微信小程序实现image图片自适应宽度

<image src="../uploads/2.jpg" bindload="imageLoad"
style="width:{{imgwidth}}rpx; height:{{imgheight }}rpx;"></image>

//获取应用实例 
var app = getApp()
Page({
	data: {
		screenWidth: 0,
		screenHeight:0,
		imgwidth:0,
		imgheight:0,
	},
	onLoad: function() {
		var _this = this;
		wx.getSystemInfo({
			success: function(res) {
				_this.setData({
					screenHeight: res.windowHeight,
					screenWidth: res.windowWidth,
				});
			}
		});

	},
  	imageLoad: function(e) {
  		var _this=this;
		var $width=e.detail.width,    //获取图片真实宽度
		    $height=e.detail.height,
		    ratio=$width/$height;   //图片的真实宽高比例
		var viewWidth=500,           //设置图片显示宽度，
            viewHeight=500/ratio;    //计算的高度值   
		this.setData({
			imgwidth:viewWidth,
		    imgheight:viewHeight
		})
  	}
})