the new NSMutableArray() call is translated to a [[NSMutableArray alloc] init] call by the iOS Runtime. This instance is then wrapped in a JavaScript object and stored in the array variable. It contains all public properties and methods exposed by NSMutableArray (and its base classes) in its prototype chain. While the addObject(object) call is straightforward, calling Objective-C methods with more arguments follows several simple rules that define how Objective-C selectors are mapped to JavaScript functions. Let's consider the following NSMutableArray selector: replaceObjectsInRange:withObjectsFromArray:range:. In JavaScript it is represented by the following function: replaceObjectsInRangeWithObjectsFromArrayRange(objectsToRange, sourceArray, sourceRange) (argument names are arbitrary). Note that the function name is generated by appending the names of the arguments as defined by the Objective-C selector by starting with a small letter for the first argument and appending each subsequent with a capital letter.

NSDictionary

You will most probably encounter methods accepting NSDictionary instances as parameters. There are few ways of creating an NSDictionary instance:

Using NSDictionary and passing arrays for keys and values.

let dict = new NSDictionary([".example.com", "cookieName", "/", "cookieValue"], [NSHTTPCookieDomain, NSHTTPCookieName, NSHTTPCookiePath,NSHTTPCookieValue]);
let cookie = NSHTTPCookie.cookieWithProperties(dict);

let dict = new NSDictionary([".example.com", "cookieName", "/", "cookieValue"], [NSHTTPCookieDomain, NSHTTPCookieName, NSHTTPCookiePath,NSHTTPCookieValue]);
let cookie = NSHTTPCookie.cookieWithProperties(dict);

Using JSON literals

let cookie = NSHTTPCookie.cookieWithProperties({[NSHTTPCookieDomain]:".example.com", [NSHTTPCookieName]:"cookieName", [NSHTTPCookiePath]:"/", [NSHTTPCookieValue]:"cookieValue"});

let cookie = NSHTTPCookie.cookieWithProperties({[NSHTTPCookieDomain]:".example.com", [NSHTTPCookieName]:"cookieName", [NSHTTPCookiePath]:"/", [NSHTTPCookieValue]:"cookieValue"});

In the second example we are passing a JSON literal to the method.NSHTTPCookieDomain is a variable and we need to use a computed property name in order to have its value (otherwise we are getting "NSHTTPCookieDomain" as key).

Working With Classes And Objects on Android

The following code snippet demonstrates how an instance of the android.widget.Button is created in JavaScript:

let context = ...;
let button = new android.widget.Button(context);
button.setText("My Button"); // "My Button" is converted to java.lang.String

let context = ...;
let button = new android.widget.Button(context);
button.setText("My Button"); // "My Button" is converted to java.lang.String

As you can see, the native Java types are exposed through their corresponding packages. In other words, to access a native Java type, you simply need to know the package it is contained in and explicitly state it. Native Java methods are accessed in the same way as regular JavaScript methods: by using the method identifier and supplying the required arguments. You can read more about Java packages on Android here.

Undefined and Null

JavaScript Undefined & Null map to Java null pointer and Objective-C nil. Native null values map to JavaScript null.

console.log(NSStringFromClass(null)); // null

console.log(NSStringFromClass(null)); // null

Android

let context = ...;
const button = new android.widget.Button(context);
button.setOnClickListener(undefined); // the Java call will be made using the null keyword

let context = ...;
const button = new android.widget.Button(context);
button.setOnClickListener(undefined); // the Java call will be made using the null keyword

IntelliSense and Access to the Native APIs via TypeScript

To have access and Intellisense for the native APIs, you have to add a developer dependency to @nativescript/types.

Steps to install and enable

npm install @nativescript/types --save-dev

Note: Always install the plugin as a devDependency (npm i @nativescript/types --save-dev flag) to avoid bringing the enormously big declaration files in the output built file.

Create reference.d.ts in the root project directory and add the following:

/// <reference path="node_modules/@nativescript/types/index.d.ts" />

By default, the file android.d.ts comes with typings generated for API level 17. As an Android developer, you might need access to a specific class, method, or property introduced in a newer API level. The @nativescript/types plugin comes with generated typings for all API levels from 17 to 27 including the related typings from the respective support library. To use typings for a specific Android level replace the reference to the default declaration file with the preferred one. The files for each API level comes postfixed with a dash followed by the number of the API level (e.g. for API 21 the file is named android-21.d.ts).

For example, let's assume you are developing an application for API 21+ and you need typings generated for that API level:

 /// <reference path="node_modules/@nativescript/types-android/lib/android-21.d.ts" />

Note: Proceed with caution when using functionalities introduced in newer API level. If you attempt to use a class, method, or property from a newer API level on a device with a lower API, the application will crash.

Modify tsconfig.json to contain the following settings:

{
  "compilerOptions": {
    ...
    "module": "esnext",
    "target": "es2015",
    "moduleResolution": "node",
    "lib": ["es2018", "dom"],
}

Note that d.ts files require a lot of memory and CPU. Consider adding skipLibCheck option to tsconfig.json.

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