context.drawImage

微信小程序context.drawImage图片显示不全

 var a = wx.createCanvasContext("myCanvas");
        a.drawImage(that.data.imageSrc, 0, 0, 400,  400);
        a.draw(true,function () {
            wx.canvasToTempFilePath({
                x: 10,
                y: 10,
                width: 300,
                height: 200,
                destWidth: 600,
                destHeight: 300,
                quality: 1,
                canvasId: "myCanvas",
                success: function (t) {
                  
                },
                fail: function() {
                    
                }
            });
        });

canvasId应该是旧版的了，新版是传canvas实例