如何根据来自本地存储的另一个数据响应延迟 XMLHttpRequest

Question

//function to download pps_compress.txt from cloud server for global parameter set PPs
function get_pps_params()
{

    chrome.storage.local.get(['ppsParams'], function(result) {
        if (typeof(result) !== "undefined" && result != null){
            ppsParams = _base64ToArrayBuffer(result.ppsParams);
            console.log(ppsParams);
            dfd_pps.resolve();
            return;
        }
    });

    if(ppsParams == null)
    {
        var oReq = new XMLHttpRequest();
        oReq.open("GET", CLOUD_SERVER + 'get_pps_params', true);
        oReq.responseType = "arraybuffer";

        oReq.onload = function (oEvent) {
            console.log("Got pps params compressed!");
            ppsParams = oReq.response; // Note: not oReq.responseText
            chrome.storage.local.set({ppsParams: _arrayBufferToBase64(ppsParams)});
            dfd_pps.resolve();
        };

        oReq.send();
    }
}

在上面的代码中，我试图从本地存储中获取一些参数到变量 ppsParams 中，但我想仅在本地存储请求失败时才向 CLOUD_SERVER 发送请求，现在两者都在执行，所以我怎么能推迟 XMLHttpRequest。

Answer 1

看起来chrome.storage.local.get是异步的，所以移动XMLHttpRequest代码如下所示

function get_pps_params()
{

    chrome.storage.local.get(['ppsParams'], function(result) {
        if (typeof(result) !== "undefined" && result != null){
            ppsParams = _base64ToArrayBuffer(result.ppsParams);
            console.log(ppsParams);
            dfd_pps.resolve();
        }
        if(ppsParams == null)
        {
            var oReq = new XMLHttpRequest();
            oReq.open("GET", CLOUD_SERVER + 'get_pps_params', true);
            oReq.responseType = "arraybuffer";

            oReq.onload = function (oEvent) {
                console.log("Got pps params compressed!");
                ppsParams = oReq.response; // Note: not oReq.responseText
                chrome.storage.local.set({ppsParams: _arrayBufferToBase64(ppsParams)});
                dfd_pps.resolve();
            };

            oReq.send();
        }
        return;
    });
}