<view class="story">
	<canvas hidden="{{radarImg}}" canvas-id="canvas" type="2d" id="canvas" style="width: 100vw; height: calc(100vw / 750 * 1080);"></canvas>
	<image hidden="{{!radarImg}}" src="{{radarImg}}" style="width: 100vw; height: calc(100vw / 750 * 1080);" />
</view>

openDialog: () => {
		console.log('openDialog()');
		let ctx = wx.createCanvasContext('canvas');
		// that.setData({
		// 	radarImg: 'tempFilePath',
		// 	show: true
		// });
		// console.log(that);
		ctx.draw(true, setTimeout(()=>{
			wx.canvasToTempFilePath({
				canvasId: 'canvas',
				success: function(res) {
					console.log(res);
					let tempFilePath = res.tempFilePath;
					that.setData({
						radarImg: tempFilePath,
						show: true
					});
				},
				fail: function(res) {
					console.log(res);
				}
			}, that);
		},500))
	},

that是在全局里定义的，在onload() 里 赋值 that = this；