Chrome扩展名null类型错误

Question

所以我有一个chrome扩展，通过xml文档检查程序的更新。

Error in event handler for 'tabs.onActivated': Cannot call method 'getElementsByTagName' of null TypeError: Cannot call method 'getElementsByTagName' of null 
at isUpdateAvailable (chrome-extension://bdhjocmpiogdmlfpbpppiffcjbonbocg/background.js:16:17) 
at chrome-extension://bdhjocmpiogdmlfpbpppiffcjbonbocg/background.js:84:5 
at chrome.Event.dispatch (event_bindings:237:41) 
at Object.chromeHidden.Event.dispatchJSON (event_bindings:151:5 

代码：

function loadXMLDoc(dname) { 
    if (window.XMLHttpRequest) {  
     xhttp=new XMLHttpRequest(); 
    } else { 
     xhttp=new ActiveXObject("Microsoft.XMLHTTP"); 
    } 
    xhttp.open("GET",dname,false); 
    xhttp.send(); 
    return xhttp; 
} 

function isUpdateAvailable(type, build) { 
    var buildNumber = localStorage["version" + build + type]; 
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + build + "/view/latest-" + type + "/"); 
    var xmlDoc = xml.responseXML; 
    var x = xmlDoc.getElementsByTagName("title")[0]; 
    var y = x.childNodes[0]; 
    var txt = y.nodeValue; 
    if(txt == buildNumber) { 
     return true; 
    } 
    xml.close(); 
    return false; 
} 

function notify(type, build) { 
    window.webkitNotifications.createNotification('icon.png', getTitle(type, build), getDescription(type, build)); 
} 

function getTitle(type, build) { 
    var title; 
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/"); 
    var xmlDoc=xml.responseXML; 
    var name=xmlDoc.getElementsByTagName("name")[0]; 
    var version=xmlDoc.getElementsByTagName("version")[0]; 
    title+=name; 
    title+=" version "; 
    title+=version; 
    title+=" is out!"; 
    xml.close(); 
    return title; 
} 

function getDescription(type, build) { 
    var desc; 
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/"); 
    var xmlDoc=xml.responseXML; 
    var name=xmlDoc.getElementsByTagName("name")[0]; 
    desc+="There is a new update for "; 
    desc+=name; 
    desc+=". You can download it here: "; 
    var downloadLink; 
    get_short_url(long_url, function(short_url) {downloadLink=short_url;}); 
    desc+=downloadLink; 
    xml.close(); 
    return desc; 
} 

function get_short_url(long_url, func) { 
    var login = "kezz101"; 
    var api_key = "R_d68d87d13b42412a56be9bd9711c4dc4"; 
    $.getJSON("http://api.bitly.com/v3/shorten?callback=?", 
     { 
      "format": "json", 
      "apiKey": api_key, 
      "login": login, 
      "longUrl": long_url 
     }, 
     function(response) { 
      func(response.data.url); 
     } 
    ); 
} 

chrome.tabs.onActivated.addListener(function() { 
    var type = localStorage["type"]; 
    var build = localStorage["build"]; 
    if(!type) { 
     type = "rb"; 
    } 
    if(!build) { 
     build = "craftbukkit"; 
    } 
    if(isUpdateAvailable(type, build)) { 
     notify(type, build); 
     var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/"); 
     var xmlDoc=xml.responseXML; 
     localStorage["version" + build + type] = xmlDoc.getElementsByTagName("build_number")[0]; 
     xml.close(); 
    } 
}); 

Answer 1

使用同步的Ajax，这一直是disabled for extensions and apps然而，当我打开一个新标签，我得到这个错误。您应该改为使用异步Ajax并将回调传递到loadXMLDoc：

function loadXMLDoc(dname, callback) { 
    if (window.XMLHttpRequest) {  
     xhttp=new XMLHttpRequest(); 
    } else { 
     xhttp=new ActiveXObject("Microsoft.XMLHTTP"); 
    } 
    xhttp.open("GET",dname); 

    xhttp.onload = function() { 
     callback(xhttp); 
    } 

    xhttp.send(); 
} 

function isUpdateAvailable(type, build, callback) { 
    var buildNumber = localStorage["version" + build + type]; 
    loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + build + "/view/latest-" + type + "/", function(xml) { 
     var xmlDoc = xml.responseXML; 
     var x = xmlDoc.getElementsByTagName("title")[0]; 
     var y = x.childNodes[0]; 
     var txt = y.nodeValue; 
     if(txt == buildNumber) { 
      callback(true); 
     } else { 
      xml.close(); 
      callback(false); 
     } 
    }); 
} 

... 

chrome.tabs.onActivated.addListener(function() { 
    var type = localStorage["type"]; 
    var build = localStorage["build"]; 
    if(!type) { 
     type = "rb"; 
    } 
    if(!build) { 
     build = "craftbukkit"; 
    } 
    isUpdateAvailable(type, build, function(isAvail) { 
     if(isAvail) { 
      notify(type, build); 
      var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/", function(xml) { 
       var xmlDoc=xml.responseXML; 
       localStorage["version" + build + type] = xmlDoc.getElementsByTagName("build_number")[0]; 
       xml.close(); 
      }); 
     } 
    }); 
});