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<!DOCTYPE html><html><head><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"><title>React Native | A framework for building native apps using React</title><meta name="viewport" content="width=device-width"><meta property="og:title" content="React Native | A framework for building native apps using React"><meta property="og:type" content="website"><meta property="og:url" content="http://facebook.github.io/react-native/index.html"><meta property="og:image" content="http://facebook.github.io/react-native/img/opengraph.png?2"><meta property="og:description" content="A framework for building native apps using React"><link rel="shortcut icon" href="/react-native/img/favicon.png?2"><link rel="stylesheet" href="/react-native/css/react-native.css"><script type="text/javascript" src="//use.typekit.net/vqa1hcx.js"></script><script type="text/javascript">try{Typekit.load();}catch(e){}</script></head><body><div class="container"><div class="nav-main"><div class="wrap"><a class="nav-home" href="/react-native/"><img src="/react-native/img/header_logo.png">React Native</a><div class="nav-site-wrapper"><ul class="nav-site nav-site-internal"><li><a href="/react-native/docs/getting-started.html" class="active">Docs</a></li><li><a href="/react-native/support.html" class="">Support</a></li></ul><ul class="nav-site nav-site-external"><li><a href="http://github.com/facebook/react-native" class="">GitHub</a></li><li><a href="http://facebook.github.io/react" class="">React</a></li></ul></div></div></div><section class="content wrap documentationContent"><div class="nav-docs"><div class="nav-docs-section"><h3>Quick Start</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/getting-started.html#content">Getting Started</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/tutorial.html#content">Tutorial</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/videos.html#content">Videos</a></li></ul></div><div class="nav-docs-section"><h3>Guides</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/style.html#content">Style</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/gesture-responder-system.html#content">Gesture Responder System</a></li><li><a style="margin-left:0;" class="active" href="/react-native/docs/nativemodulesios.html#content">Native Modules (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linking-libraries.html#content">Linking Libraries</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/debugging.html#content">Debugging</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/testing.html#content">Testing</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/runningondevice.html#content">Running On Device</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/embedded-app.html#content">Integration with Existing App</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/javascript-environment.html#content">JavaScript Environment</a></li></ul></div><div class="nav-docs-section"><h3>components</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/activityindicatorios.html#content">ActivityIndicatorIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/datepickerios.html#content">DatePickerIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/image.html#content">Image</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/listview.html#content">ListView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/mapview.html#content">MapView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigator.html#content">Navigator</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigatorios.html#content">NavigatorIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pickerios.html#content">PickerIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/scrollview.html#content">ScrollView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/sliderios.html#content">SliderIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/switchios.html#content">SwitchIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/tabbarios.html#content">TabBarIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/text.html#content">Text</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/textinput.html#content">TextInput</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchablehighlight.html#content">TouchableHighlight</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchableopacity.html#content">TouchableOpacity</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchablewithoutfeedback.html#content">TouchableWithoutFeedback</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/view.html#content">View</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/webview.html#content">WebView</a></li></ul></div><div class="nav-docs-section"><h3>apis</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/alertios.html#content">AlertIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/appregistry.html#content">AppRegistry</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/appstateios.html#content">AppStateIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/asyncstorage.html#content">AsyncStorage</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/cameraroll.html#content">CameraRoll</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/interactionmanager.html#content">InteractionManager</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/layoutanimation.html#content">LayoutAnimation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linkingios.html#content">LinkingIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/netinfo.html#content">NetInfo</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/panresponder.html#content">PanResponder</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pixelratio.html#content">PixelRatio</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pushnotificationios.html#content">PushNotificationIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/statusbarios.html#content">StatusBarIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/stylesheet.html#content">StyleSheet</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/vibrationios.html#content">VibrationIOS</a></li></ul></div><div class="nav-docs-section"><h3>Polyfills</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/flexbox.html#content">Flexbox</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/geolocation.html#content">Geolocation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/network.html#content">Network</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/timers.html#content">Timers</a></li></ul></div></div><div class="inner-content"><a id="content"></a><h1>Native Modules (iOS)</h1><div><p>Sometimes an app needs access to platform API, and React Native doesn't have a corresponding wrapper yet. Maybe you want to reuse some existing Objective-C or C++ code without having to reimplement it in JavaScript. Or write some high performance, multi-threaded code such as image processing, network stack, database or rendering.</p><p>We designed React Native such that it is possible for you to write real native code and have access to the full power of the platform. This is a more advanced feature and we don't expect it to be part of the usual development process, however it is essential that it exists. If React Native doesn't support a native feature that you need, you should be able to build it yourself.</p><p>This is a more advanced guide that shows how to build a native module. It assumes the reader knows Objective-C (Swift is not supported yet) and core libraries (Foundation, UIKit).</p><h2><a class="anchor" name="ios-calendar-module-example"></a>iOS Calendar module example <a class="hash-link" href="#ios-calendar-module-example">#</a></h2><p>This guide will use the <a href="https://developer.apple.com/library/mac/documentation/DataManagement/Conceptual/EventKitProgGuide/Introduction/Introduction.html" target="_blank">iOS Calendar API</a> example. Let's say we would like to be able to access the iOS calendar from JavaScript.</p><p>A native module is just an Objective-C class that implements the <code>RCTBridgeModule</code> protocol. If you are wondering, RCT is a shorthand for ReaCT.</p><div class="prism language-javascript"><span class="token comment" spellcheck="true">// CalendarManager.h
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</span>#import <span class="token string">"RCTBridgeModule.h"</span>
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@interface <span class="token class-name">CalendarManager</span> <span class="token punctuation">:</span> NSObject <RCTBridgeModule<span class="token operator">></span>
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@end</div><p>In addition to implementing the <code>RCTBridgeModule</code> protocol, your class must also include the <code>RCT_EXPORT_MODULE()</code> macro. This takes an optional argument that specifies the name that the module will accessed by in your JavaScript code (more on this later). If you do not specify a name, the JavaScript module name will match the Objective-C class name.</p><div class="prism language-javascript"><span class="token comment" spellcheck="true">// CalendarManager.m
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</span>@implementation CalendarManager
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<span class="token function">RCT_EXPORT_MODULE<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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@end</div><p>React Native will not expose any methods of <code>CalendarManager</code> to JavaScript unless explicitly told to. This is done using the <code>RCT_EXPORT_METHOD()</code> macro:</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name location<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>location<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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<span class="token function">RCTLogInfo<span class="token punctuation">(</span></span>@<span class="token string">"Pretending to create an event %@ at %@"</span><span class="token punctuation">,</span> name<span class="token punctuation">,</span> location<span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p>Now, from your JavaScript file you can call the method like this:</p><div class="prism language-javascript"><span class="token keyword">var</span> CalendarManager <span class="token operator">=</span> <span class="token function">require<span class="token punctuation">(</span></span><span class="token string">'NativeModules'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>CalendarManager<span class="token punctuation">;</span>
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CalendarManager<span class="token punctuation">.</span><span class="token function">addEvent<span class="token punctuation">(</span></span><span class="token string">'Birthday Party'</span><span class="token punctuation">,</span> <span class="token string">'4 Privet Drive, Surrey'</span><span class="token punctuation">)</span><span class="token punctuation">;</span></div><blockquote><p><strong>NOTE</strong>: JavaScript method names</p><p>The name of the method exported to JavaScript is the native method's name up to the first colon. React Native also defines a macro called <code>RCT_REMAP_METHOD()</code> to specify the JavaScript method's name. This is useful when multiple native methods are the same up to the first colon and would have conflicting JavaScript names.</p></blockquote><p>The return type of bridge methods is always <code>void</code>. React Native bridge is asynchronous, so the only way to pass a result to JavaScript is by using callbacks or emitting events (see below).</p><h2><a class="anchor" name="argument-types"></a>Argument types <a class="hash-link" href="#argument-types">#</a></h2><p><code>RCT_EXPORT_METHOD</code> supports all standard JSON object types, such as:</p><ul><li>string (<code>NSString</code>)</li><li>number (<code>NSInteger</code>, <code>float</code>, <code>double</code>, <code>CGFloat</code>, <code>NSNumber</code>)</li><li>boolean (<code>BOOL</code>, <code>NSNumber</code>)</li><li>array (<code>NSArray</code>) of any types from this list</li><li>map (<code>NSDictionary</code>) with string keys and values of any type from this list</li><li>function (<code>RCTResponseSenderBlock</code>)</li></ul><p>But it also works with any type that is supported by the <code>RCTConvert</code> class (see <a href="https://github.com/facebook/react-native/blob/master/React/Base/RCTConvert.h" target="_blank"><code>RCTConvert</code></a> for details). The <code>RCTConvert</code> helper functions all accept a JSON value as input and map it to a native Objective-C type or class. </p><p>In our <code>CalendarManager</code> example, we to pass the event date to the native method. We can't send JavaScript Date objects over the bridge, so we need to convert the date to a string or number. We could write our native function like this:</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name location<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>location date<span class="token punctuation">:</span><span class="token punctuation">(</span>NSNumber <span class="token operator">*</span><span class="token punctuation">)</span>secondsSinceUnixEpoch<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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NSDate <span class="token operator">*</span>date <span class="token operator">=</span> <span class="token punctuation">[</span>RCTConvert NSDate<span class="token punctuation">:</span>secondsSinceUnixEpoch<span class="token punctuation">]</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p>or like this:</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name location<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>location date<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>ISO8601DateString<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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NSDate <span class="token operator">*</span>date <span class="token operator">=</span> <span class="token punctuation">[</span>RCTConvert NSDate<span class="token punctuation">:</span>ISO8601DateString<span class="token punctuation">]</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p>But by using the automatic type conversion feature, we can skip the manual conversion step completely, and just write:</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name location<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>location date<span class="token punctuation">:</span><span class="token punctuation">(</span>NSDate <span class="token operator">*</span><span class="token punctuation">)</span>date<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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<span class="token comment" spellcheck="true"> // Date it ready to use!
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</span><span class="token punctuation">}</span></div><p>You would then call this from JavaScript by using:</p><div class="prism language-javascript">CalendarManager<span class="token punctuation">.</span><span class="token function">addEvent<span class="token punctuation">(</span></span><span class="token string">'Birthday Party'</span><span class="token punctuation">,</span> date<span class="token punctuation">.</span><span class="token function">toTime<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true"> // passing date as number of seconds since Unix epoch</span></div><p>or</p><div class="prism language-javascript">CalendarManager<span class="token punctuation">.</span><span class="token function">addEvent<span class="token punctuation">(</span></span><span class="token string">'Birthday Party'</span><span class="token punctuation">,</span> date<span class="token punctuation">.</span><span class="token function">toISOString<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true"> // passing date as ISO-8601 string</span></div><p>As <code>CalendarManager.addEvent</code> method gets more and more complex, the number of arguments will grow. Some of them might be optional. In this case it's worth considering changing the API a little bit to accept a dictionary of event attributes, like this:</p><div class="prism language-javascript">#import <span class="token string">"RCTConvert.h"</span>
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<span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name details<span class="token punctuation">:</span><span class="token punctuation">(</span>NSDictionary <span class="token operator">*</span><span class="token punctuation">)</span>details<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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NSString <span class="token operator">*</span>location <span class="token operator">=</span> <span class="token punctuation">[</span>RCTConvert NSString<span class="token punctuation">:</span>details<span class="token punctuation">[</span>@<span class="token string">"location"</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
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NSDate <span class="token operator">*</span>time <span class="token operator">=</span> <span class="token punctuation">[</span>RCTConvert NSDate<span class="token punctuation">:</span>details<span class="token punctuation">[</span>@<span class="token string">"time"</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
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<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
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<span class="token punctuation">}</span></div><p>and call it from JavaScript:</p><div class="prism language-javascript">CalendarManager<span class="token punctuation">.</span><span class="token function">addEvent<span class="token punctuation">(</span></span><span class="token string">'Birthday Party'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
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location<span class="token punctuation">:</span> <span class="token string">'4 Privet Drive, Surrey'</span><span class="token punctuation">,</span>
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time<span class="token punctuation">:</span> date<span class="token punctuation">.</span><span class="token function">toTime<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">,</span>
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description<span class="token punctuation">:</span> <span class="token string">'...'</span>
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<span class="token punctuation">}</span><span class="token punctuation">)</span></div><blockquote><p><strong>NOTE</strong>: About array and map</p><p>Objective-C doesn't provide any guarantees about the types of values in these structures. Your native module might expect an array of strings, but if JavaScript calls your method with an array containing numbers and strings, you'll get an <code>NSArray</code> containing a mix of <code>NSNumber</code> and <code>NSString</code>. For arrays, <code>RCTConvert</code> provides some typed collections you can use in your method declaration, such as <code>NSStringArray</code>, or <code>UIColorArray</code>. For maps, it is the developer's responsibility to check the value types individually by manually calling <code>RCTConvert</code> helper methods.</p></blockquote><h2><a class="anchor" name="callbacks"></a>Callbacks <a class="hash-link" href="#callbacks">#</a></h2><blockquote><p><strong>WARNING</strong></p><p>This section is more experimental than others, we don't have a set of best practices around callbacks yet.</p></blockquote><p>Native module also supports a special kind of argument- a callback. In most cases it is used to provide the function call result to JavaScript.</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>findEvents<span class="token punctuation">:</span><span class="token punctuation">(</span>RCTResponseSenderBlock<span class="token punctuation">)</span>callback<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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NSArray <span class="token operator">*</span>events <span class="token operator">=</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
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<span class="token function">callback<span class="token punctuation">(</span></span>@<span class="token punctuation">[</span><span class="token punctuation">[</span>NSNull <span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">,</span> events<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p><code>RCTResponseSenderBlock</code> accepts only one argument - an array of arguments to pass to the JavaScript callback. In this case we use node's convention to set first argument to error and the rest - to the result of the function.</p><div class="prism language-javascript">CalendarManager<span class="token punctuation">.</span><span class="token function">findEvents<span class="token punctuation">(</span></span><span class="token punctuation">(</span>error<span class="token punctuation">,</span> events<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span>
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<span class="token keyword">if</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
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console<span class="token punctuation">.</span><span class="token function">error<span class="token punctuation">(</span></span>error<span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
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<span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">setState<span class="token punctuation">(</span></span><span class="token punctuation">{</span>events<span class="token punctuation">:</span> events<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span>
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<span class="token punctuation">}</span><span class="token punctuation">)</span></div><p>Native module is supposed to invoke its callback only once. It can, however, store the callback as an ivar and invoke it later. This pattern is often used to wrap iOS APIs that require delegate. See <a href="https://github.com/facebook/react-native/blob/master/React/Modules/RCTAlertManager.m" target="_blank"><code>RCTAlertManager</code></a>.</p><p>If you want to pass error-like object to JavaScript, use <code>RCTMakeError</code> from <a href="https://github.com/facebook/react-native/blob/master/React/Base/RCTUtils.h" target="_blank"><code>RCTUtils.h</code></a>.</p><h2><a class="anchor" name="implementing-a-native-module"></a>Implementing a native module <a class="hash-link" href="#implementing-a-native-module">#</a></h2><p>The native module should not have any assumptions about what thread it is being called on. React Native invokes native modules methods on a separate serial GCD queue, but this is an implementation detail and might change. If the native module needs to call main-thread-only iOS API, it should schedule the operation on the main queue:</p><div class="prism language-javascript"><span class="token function">RCT_EXPORT_METHOD<span class="token punctuation">(</span></span>addEvent<span class="token punctuation">:</span><span class="token punctuation">(</span>NSString <span class="token operator">*</span><span class="token punctuation">)</span>name callback<span class="token punctuation">:</span><span class="token punctuation">(</span>RCTResponseSenderBlock<span class="token punctuation">)</span>callback<span class="token punctuation">)</span>
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<span class="token punctuation">{</span>
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<span class="token function">dispatch_async<span class="token punctuation">(</span></span><span class="token function">dispatch_get_main_queue<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token operator">^</span><span class="token punctuation">{</span>
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<span class="token comment" spellcheck="true"> // Call iOS API on main thread
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</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
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<span class="token comment" spellcheck="true"> // You can invoke callback from any thread/queue
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</span> <span class="token function">callback<span class="token punctuation">(</span></span>@<span class="token punctuation">[</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p>The same way if the operation can take a long time to complete, the native module should not block. It is a good idea to use <code>dispatch_async</code> to schedule expensive work on background queue.</p><h2><a class="anchor" name="exporting-constants"></a>Exporting constants <a class="hash-link" href="#exporting-constants">#</a></h2><p>Native module can export constants that are instantly available to JavaScript at runtime. This is useful to export some initial data that would otherwise require a bridge round-trip.</p><div class="prism language-javascript"><span class="token operator">-</span> <span class="token punctuation">(</span>NSDictionary <span class="token operator">*</span><span class="token punctuation">)</span>constantsToExport
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<span class="token punctuation">{</span>
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<span class="token keyword">return</span> @<span class="token punctuation">{</span> @<span class="token string">"firstDayOfTheWeek"</span><span class="token punctuation">:</span> @<span class="token string">"Monday"</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span></div><p>JavaScript can use this value right away:</p><div class="prism language-javascript">console<span class="token punctuation">.</span><span class="token function">log<span class="token punctuation">(</span></span>CalendarManager<span class="token punctuation">.</span>firstDayOfTheWeek<span class="token punctuation">)</span><span class="token punctuation">;</span></div><p>Note that the constants are exported only at initialization time, so if you change <code>constantsToExport</code> value at runtime it won't affect JavaScript environment.</p><h2><a class="anchor" name="sending-events-to-javascript"></a>Sending events to JavaScript <a class="hash-link" href="#sending-events-to-javascript">#</a></h2><p>The native module can signal events to JavaScript without being invoked directly. The easiest way to do this is to use <code>eventDispatcher</code>:</p><div class="prism language-javascript">#import <span class="token string">"RCTBridge.h"</span>
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#import <span class="token string">"RCTEventDispatcher.h"</span>
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@implementation CalendarManager
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@synthesize bridge <span class="token operator">=</span> _bridge<span class="token punctuation">;</span>
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<span class="token operator">-</span> <span class="token punctuation">(</span>void<span class="token punctuation">)</span>calendarEventReminderReceived<span class="token punctuation">:</span><span class="token punctuation">(</span>NSNotification <span class="token operator">*</span><span class="token punctuation">)</span>notification
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<span class="token punctuation">{</span>
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NSString <span class="token operator">*</span>eventName <span class="token operator">=</span> notification<span class="token punctuation">.</span>userInfo<span class="token punctuation">[</span>@<span class="token string">"name"</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
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<span class="token punctuation">[</span>self<span class="token punctuation">.</span>bridge<span class="token punctuation">.</span>eventDispatcher sendAppEventWithName<span class="token punctuation">:</span>@<span class="token string">"EventReminder"</span>
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body<span class="token punctuation">:</span>@<span class="token punctuation">{</span>@<span class="token string">"name"</span><span class="token punctuation">:</span> eventName<span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
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<span class="token punctuation">}</span>
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@end</div><p>JavaScript code can subscribe to these events:</p><div class="prism language-javascript"><span class="token keyword">var</span> subscription <span class="token operator">=</span> DeviceEventEmitter<span class="token punctuation">.</span><span class="token function">addListener<span class="token punctuation">(</span></span>
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<span class="token string">'EventReminder'</span><span class="token punctuation">,</span>
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<span class="token punctuation">(</span>reminder<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> console<span class="token punctuation">.</span><span class="token function">log<span class="token punctuation">(</span></span>reminder<span class="token punctuation">.</span>name<span class="token punctuation">)</span>
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<span class="token punctuation">)</span><span class="token punctuation">;</span>
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<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token comment" spellcheck="true">
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// Don't forget to unsubscribe
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</span>subscription<span class="token punctuation">.</span><span class="token function">remove<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span></div><p>For more examples of sending events to JavaScript, see <a href="https://github.com/facebook/react-native/blob/master/Libraries/Geolocation/RCTLocationObserver.m" target="_blank"><code>RCTLocationObserver</code></a>.</p></div><div class="docs-prevnext"><a class="docs-next" href="linking-libraries.html#content">Next →</a></div></div></section><footer class="wrap"><div class="right">© 2015 Facebook Inc.</div></footer></div><div id="fb-root"></div><script>
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