我正在编写一个与 gmail api 交互的 chrome 扩展程序(chrome 45 是我的版本)并且我在从 background.js 向我的内容脚本发送消息时遇到问题。异步方面是问题所在。回调后如何获取要发送的消息？

//---------in content script--------- 
chrome.runtime.sendMessage({ messageId: _id }, function (response) {     
    console.log('the respose.messagePayload is: ' + response.messagePayload);  
}); 
 
//---------in background script--------- 
chrome.runtime.onMessage.addListener(function (request, sender, sendResponse) { 
    getMessage('me', request.messageId, function (payload) { 
        //i want to send my response here 
        //this executes and grabs the payload of data from the api, but isn't sending back the message to the content-script 
        sendResponse({ messagePayload: payload });  
    }); 
    //this synchronous call sends a message to the content-script 
    //sendResponse({ messagePayload: "payload" }); 
    return true; 
}); 
 
function getMessage(userId, messageId,callback) { 
    var request = gapi.client.gmail.users.messages.get({ 
        'userId': userId, 
        'id': messageId 
    }); 
    request.execute(callback); 
} 

Chrome Extension Message passing: response not sent

请您参考如下方法:

您应该在回调中发送 sendResponse 函数。

getMessage('me', request.messageId, sendResponse); 

然后在您的 getMessage 调用完成时执行它。

function getMessage(userId, messageId, sendResponse) { 
    var request = gapi.client.gmail.users.messages.get({ 
        'userId': userId, 
        'id': messageId 
    }); 
 
    request.execute(function(response) { 
        sendResponse({messagePayload: response.payload}); 
    }); 
} 

另一个可能的解决方案:

1. 从 sender 对象中获取 tab id。
2. 只需将选项卡 ID 发送到您的 getMessage 函数，而不是回调函数。
3. 现在，您在 getMessage 函数中的回调函数会将有效负载发送回您的内容脚本。
4. 在您的内容脚本中添加一个类似的 onMessage 监听器，它将接收您的有效载荷，然后对有效载荷执行您想要执行的操作。

你的代码应该是这样的:

//---------in background script--------- 
chrome.runtime.onMessage.addListener(function (request, sender,sendResponse) { 
    getMessage('me', request.messageId, sender.tab.id); 
}); 
 
function getMessage(userId, messageId, tabId) { 
    var request = gapi.client.gmail.users.messages.get({ 
        'userId': userId, 
        'id': messageId 
    }); 
    request.execute(function(response) { 
        // Whatever response you want to sent 
        // Below URL explains your response object 
        // https://developers.google.com/gmail/api/v1/reference/users/messages#resource 
        // Assuming you want to send payload 
        chrome.tabs.sendMessage(tab.id, {messagePayload: response.payload}); 
    }); 
} 
 
//---------in content script--------- 
chrome.runtime.sendMessage({ messageId: _id }); 
 
chrome.runtime.onMessage.addListener(function (request, sender,sendResponse) { 
    // Just to verify that the request is from the getMessage callback 
    // Because this will listen to all request from your extension 
    if (request.messagePayload) { 
        console.log('the respose.messagePayload is: ' + request.messagePayload); 
    } 
}); 

希望对您有所帮助。