如何将屏幕截图从扩展程序移动到内容脚本？

Question

So the idea goes like this... all i want to do is call an extension background page as soon as a specific content script is loaded. (in this case when a page is loaded) I have something like this in my content script.

var port = chrome.extension.connect({name: "screenshot"});
port.postMessage({request: "screenshot"});
port.onMessage.addListener(function(msg) {
    var img = document.createElement('img');
    img.src = msg.response;
    document.body.appendChild(img);
});

And something like this in my background.js page

function takeScreenshot(){
    chrome.tabs.captureVisibleTab(null, function(img) {
        return img;

    });
}
chrome.extension.onConnect.addListener(function(port) {
    console.assert(port.name == "screenshot");
    port.onMessage.addListener(function(msg) {
    if (msg.request == "screenshot"){
        port.postMessage({response: takeScreenshot() });
    }
    });
});

But i am having no luck... I get the image to show up on the body but it doesnt actually pass the image when returning img from takeScreenshot

Answer 1

The problem is you are trying to achieve synchronization between synchronous and asynchronous events in background page

a) chrome.tabs.captureVisibleTab() is asynchronous

b) port.postMessage() is synchronous

When port.postMessage({response: takeScreenshot() }); is called it does not wait for takeScreenshot() call back ie; return img; so an empty JSON object is sent to content script

Avoid it with posting message after call back returns

chrome.extension.onConnect.addListener(function(port) {
    console.assert(port.name == "screenshot");
    port.onMessage.addListener(function(msg) {
    if (msg.request == "screenshot"){
        chrome.tabs.captureVisibleTab(null,{format:"jpeg",quality:100},function(img) {
        //post message only after call back return with Data URL
        port.postMessage(img);
        });
    }
    });
});

Working Version Demonstration

manifest.json

{
"name":"Screen Shot Demo",
"description":"This demonstrates Screen Shot API",
"manifest_version":2,
"version":"1",
"permissions":["tabs","<all_urls>"],
"background":{
    "scripts":["background.js"]
},
"content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["myscript.js"]
    }
  ]
}

background.js

chrome.extension.onConnect.addListener(function(port) {
    console.assert(port.name == "screenshot");
    port.onMessage.addListener(function(msg) {
    if (msg.request == "screenshot"){
        chrome.tabs.captureVisibleTab(null,{format:"jpeg",quality:100},function(img) {
        //post message only after call back return with Data URL
        port.postMessage(img);
        });
    }
    });
});

myscript.js (contentscript)

var port = chrome.extension.connect({
    name: "screenshot"
});
port.postMessage({
    request: "screenshot"
});
port.onMessage.addListener(function (msg) {
    var img = document.createElement('img');
    img.src = msg;
    document.body.appendChild(img);
});

Let me know if you need more information.