在chrome extension的文档里有这样一句话 sendResponse was called synchronously. If you want to asynchronously use sendResponse, add return true; to the onMessage event handler.
第一遍看文档的时候，刚好理解反了...后来被坑了一次，sendMessage后死活没有返回，才想起文档似乎有这么一句。
下面是一个例子:
比如在contentscript sendMessage
// in contentscript.js
chrome.runtime.sendMessage({to: 'background', subject: 'asyncCall'}, function(data){
console.log(data);
});
background里面的监听代码如下：
chrome.runtime.onMessage.addListener( function (request, sender, sendResponse) {
if(request.subject == "asyncCall"){
setTimeout(function(){
sendResponse('called')
}, 1000);
}
});
但log出来的结果是undefined.
因为response需要被异步调用，而文档中说sendResponse was called synchronously.
想要返回结果，我们需要加上return true，代码如下
chrome.runtime.onMessage.addListener( function (request, sender, sendResponse) {
if(request.subject == "asyncCall"){
setTimeout(function(){
sendResponse('called')
}, 1000);
return true;
}
});