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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><li><a href="https://github.com/facebook/react-native/releases" class="">Releases</a></li></ul><ul class="nav-site nav-site-external"><li><a href="https://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="" href="/react-native/docs/animations.html#content">Animations</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/accessibility.html#content">Accessibility (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/native-modules-ios.html#content">Native Modules (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/native-components-ios.html#content">Native UI Components (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/direct-manipulation.html#content">Direct Manipulation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linking-libraries-ios.html#content">Linking Libraries (iOS)</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/running-on-device-ios.html#content">Running On Device (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/embedded-app-ios.html#content">Integrating with Existing Apps (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/javascript-environment.html#content">JavaScript Environment</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigator-comparison.html#content">Navigator Comparison</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/segmentedcontrolios.html#content">SegmentedControlIOS</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/tabbarios-item.html#content">TabBarIOS.Item</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/actionsheetios.html#content">ActionSheetIOS</a></li><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="active" 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>PixelRatio</h1><div><div><p>PixelRatio class gives access to the device pixel density.</p><p>There are a few use cases for using PixelRatio:</p><h3><a class="anchor" name="displaying-a-line-that-s-as-thin-as-the-device-permits"></a>Displaying a line that's as thin as the device permits <a class="hash-link" href="#displaying-a-line-that-s-as-thin-as-the-device-permits">#</a></h3><p>A width of 1 is actually pretty thick on devices with high pixel density
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(such as iPhone 4+ and many Android devices), we can make one that's
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thinner using a width of <code>1 / PixelRatio.get()</code>.
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It's a technique that works on all the devices independent of their
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pixel density.</p><div class="prism language-javascript">style<span class="token operator">=</span><span class="token punctuation">{</span><span class="token punctuation">{</span> borderWidth<span class="token punctuation">:</span> <span class="token number">1</span> <span class="token operator">/</span> PixelRatio<span class="token punctuation">.</span><span class="token keyword">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">}</span></div><h3><a class="anchor" name="fetching-a-correctly-sized-image"></a>Fetching a correctly sized image <a class="hash-link" href="#fetching-a-correctly-sized-image">#</a></h3><p>You should get a higher resolution image if you are on a high pixel density
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device. A good rule of thumb is to multiply the size of the image you display
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by the pixel ratio.</p><div class="prism language-javascript"><span class="token keyword">var</span> image <span class="token operator">=</span> <span class="token function">getImage<span class="token punctuation">(</span></span><span class="token punctuation">{</span>
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width<span class="token punctuation">:</span> PixelRatio<span class="token punctuation">.</span><span class="token function">getPixelSizeForLayoutSize<span class="token punctuation">(</span></span><span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
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height<span class="token punctuation">:</span> PixelRatio<span class="token punctuation">.</span><span class="token function">getPixelSizeForLayoutSize<span class="token punctuation">(</span></span><span class="token number">100</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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<Image source<span class="token operator">=</span><span class="token punctuation">{</span>image<span class="token punctuation">}</span> style<span class="token operator">=</span><span class="token punctuation">{</span><span class="token punctuation">{</span>width<span class="token punctuation">:</span> <span class="token number">200</span><span class="token punctuation">,</span> height<span class="token punctuation">:</span> <span class="token number">100</span><span class="token punctuation">}</span><span class="token punctuation">}</span> <span class="token operator">/</span><span class="token operator">></span></div></div><span><h3><a class="anchor" name="methods"></a>Methods <a class="hash-link" href="#methods">#</a></h3><div class="props"><div class="prop"><h4 class="propTitle"><a class="anchor" name="get"></a><span class="propType">static </span>get<span class="propType">()</span> <a class="hash-link" href="#get">#</a></h4><div><p>Returns the device pixel density. Some examples:</p><ul><li>PixelRatio.get() === 1<ul><li>mdpi Android devices (160 dpi)</li></ul></li><li>PixelRatio.get() === 1.5<ul><li>hdpi Android devices (240 dpi)</li></ul></li><li>PixelRatio.get() === 2<ul><li>iPhone 4, 4S</li><li>iPhone 5, 5c, 5s</li><li>iPhone 6</li><li>xhdpi Android devices (320 dpi)</li></ul></li><li>PixelRatio.get() === 3<ul><li>iPhone 6 plus</li><li>xxhdpi Android devices (480 dpi)</li></ul></li><li>PixelRatio.get() === 3.5<ul><li>Nexus 6</li></ul></li></ul></div></div><div class="prop"><h4 class="propTitle"><a class="anchor" name="getfontscale"></a><span class="propType">static </span>getFontScale<span class="propType">()</span> <a class="hash-link" href="#getfontscale">#</a></h4><div><p>Returns the scaling factor for font sizes. This is the ratio that is used to calculate the
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absolute font size, so any elements that heavily depend on that should use this to do
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calculations.</p><p>If a font scale is not set, this returns the device pixel ratio.</p><p>Currently this is only implemented on Android and reflects the user preference set in
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Settings > Display > Font size, on iOS it will always return the default pixel ratio.
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@platform android</p></div></div><div class="prop"><h4 class="propTitle"><a class="anchor" name="getpixelsizeforlayoutsize"></a><span class="propType">static </span>getPixelSizeForLayoutSize<span class="propType">(layoutSize: number)</span> <a class="hash-link" href="#getpixelsizeforlayoutsize">#</a></h4><div><p>Converts a layout size (dp) to pixel size (px).</p><p>Guaranteed to return an integer number.</p></div></div><div class="prop"><h4 class="propTitle"><a class="anchor" name="startdetecting"></a><span class="propType">static </span>startDetecting<span class="propType">()</span> <a class="hash-link" href="#startdetecting">#</a></h4><div><p>// No-op for iOS, but used on the web. Should not be documented.</p></div></div></div></span></div><div><h3><a class="anchor" name="description"></a><a class="edit-github" href="https://github.com/facebook/react-native/blob/master/docs/PixelRatio.md">Edit on GitHub</a>Description <a class="hash-link" href="#description">#</a></h3><div><h2><a class="anchor" name="pixel-grid-snapping"></a>Pixel Grid Snapping <a class="hash-link" href="#pixel-grid-snapping">#</a></h2><p>In iOS, you can specify positions and dimensions for elements with arbitrary precision, for example 29.674825. But, ultimately the physical display only have a fixed number of pixels, for example 640×960 for iphone 4 or 750×1334 for iphone 6. iOS tries to be as faithful as possible to the user value by spreading one original pixel into multiple ones to trick the eye. The downside of this technique is that it makes the resulting element look blurry.</p><p>In practice, we found out that developers do not want this feature and they have to work around it by doing manual rounding in order to avoid having blurry elements. In React Native, we are rounding all the pixels automatically.</p><p>We have to be careful when to do this rounding. You never want to work with rounded and unrounded values at the same time as you're going to accumulate rounding errors. Having even one rounding error is deadly because a one pixel border may vanish or be twice as big.</p><p>In React Native, everything in JS and within the layout engine work with arbitrary precision numbers. It's only when we set the position and dimensions of the native element on the main thread that we round. Also, rounding is done relative to the root rather than the parent, again to avoid accumulating rounding errors.</p></div></div><div class="docs-prevnext"><a class="docs-next" href="pushnotificationios.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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