var count = 0;

/**
 * 下载图片
 * @param {String} url 图片下载地址
 */
function downloadImage(url, callSuccess) {
	let xhr = new XMLHttpRequest();
	xhr.open('get', url, true);
	xhr.responseType = "blob"; //设置返回类型,此处我用于下载文件 所以返回blob
	xhr.onload = function() {
		// 请求完成
		if (this.status === 200) {
			var blob = this.response;
			var bmpPromise = createImageBitmap(blob, {
				imageOrientation: "none",
				premultiplyAlpha: "none",
				colorSpaceConversion: "default",
			});
			bmpPromise.then(function(image) {
				callSuccess(image);
			})
		} else {
			console.log(url + ' Not found');
		}
	}
	/* 发送请求 */
	xhr.send();
}

/* 接收主线程发送的文件下载请求 */
onmessage = function(event) {
	this._data = event.data;
	let url = this._data.url + '/tile/' + this._data.level + '/' + this._data.y + '/' + this._data.x;
	/* 发送请求下载文件 */
	downloadImage(url, function(image) {
		postMessage({
			key: this._data.key,
			image: image,
		});
	})
}