在canvas.ToBlob()异步函数之外访问blob值 [英] access blob value outside of canvas.ToBlob() async function

问题描述

我正在使用HTMLCanvas元素，该元素返回async toBlob()函数外部的blob对象.该函数不会返回输出值，因此我试图在外部声明一个变量并通过命令访问它.

I'm working with HTMLCanvas element that return the blob object outside of the async toBlob() function. This function doesn't return an output value, so I'm trying to declare a variable outside and access it through the command.

在这种情况下如何使用JS Promise?

How I can use JS Promise for this scenario?

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

推荐答案

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

You can use Promise constructor, pass Blob instance to resolve(), access Promise value at .then()

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)
});

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