安卓，ctx.draw生成图片白屏

根据下方代码，生成图片时有概率白屏，初步调试后发现和SETTIMEOUT的时间有关（手机内存越小发生几率越高，TIMEOUT300ms时有总体4%的发生率，2000MS时有总体2%的发生率），是否有优化/解决方案？

return new Promise((resolve, reject) => {
var that = this;
uni.getImageInfo({
src: list[index].url,
success: (ress) => {
let maxWidth = 300;
let maxHeight = 300;
let img = ress;
let targetWidth
let targetHeight
if (img.width > maxWidth && img.height > maxHeight) {
const rate = Math.min(maxWidth / img.width, maxHeight / img.height)
targetWidth = img.width rate
targetHeight = img.height rate
} else if (img.width > maxWidth) {
targetWidth = maxWidth
targetHeight = (maxWidth / img.width) img.height
} else if (img.height > maxHeight) {
targetHeight = maxHeight
targetWidth = (maxHeight / img.height) img.width
} else {
targetWidth = img.width
targetHeight = img.height
}

                  let ctx = uni.createCanvasContext('firstCanvas', that); //创建画布  
                  //将图片src放到cancas内，宽高为图片大小  
                  ctx.drawImage(list[index].url, 0, 0, targetWidth, targetHeight)  
                  ctx.setFontSize(12) //字体大小  
                  ctx.setFillStyle('#ffffff') //字体颜色  
                  let textToWidth = targetWidth * 0.02;  //字体宽度  
                  let textToHeight = targetHeight * 0.85; //字体高度  
                  ctx.fillText(that.CurentTime(), textToWidth, textToHeight) // 设置字体  
                  ctx.draw(false, setTimeout(()=>{  
                      uni.canvasToTempFilePath({//将画布中内容转成图片，即水印与图片合成  
                          width: targetWidth,   // 画布宽  
                          height: textToHeight, // 画布高  
                          canvasId: 'firstCanvas',  
                          quality: 0.5,  
                          fileType: 'jpg',  
                          success: (res) => {  
                              list[index].url = res.tempFilePath  
                              resolve(true);  
                          },  
                          fail: (e)=> {  
                              uni.showModal({  
                                  title: "图片转化失败,原因(" + JSON.stringify(e) + ")",  
                                  showCancel: false,  
                              });  
                          },  
                          complete: ()=> {  
                              // console.log("complete");  
                          },  
                      })  
                  },2000))  
              }  
          })  
      })