Javascript 将 URL 转换为 BASE64 图像

Question

我正在构建一个 Ionic2 应用程序。我正在尝试将图像 url 转换为 base64 图像。我找到了 this我正在尝试使用它。

我有以下代码:

var imgUrl = 'https://www.google.de/images/srpr/logo11w.png';
let base64image = this.getBase64Image(imgUrl);
console.log(base64image);

和

public getBase64Image(imgUrl) {
    var img = new Image();
    img.src = imgUrl;
    img.setAttribute('crossOrigin', 'anonymous');
    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;
    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);
    var dataURL = canvas.toDataURL("image/png");
    return dataURL.replace(/^data:image\/(png|jpg);base64,/, "");
}

但是，它输出以下内容:

data:,

我没有收到任何错误，但需要一个 base64 图像。

我的代码一定是不正确的。任何人都可以建议如何将 url 转换为 base64 图像吗？

谢谢

更新

感谢下面各位的反馈，我已经听从了他们的建议来加载图片。现在我有以下代码:

public getBase64Image(imgUrl): Promise<string> {
    return new Promise<string>(resolve => {
        var img = new Image();
        img.src = imgUrl;
        img.setAttribute('crossOrigin', 'anonymous');
        img.onload = (() => {
            var canvas = document.createElement("canvas");
            canvas.width = img.width;
            canvas.height = img.height;
            var ctx = canvas.getContext("2d");
            ctx.drawImage(img, 0, 0);
            var dataURL = canvas.toDataURL("image/png");
            //console.log('UgetBase64Image.dataURL ', dataURL);
            resolve(dataURL.replace(/^data:image\/(png|jpg);base64,/, ""));
        });
    });
}

用法:

                                let promise64: Promise<string> = this.getBase64Image(personModel.avatar);
                                promise64.then((data) => {
                                    personModel.avatar64 = data;

                                });

当我运行 console.log 时，这似乎确实创建了一个 base64 图像。

但是，我确实收到以下错误:

Error: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.
    at HTMLImageElement.img.onload (utilityService.ts:80)

第 80 行:var dataURL = canvas.toDataURL("image/png");

我原以为下面的代码可以解决这个安全问题，但无济于事:

img.setAttribute('crossOrigin', 'anonymous');

更多信息:

完整错误:

:8100/iVBORw0KGgoAAAANSUhEUgAAAbgAAAG5CAYAAAD8liEWAAAgAElEQVR4Xty9B3NkR5Ksm…bNkFj80enI0JnJ80+gTsx2sbrX9zhp7k1oOOPZ5K7Oh/AvN0hP6tzZ6QAAAAAElFTkSuQmCC:1 GET http://localhost:8100/iVBORw0KGgoAAAANSUhEUgAAAbgAAAG5CAYAAAD8liEWAAAgAElEQ…t3bNkFj80enI0JnJ80+gTsx2sbrX9zhp7k1oOOPZ5K7Oh/AvN0hP6tzZ6QAAAAAElFTkSuQmCC net::ERR_EMPTY_RESPONSE
polyfills.js:3 POST http://localhost:8080/jbosswildfly-1.0/person/updatetime 400 (Bad Request)
e @ polyfills.js:3
t.scheduleTask @ polyfills.js:3
e.scheduleMacroTask @ polyfills.js:3
(anonymous) @ polyfills.js:3
send @ VM9549:3
(anonymous) @ xhr_backend.js:117
Observable.subscribe @ Observable.js:45
MapOperator.call @ map.js:54
Observable.subscribe @ Observable.js:42
(anonymous) @ personService.ts:141
t @ polyfills.js:3
PersonService.updateTimeStamps @ personService.ts:140
(anonymous) @ searchjobsParent.ts:109
t.invoke @ polyfills.js:3
onInvoke @ ng_zone.js:236
t.invoke @ polyfills.js:3
onInvoke @ ng_zone.js:236
t.invoke @ polyfills.js:3
e.run @ polyfills.js:3
(anonymous) @ polyfills.js:3
t.invokeTask @ polyfills.js:3
onInvokeTask @ ng_zone.js:227
t.invokeTask @ polyfills.js:3
onInvokeTask @ ng_zone.js:227
t.invokeTask @ polyfills.js:3
e.runTask @ polyfills.js:3
i @ polyfills.js:3
polyfills.js:3 GET http://localhost:8080/jbosswildfly-1.0/person/list/favouritejob/null/0/ 400 (Bad Request)

EXCEPTION: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.
ErrorHandler.handleError @ error_handler.js:47
IonicErrorHandler.handleError @ ionic-error-handler.js:56
next @ application_ref.js:272
schedulerFn @ async.js:82
SafeSubscriber.__tryOrUnsub @ Subscriber.js:223
SafeSubscriber.next @ Subscriber.js:172
Subscriber._next @ Subscriber.js:125
Subscriber.next @ Subscriber.js:89
Subject.next @ Subject.js:55
EventEmitter.emit @ async.js:74
NgZone.triggerError @ ng_zone.js:278
onHandleError @ ng_zone.js:257
t.handleError @ polyfills.js:3
e.runTask @ polyfills.js:3
invoke @ polyfills.js:3
error_handler.js:52 ORIGINAL STACKTRACE:
ErrorHandler.handleError @ error_handler.js:52
IonicErrorHandler.handleError @ ionic-error-handler.js:56
next @ application_ref.js:272
schedulerFn @ async.js:82
SafeSubscriber.__tryOrUnsub @ Subscriber.js:223
SafeSubscriber.next @ Subscriber.js:172
Subscriber._next @ Subscriber.js:125
Subscriber.next @ Subscriber.js:89
Subject.next @ Subject.js:55
EventEmitter.emit @ async.js:74
NgZone.triggerError @ ng_zone.js:278
onHandleError @ ng_zone.js:257
t.handleError @ polyfills.js:3
e.runTask @ polyfills.js:3
invoke @ polyfills.js:3
error_handler.js:53 Error: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.
    at HTMLImageElement.img.onload (utilityService.ts:82)
    at HTMLImageElement.n [as _onload] (polyfills.js:2)
    at t.invokeTask (polyfills.js:3)
    at Object.onInvokeTask (ng_zone.js:227)
    at t.invokeTask (polyfills.js:3)
    at e.runTask (polyfills.js:3)
    at HTMLImageElement.invoke (polyfills.js:3)

Answer 1

图像实例在图像完全加载时触发 onload 事件。有了这个，另一个问题是处理异步函数。为了能够使用 getBase64Image 所使用的功能，必须使用回调函数。没有回调函数，函数返回undefined

let base64image = this.getBase64Image(imgUrl);
console.log(base64image); // undefined

调整后的功能

public getBase64Image(imgUrl, callback) {

    var img = new Image();

    // onload fires when the image is fully loadded, and has width and height

    img.onload = function(){

      var canvas = document.createElement("canvas");
      canvas.width = img.width;
      canvas.height = img.height;
      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, 0, 0);
      var dataURL = canvas.toDataURL("image/png"),
          dataURL = dataURL.replace(/^data:image\/(png|jpg);base64,/, "");

      callback(dataURL); // the base64 string

    };

    // set attributes and src 
    img.setAttribute('crossOrigin', 'anonymous'); //
    img.src = imgUrl;

}

用法:

this.getBase64Image(imgUrl, function(base64image){
     console.log(base64image);
});