js canvas转base64编码图片不全

document.choose('image',function(err,uri){
            let ctx = that.$element('drawImage').getContext('2d');
            let img = new Image();
            img.src = uri;
            img.onload = function() {
                console.log('Image load success');
                ctx.drawImage(img, 0, 0, 320, 250);
            }

        }).then(function(data){
            console.log(data)
        })
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let el =this.$refs.canvas;
        let base = el.toDataURL('image/png');
        console.log(base)
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