React native 分辨率适配(px,dp)

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》