javascript-for循环内的回调完成后,如何执行函数？

我在搜索函数中有一个for循环,该函数执行一个在循环内执行回调的回调,并且我想在循环后以及所有回调完成后执行BUILD()函数.我不确定该怎么做,因为循环会在所有回调完成之前完成.回调是向我发送数据的API请求,我想对该数据进行BUILD()处理.

我阅读了deferred的内容,因此我尝试将for循环放入延迟的函数中,然后在’.then(…)’上调用BUILD().但这似乎不起作用-我想我理解错了.

救命？！

请注意,这使用的是Google Maps Places API(search和getDetails).

var types = {
    'gym' : 'fitness, gym',
    'grocery_or_supermarket': ''
}

function search() {
    for (var key in types) {
         var request = { ... };
         service.search(request, searchCallback);
    }
    // PROBLEM AREA
    BUILD();
}

function searchCallback(results, status) {
    for (var i = 0; i < results.length; i++) {
        var request = { ... };
        service.getDetails(request, detailsCallback);
    }
}

function detailsCallback(place, status) {
    // add place marker to maps and assign info window and info window event
}

解决方法:

只需对您的代码进行少量修改,就可以实现.

var total = 1337; // Some number
var internal_counter = 0;
var fn_callback = function() {
    searchCallback.apply(this, arguments);
    if (++internal_counter === total) {
        BUILD();
    }
};
for (var i=0; i<total; i++) {
    service.search(request, fn_callback);
    ...

说明

首先,我们创建一个局部函数和变量.

>变量是一个计数器,在调用回调时增加.
>该函数传递给异步方法(service.search),该方法调用原始回调.增加计数器后,对照保存迭代总数的变量检查计数器的值.如果它们相等,则调用修整功能(BUILD).

一个复杂的情况：处理嵌套的回调.

var types = { '...' : ' ... ' };

function search() {
    var keys = Object.keys(types);
    var total = keys.length;
    // This counter keeps track of the number of completely finished callbacks
    //  (search_callback has run AND all of its details_callbacks has run)
    var internal_counter = 0;

    for (var i=0; i<total; i++) {
        var request = { '...' : ' ... ' };
        services.search(request, fn_searchCallback);
    }

    // LOCAL Function declaration (which references `internal_counter`)
    function fn_searchCallback(results, status) {
        // Create a local counter for the callbacks
        // I'm showing another way of using a counter: The opposite way
        // Instead of counting the # of finished callbacks, count the number
        //  of *pending* processes. When this counter reaches zero, we're done.
        var local_counter = results.length;
        for (var i=0; i<results.length; i++) {
            service.getDetails(request, fn_detailsCallback);
        }
        // Another LOCAL function (which references `local_counter`)
        function fn_detailsCallback(result, status) {

            // Run the function logic of detailsCallback (from the question)
            // " ... add place marker to maps and assign info window ... "

            // Reduce the counter of pending detailsCallback calls.
            //   If it's zero, all detailsCallbacks has run.
            if (--local_counter === 0) {
                // Increase the "completely finished" counter
                //  and check if we're finished.
                if (++internal_counter === total) {
                    BUILD();
                }
            }
        } // end of fn_detailsCallback
    } // end of fn_searchCallback
}

注释中说明了功能逻辑.我在本节的标题前加上“复杂”,因为该函数使用了嵌套的局部函数和变量.视觉解释：

var types, BUILD;
function search
    var keys, total, internal_counter, fn_searchCallback;
    function fn_searchCallback
        var result, status; // Declared in the formal arguments
        var local_counter, i, fn_detailsCallback;
        function fn_detailsCallback
            var result, status; // Declared in the formal arguments

在上图中,每个缩进级别表示一个新的scope ^{Explanaation on MDN}.
例如,当一个函数被调用42次时,将创建42个新的本地范围,它们共享相同的父范围.在范围内,父范围看不到declared variables.尽管父范围内的变量可以由“子”范围内的变量读取和更新,但前提是您不声明具有相同名称的变量.我的答案功能中使用了此功能.

标签：callback,google-maps-api-3,javascript
来源： https://codeday.me/bug/20191201/2078444.html