当调用使用回调的函数（在循环中）时，如何传递变量并使它们不改变？

Question

我在循环中为数组中的一堆 appId 调用 sendMessage。当我的回调被调用时，我需要检查错误，然后将出现错误的 appId 列入“黑名单”。问题是我尝试过的每一种方法都会导致回调中的 appId 在它被调用时发生变化！所以错误的 appId 会被列入黑名单。

我尝试了三个版本（见下文）。一个从不列入黑名单，另外两个做错了：

**这个黑名单错误的一个**

for ( var appName in apps )
{
    var app = apps[ appName ];

    var appId = app[ "appId" ];

    //Send message to that app
    chrome.runtime.sendMessage(
        app[ "appId" ],
        app,
        function (response)
        {
            var lastError = chrome.runtime.lastError;

            //Want to blacklist apps here
            if ( lastError && lastError.message.indexOf( "not exist" ) !== -1 )
            {
                //This blacklists the wrong one!
                myGlobalObject.addToTimeout( appId );
            }
        }
    );
}

**这个也黑名单错误的一个**

for ( var appName in apps )
{
    var app = apps[ appName ];

    var appId = app[ "appId" ];

    //Send message to that app
    chrome.runtime.sendMessage(
        app[ "appId" ],
        app,
        function (response)
        {
            var lastError = chrome.runtime.lastError;

            //Want to blacklist apps here
            if ( lastError && lastError.message.indexOf( "not exist" ) !== -1 )
            {
                //This blacklists the wrong one!
                myGlobalObject.addToTimeout( this[ "appId" ] );
            }
        }.bind( app )
    );
}

** 这个永远不会黑名单 **

for ( var appName in apps )
{
    var app = apps[ appName ];

    //Send message to that app
    chrome.runtime.sendMessage(
        app[ "appId" ],
        app,
        function (response)
        {
            var lastError = chrome.runtime.lastError;

            //Want to blacklist apps here
            if ( lastError && lastError.message.indexOf( "not exist" ) !== -1 )
            {
                //Somehow this NEVER blacklists it!
                myGlobalObject.addToTimeout( app[ "appId" ] );
            }
        }
    );
}

Answer 1

对于稍后调用的回调，您需要在闭包中“冻结”变量的值。有几种方法可以做到这一点，这是一种使用立即调用函数的方法，该函数将变量作为参数传递，它们在该函数闭包中被捕获，因此它们的值在回调期间保持你想要的值：

for ( var appName in apps )
{
    var app = apps[ appName ];

    var appId = app[ "appId" ];

    // this creates a closure which captures the values of your variables
    // and holds them at the desired value until the callback is called
    // a separate and unique closure will be created
    // for each cycle of the for loop
    (function(appID) {

        //Send message to that app
        chrome.runtime.sendMessage(
            appId,
            app,
            function (response)
            {
                // since this is called some time later
                // variables outside this may have different values
                // as the outer for loop continued to run
                var lastError = chrome.runtime.lastError;

                //Want to blacklist apps here
                if ( lastError && lastError.message.indexOf( "not exist" ) !== -1 )
                {
                    //This blacklists the wrong one!
                    myGlobalObject.addToTimeout( appId );
                }
            }
        );
    })(appID);
}