Question

我正在尝试使用NodeJS从MySQL INSERT查询中存储affectedRows的详细信息。我的思绪正在融化，试图理解回调和承诺。作为一个单人开发团队，我想联系并询问关于如何在foreach循环中应用回调的最清楚的解释。

从这几行代码中可以清楚地看到目标;将数据存储在affected_rows []数组中。

var affected_rows = [];

asset_array.forEach(function(asset) { // Populate the asset table
    var query_string = "INSERT IGNORE INTO " + asset_table + " SET symbol = '" + asset[0] + "', name = '" + asset[1] + "'";
    connection.query(query_string, function(err, rows, fields) {
        if (err) throw err;
        if ( rows.affectedRows > 0 ) {
            data_to_push = [asset_table, asset[0], asset[1]];
            affected_rows.push(data_to_push);
        }
    });
});

console.log(affected_rows); // [] for obvious async reasons

Answer 1

一个选项是在函数内处理asset_array并将回调传递给它，当循环通过asset_array时，检查当前索引是否与asset_array长度（-1）匹配。如果是，请拨打回叫。

    var affected_rows = [];

    function processAssets(cb) {
        var array_len = asset_array_len.length
        asset_array.forEach(function(asset, index) {
            var query_string = 'INSERT IGNORE INTO ' + asset_table + ' SET symbol = \'' + asset[0] + '\', name = \'' + asset[1] + '\'';
            connection.query(query_string, function(err, rows, fields) {
                if (err) throw err

                if (rows.affectedRows > 0) {
                    data_to_push = [asset_table, asset[0], asset[1]];
                    affected_rows.push(data_to_push);
                }
                if (index === (array_len - 1)) cb()
            });
        });
    }

    processAssets(function() {
        console.log(affected_rows)
    })

Answer 2

建议您查看异步Queue。

您可以像这样更改代码以使用它。

//2nd Step - Perform each task and then call callback() to move to next task
var q = async.queue(function(query_string, callback) {
    connection.query(query_string, function(err, rows, fields) {
        if (err) throw err;
        if ( rows.affectedRows > 0 ) {
            data_to_push = [asset_table, asset[0], asset[1]];
            affected_rows.push(data_to_push);
        }

        callback(); //call next task
    });
}, 2); //here 2 means concurrency ie 2 tasks will run in parallel

//Final Step - Drain gives you end of queue which means all tasks have finished processing
q.drain = function() {
    //Do whatever you want after all tasks are finished
};

//1st Step - create a queue of all tasks that you need to perform
for (var i = 0; i < asset_array.length ; i++) {
    var query_string = "INSERT IGNORE INTO " + asset_table + " SET symbol = '" + asset[0] + "', name = '" + asset[1] + "'";
    q.push(query_string);
}

循环遍历MySQL行并将结果存储在数组中

2 个答案: