react-native/Libraries/Utilities/PixelRatio.js

65 lines
1.7 KiB
JavaScript

/**
* Copyright (c) 2015-present, Facebook, Inc.
* All rights reserved.
*
* This source code is licensed under the BSD-style license found in the
* LICENSE file in the root directory of this source tree. An additional grant
* of patent rights can be found in the PATENTS file in the same directory.
*
* @providesModule PixelRatio
* @flow
*/
'use strict';
var Dimensions = require('Dimensions');
/**
* PixelRatio class gives access to the device pixel density.
*
* There are a few use cases for using PixelRatio:
*
* ### Displaying a line that's as thin as the device permits
*
* A width of 1 is actually pretty thick on an iPhone 4+, we can do one that's
* thinner using a width of `1 / PixelRatio.get()`. It's a technique that works
* on all the devices independent of their pixel density.
*
* ```
* style={{ borderWidth: 1 / PixelRatio.get() }}
* ```
*
* ### Fetching a correctly sized image
*
* You should get a higher resolution image if you are on a high pixel density
* device. A good rule of thumb is to multiply the size of the image you display
* by the pixel ratio.
*
* ```
* var image = getImage({
* width: 200 * PixelRatio.get(),
* height: 100 * PixelRatio.get()
* });
* <Image source={image} style={{width: 200, height: 100}} />
* ```
*/
class PixelRatio {
/**
* Returns the device pixel density. Some examples:
*
* - PixelRatio.get() === 2
* - iPhone 4, 4S
* - iPhone 5, 5c, 5s
* - iPhone 6
* - PixelRatio.get() === 3
* - iPhone 6 plus
*/
static get(): number {
return Dimensions.get('window').scale;
}
}
// No-op for iOS, but used on the web. Should not be documented.
PixelRatio.startDetecting = function() {};
module.exports = PixelRatio;