PROSAGA码农传奇-时空索引-在QML中下载并转换图像到数据URI

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  <P>
    该
    <a href="https://bugreports.qt.io/browse/QTBUG-53709" rel="nofollow noreferrer">
      Qt错误报告
    </A>
     对于没有实现的XMLHttpRequest overrideMimeType方法（eyllanesc的注释链接指向我）表明对arraybuffers有一些支持。基于此并查看QQmlXMLHttpRequest源代码，我能够实现这一点。这是一个示例（我有一个包含我自己的Base64编码器，因为Qt.btoa不起作用）：
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   <pre>
    <code>
      import QtQuick 2.7
import ArcGIS.AppFramework 1.0

App {
    id: app
    width: 400
    height: 640

Component.onCompleted: {
        var url = 'https://firebasestorage.googleapis.com/v0/b/christophereby-3733b.appspot.com/o/icon1.png?alt=media';
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url, true);
        xhr.responseType = 'arraybuffer';
        xhr.onreadystatechange = function() {
            if (xhr.readyState === XMLHttpRequest.DONE) {
                if (xhr.status === 200) {
                    var response = new Uint8Array(xhr.response);
                    var raw = "";
                    for (var i = 0; i < response.byteLength; i++) {
                        raw += String.fromCharCode(response[i]);
                    }

//FROM https://cdnjs.cloudflare.com/ajax/libs/Base64/1.0.1/base64.js
                    function base64Encode (input) {
                        var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
                        var str = String(input);
                        for (
                            // initialize result and counter
                            var block, charCode, idx = 0, map = chars, output = '';
                            str.charAt(idx | 0) || (map = '=', idx % 1);
                            output += map.charAt(63 & block >> 8 - idx % 1 * 8)
                            ) {
                            charCode = str.charCodeAt(idx += 3/4);
                            if (charCode > 0xFF) {
                                throw new Error("Base64 encoding failed: The string to be encoded contains characters outside of the Latin1 range.");
                            }
                            block = block << 8 | charCode;
                        }
                        return output;
                    }
                    var image = 'data:image/png;base64,' + base64Encode(raw);
                    img.source = image;
                }
            }
        }
        xhr.send();
    }
    Image {
        id: img
    }
}

</code>
  </pre>
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