121 lines
4.1 KiB
JavaScript
121 lines
4.1 KiB
JavaScript
/**
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* Copyright (c) 2015-present, Facebook, Inc.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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* @providesModule PixelRatio
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* @flow
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*/
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'use strict';
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var Dimensions = require('Dimensions');
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/**
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* PixelRatio class gives access to the device pixel density.
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*
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* ## Fetching a correctly sized image
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*
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* 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.
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*
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* ```
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* var image = getImage({
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* width: PixelRatio.getPixelSizeForLayoutSize(200),
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* height: PixelRatio.getPixelSizeForLayoutSize(100),
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* });
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* <Image source={image} style={{width: 200, height: 100}} />
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* ```
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*
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* ## Pixel grid snapping
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*
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* In iOS, you can specify positions and dimensions for elements with arbitrary
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* precision, for example 29.674825. But, ultimately the physical display only
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* have a fixed number of pixels, for example 640×960 for iPhone 4 or 750×1334
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* for iPhone 6. iOS tries to be as faithful as possible to the user value by
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* spreading one original pixel into multiple ones to trick the eye. The
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* downside of this technique is that it makes the resulting element look
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* blurry.
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*
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* In practice, we found out that developers do not want this feature and they
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* have to work around it by doing manual rounding in order to avoid having
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* blurry elements. In React Native, we are rounding all the pixels
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* automatically.
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*
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* We have to be careful when to do this rounding. You never want to work with
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* rounded and unrounded values at the same time as you're going to accumulate
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* rounding errors. Having even one rounding error is deadly because a one
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* pixel border may vanish or be twice as big.
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*
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* In React Native, everything in JavaScript and within the layout engine works
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* with arbitrary precision numbers. It's only when we set the position and
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* dimensions of the native element on the main thread that we round. Also,
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* rounding is done relative to the root rather than the parent, again to avoid
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* accumulating rounding errors.
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*
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*/
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class PixelRatio {
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/**
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* Returns the device pixel density. Some examples:
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*
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* - PixelRatio.get() === 1
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* - mdpi Android devices (160 dpi)
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* - PixelRatio.get() === 1.5
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* - hdpi Android devices (240 dpi)
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* - PixelRatio.get() === 2
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* - iPhone 4, 4S
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* - iPhone 5, 5c, 5s
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* - iPhone 6
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* - xhdpi Android devices (320 dpi)
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* - PixelRatio.get() === 3
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* - iPhone 6 plus
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* - xxhdpi Android devices (480 dpi)
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* - PixelRatio.get() === 3.5
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* - Nexus 6
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*/
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static get(): number {
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return Dimensions.get('window').scale;
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}
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/**
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* 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.
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*
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* If a font scale is not set, this returns the device pixel ratio.
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*
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* 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
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*/
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static getFontScale(): number {
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return Dimensions.get('window').fontScale || PixelRatio.get();
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}
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/**
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* Converts a layout size (dp) to pixel size (px).
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*
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* Guaranteed to return an integer number.
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*/
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static getPixelSizeForLayoutSize(layoutSize: number): number {
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return Math.round(layoutSize * PixelRatio.get());
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}
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/**
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* Rounds a layout size (dp) to the nearest layout size that corresponds to
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* an integer number of pixels. For example, on a device with a PixelRatio
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* of 3, `PixelRatio.roundToNearestPixel(8.4) = 8.33`, which corresponds to
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* exactly (8.33 * 3) = 25 pixels.
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*/
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static roundToNearestPixel(layoutSize: number): number {
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var ratio = PixelRatio.get();
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return Math.round(layoutSize * ratio) / ratio;
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}
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// No-op for iOS, but used on the web. Should not be documented.
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static startDetecting() {}
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}
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module.exports = PixelRatio;
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