javascript – 访问canvas.ToBlob()异步函数之外的blob值

我正在使用HTMLCanvas元素,它返回异步toBlob()函数之外的blob对象.此函数不返回输出值,因此我尝试在外部声明变量并通过命令访问它.

我如何在这种情况下使用JS Promise？

var myblob;
            canvas.toBlob(function(blob) {                         
                              myblob = blob;
                              console.log("inside " + myblob); // getting value after the console outside
                           })
 console.log( "outside " + myblob); // getting undefined   

解决方法:

您可以使用Promise构造函数,将Blob实例传递给resolve(),在.then()访问Promise值

function getCanvasBlob(canvas) {
  return new Promise(function(resolve, reject) {
    canvas.toBlob(function(blob) {
      resolve(blob)
    })
  })
}

var canvasBlob = getCanvasBlob(canvas);

canvasBlob.then(function(blob) {
  // do stuff with blob
}, function(err) {
  console.log(err)
});