PixelRatio gives you access to the device's pixel density and font scale.
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.
Pixel grid snapping
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.
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.
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.
Returns the device pixel density. Some examples:
PixelRatio.get() === 1
PixelRatio.get() === 1.5
PixelRatio.get() === 2
- iPhone SE, 6S, 7, 8
- iPhone XR
- iPhone 11
- xhdpi Android devices
PixelRatio.get() === 3
- iPhone 6S Plus, 7 Plus, 8 Plus
- iPhone X, XS, XS Max
- iPhone 11 Pro, 11 Pro Max
- Pixel, Pixel 2
- xxhdpi Android devices
PixelRatio.get() === 3.5
- Nexus 6
- Pixel XL, Pixel 2 XL
- xxxhdpi Android devices
Returns the scaling factor for font sizes. This is the ratio that is used to calculate the absolute font size, so any elements that heavily depend on that should use this to do calculations.
- on Android value reflects the user preference set in Settings > Display > Font size
- on iOS value reflects the user preference set in Settings > Display & Brightness > Text Size, value can also be updated in Settings > Accessibilty > Display & Test Size > Larger Text
If a font scale is not set, this returns the device pixel ratio.
Converts a layout size (dp) to pixel size (px).
Guaranteed to return an integer number.
Rounds a layout size (dp) to the nearest layout size that corresponds to an integer number of pixels. For example, on a device with a PixelRatio of 3,
PixelRatio.roundToNearestPixel(8.4) = 8.33, which corresponds to exactly (8.33 * 3) = 25 pixels.