React Native中使用的尺寸单位是dp(一种基于屏幕密度的抽象单位。在每英寸160点的显示器上，1dp = 1px),而设计师使用的是px, 这两种尺寸如何换算呢？官方提供了PixelRatio:
import {PixelRatio} from 'react-native';
const dp2px = dp=>PixelRatio.getPixelSizeForLayoutSize(dp);
const px2dp = px=>PixelRatio.roundToNearestPixel(px);
设计师给你一个尺寸，比如100px*200px的View，按照下面的方式可实现设计还原：
<View style={{width:px2dp(100),height:px2dp(200),backgroundColor:"red"}}/>
这个时候，你或许会说，这也太麻烦了，每个有尺寸的地方我都得转么，能不能我直接用px写，当然可以，不过需要整体加个缩放系数：
import {PixelRatio,Dimensions}} from 'react-native';
const dp2px = dp=>PixelRatio.getPixelSizeForLayoutSize(dp);
const px2dp = px=>PixelRatio.roundToNearestPixel(px);
let pxRatio = PixelRatio.get();
let {win_width,win_height} = Dimensions.get("window");
let scale = 1/pxRatio;
let width = dp2px(win_width);
let height = dp2px(win_height);
const com = props=>(
<View sytle={styles.container}>
<View style={{width:100,height:200,backgroundColor:"red"}}/>
</View>)
const styles={
container: {
width:width,
height:height,
transform:[{translateX:-width*.5},
{translateY:-height*.5},
{scale:scale},
{translateX:width*.5},
{translateY:height*.5}]
},
}
这样处理后，在根节点内，你再也不用考虑dp的问题了，直接使用px即可。
不过此时还有另外一个问题，设计尺寸是死的，屏幕大小是活的，得考虑分辨率适配啊，那在不同的分辨率下如何正确的实现设计师的设计呢？
我们将使用一种游戏经常会用到得方案，fixedWidth/fixedHeight.
fixedWidth
fixedWidth 模式是保持原始宽高比缩放应用程序内容，缩放后应用程序内容在水平和垂直方向都填满播放器窗口，但只保持应用程序内容的原始宽度不变，高度可能会改变,简言之宽度固定，高度自适应。
fixedHeight
fixedHeight 模式是保持原始宽高比缩放应用程序内容，缩放后应用程序内容在水平和垂直方向都填满播放器窗口，但只保持应用程序内容的原始高度不变，宽度可能会改变,简言之高度固定，宽度自适应。
具体如何应用呢，别急，一步步来。
先来看看如何得到屏幕的像素宽高：
import {Dimensions,PixelRatio} from 'react-native';
let {width,height} = Dimensions.get("window");
let w =dp2px(width);
let h = dp2px(height);
假定我们的设计尺寸是
let designSize = {width:750,height:1336};
按照fixedWidth、fixedHeight的定义，我们计算下新的宽高：
//fixedWidth
let scale = designSize.width/w;
let winSize = {width:designSize.width,height:h*scale};
//fixedHeight
let scale = designSize.height/h;
let winSize = {width:designSize.width*scale,height:designSize.height};
这个winsize就是最终实际用来布局的屏幕尺寸,此时我们又会多了一个分辨率适配的缩放系数，还记得我们前一个我们添加的为了使用px的缩放系数么，我们在这里做一个整合：
import {PixelRatio,Dimensions}} from 'react-native';
const dp2px = dp=>PixelRatio.getPixelSizeForLayoutSize(dp);
const px2dp = px=>PixelRatio.roundToNearestPixel(px);
let designSize = {width:750,height:1336};
let pxRatio = PixelRatio.get();
let {win_width,win_height} = Dimensions.get("window");
let width = dp2px(win_width);
let height = dp2px(win_height);
let design_scale = designSize.width/width;
height = height*design_scale
let scale = 1/pxRatio/design_scale;
const com = props=>(
<View sytle={styles.container}>
<View style={{width:100,height:200,backgroundColor:"red"}}/>
</View>)
const styles={
container: {
width:width,
height:height,
transform:[{translateX:-width*.5},
{translateY:-height*.5},
{scale:scale},
{translateX:width*.5},
{translateY:height*.5}]
},
}
在后续的开发中将不必再关注适配的问题，只需要按照设计师给的尺寸实现布局即可。
最后再附上一个工具类 Resolution.js：
import React, {Component, PropTypes} from 'react';
import {
Dimensions,
PixelRatio,
Platform,
StatusBar,
View
} from 'react-native';
let props = {};
export default class Resolution {
static get(useFixWidth = true){
return useFixWidth?{...props.fw}:{...props.fh}
}
static setDesignSize(dwidth=750,dheight=1336,dim="window"){
let designSize = {width:dwidth,height:dheight};
let navHeight = Platform.OS === 'android' ? StatusBar.currentHeight : 64;
let pxRatio = PixelRatio.get(dim);
let {width,height} = Dimensions.get(dim);
if(dim != "screen")height-=navHeight;
let w = PixelRatio.getPixelSizeForLayoutSize(width);
let h = PixelRatio.getPixelSizeForLayoutSize(height);
let fw_design_scale = designSize.width/w;
fw_width = designSize.width;
fw_height = h*fw_design_scale;
fw_scale = 1/pxRatio/fw_design_scale;
let fh_design_scale = designSize.height/h;
fh_width = w*fh_design_scale;
fh_height = designSize.height;
fh_scale = 1/pxRatio/fh_design_scale;
props.fw = {width:fw_width,height:fw_height,scale:fw_scale,navHeight};
props.fh = {width:fh_width,height:fh_height,scale:fh_scale,navHeight};
}
static FixWidthView = (p) => {
let {width,height,scale,navHeight} = props.fw;
return (
<View {...p} style={{
marginTop:navHeight,
width:width,
height:height,
backgroundColor: 'transparent',
transform:[{translateX:-width*.5},
{translateY:-height*.5},
{scale:scale},
{translateX:width*.5},
{translateY:height*.5}]
}}>
</View>
);
};
static FixHeightView = (p) => {
let {width,height,scale,navHeight} = props.fh;
return (
<View {...p} style={{
marginTop:navHeight,
width:width,
height:height,
backgroundColor: 'transparent',
transform:[{translateX:-width*.5},
{translateY:-height*.5},
{scale:scale},
{translateX:width*.5},
{translateY:height*.5}]
}}>
{p.children}
</View>
);
};
};
//init
Resolution.setDesignSize();
How to use:
import React, { Component } from 'react';
import {
AppRegistry,
StyleSheet,
Text,
Image,
View
} from 'react-native';
import Resolution from "./Resolution"
export default class tets extends Component {
render() {
return (
<Resolution.FixWidthView style={styles.container}>
<Image source={require("./assets/bg_day.jpg")} style={{position:"absolute"}}/>
<Text style={styles.welcome}>
Welcome to React Native!
</Text>
<Text style={styles.instructions}>
To get started, edit index.ios.js
</Text>
<Text style={styles.instructions}>
Press Cmd+R to reload,{'\n'}
Cmd+D or shake for dev menu
</Text>
</Resolution.FixWidthView>
);
}
}
const styles = StyleSheet.create({
container: {
flex: 0,
justifyContent: 'center',
alignItems: 'center',
// backgroundColor: '#ff0000',
},
welcome: {
fontSize: 20,
textAlign: 'center',
margin: 10,
backgroundColor:"transparent"
},
instructions: {
backgroundColor:"transparent",
textAlign: 'center',
color: 0xffff,
marginBottom: 5,
},
});
AppRegistry.registerComponent('rn_resolution', () => tets);
bg_day.jpg的尺寸是750*1500，上面的程序在所有的分辨率下图片都能正确显示。
这里有个demo：Github，如果解决你的问题了，记得给我加星哦~~
另外：不同分辨率下背景图片尺寸如何选择，移步另一篇博文：《分辨率适配的取值范围》
