如果有一张 20*10 的图片，要把它放入一个 40*30 的显示区域内，我们可以做到：

contain 模式，图片显示分辨率为20*10，四周都有空白；

cover模式，图片放大为 60*30，然后切成 40*30，丢失部分图片内容；

stretch 模式，图片放大为 40*30，丢失原始的宽、高比。

实现自定义组件 ImageEquallyEnlarg.js

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 * @flow
 */

import React,{ Component } from 'react';
import {
  AppRegistry,StyleSheet,Image,TouchableHighlight,View
} from 'react-native';

export default class ImageEqualEnlarge extends Component {

//ImageEqualEnlarge 组件的状态机变量是一个style,它将被用于定义显示图片的样式
  constructor(props) {
    super(props);
    this.state = {
      style:{}
    };
  }
 //声明必须要有的图片原始宽度与高度
  static propTypes = {
    originalWidth:React.PropTypes.number.isrequired,originalHeight:React.PropTypes.number.isrequired
  }
//此函数被挂接到组件的onLayout事件上，当组件被布局时，此函数被调用
//在此函数中计算新的宽度与高度并将其保存到组件的状态机变量中
onImageLayout(event){
    let layout = event.nativeEvent.layout;
    if(layout.width<=this.props.originalWidth) 
      return;
    if (layout.height<=this.props.originalHeight) 
      return;
    
    let originalAspectRatio = this.props.originalWidth/this.props.originalHeight;
    let currentAspectRatio = layout.width/layout.height;

    if(originalAspectRatio===currentAspectRatio) return;

    if(originalAspectRatio > currentAspectRatio){
           let newHeight = layout.width / originalAspectRatio;
           this.setState({
            style:{
              height:newHeight,}
           });
           return;
    }
    let newWidth = layout.height*originalAspectRatio;
    this.setState({
      style:{
        width:newWidth,}
    });
  }

  render() {
    return (
      <Image
      {...this.props}
      style={[this.props.style,this.state.style]}
      onLayout ={this.onImageLayout.bind(this)}
      >
        
      </Image>
    );
  }


}

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 * @flow
 */

import React,View
} from 'react-native';

import ImageEqualEnlarge from './ImageEqualEnlarge.js' 

export default class ViewProject extends Component {

  render() {
    return (
      <View style={styles.container}>
        <ImageEqualEnlarge style={styles.imageStyle}
         source={require('./img/logo.png')}
         originalWidth={120}
         originalHeight={120}
        >
        </ImageEqualEnlarge>
        <ImageEqualEnlarge style={styles.image2Style}
         source={require('./img/logo.png')}
         originalWidth={120}
         originalHeight={120}
        >
        </ImageEqualEnlarge>
      
      </View>
    );
  }


}



const styles = StyleSheet.create({
  container: {
    backgroundColor:'blue',},imageStyle:{
    width:200,height:200,backgroundColor:'red',image2Style:{
    width:300,}
 
});
AppRegistry.registerComponent('ViewProject',() => ViewProject);