Question

我是Nodejs的新手，我一直在为我的合同编写一个Web3处理程序，一切正常但全局变量在promise执行期间不会更新，即使我在console.log之后也没有任何价值然后。

...
var cityDb = {};
function gettingBalance() {

    return new Promise ( (resolve, reject) => { 

        Contract.totalSupply.call(function(error, result){
            if(!error)
                {

                resolve(result.toNumber());
                }
            else

                reject(error);
        });
    });
}


function gettingCities(cityNumber) {

    return new Promise ( (resolve,reject) => { 

        Contract.CityDB.call( cityNumber, (error, result) => {
            if(!error)
                {

                    resolve(result);

                }
            else
                reject(error);
        });

    });

}



gettingBalance()
    .then ( (result) => {
        console.log('Total after function: '+result);
        return result;
    })
        .then ( (totalSupply) => {
            console.log('loop data :'+totalSupply);

            // START SECOND PROMISE 

            for (let i=0; i <= totalSupply; i++) { 

                gettingCities(i) 
                    .then( (result) => {

                    cityDb[i] = {
                            cityName: result[0],
                            Population: result[1],
                            };


                        let j = i-1;
                        console.log('Loop ['+i+']'+ cityDb[i].cityName);

                    })
                    .catch( (error) => {
                        console.log(error);
                    });

                    if (i == totalSupply) { return;  }

            }


            // END SECOND PROMISE

        })
        .then ( (response) => {
            console.log('Test promise loop: '+ cityDb[2]);
        })
        .catch( (err) => {
            console.log('loop error: '+err);
        });

记录结果：

Total supply after function: 3
loop data :3
Test promise loop: undefined
Loop [0]0x4e657720596f726b000000000000000000000000000000000000000000000000
Loop [1]0x5061726973000000000000000000000000000000000000000000000000000000
Loop [2]0x4e61646f72000000000000000000000000000000000000000000000000000000
Loop [3]0x0000000000000000000000000000000000000000000000000000000000000000

为什么测试保证循环未定义？ cityDB没有从循环中获得更新，但循环完全按预期工作。

Answer 1

问题是你没有正确地链接承诺。您的所有then来电完成后，您的上次gettingCities并未等待。为此，您可以使用Promise.all

gettingBalance()
    .then((result) => {
        console.log('Total after function: ' + result);
        return result;
    })
    .then((totalSupply) => {
        console.log('loop data :' + totalSupply);

        // START SECOND PROMISE 
        const promises = [];

        for (let i = 0; i < totalSupply; i++) {
            // We push gettingCities promises into the array
            promises.push(gettingCities(i));    
        }

        // We pass gettingCities promises into Promise.all
        return Promise.all(promises);

    })
    .then((results) => {

        // All gettingCities results
        // We loop through each result and fill cityDb
        results.forEach((result, i) => {

            cityDb[i] = {
                cityName: result[0],
                Population: result[1],
            };

            console.log('Loop [' + i + ']' + cityDb[i].cityName);
        })

        console.log('Test promise loop: ' + cityDb[2]);
    })
    .catch((err) => {
        console.log('loop error: ' + err);
    });

您可以删除全局cityDb，并在上一个.then

中处理它

.then((results) => {

    // gettingCities results
    const cityDb = {};
    results.forEach((result, i) => {

        cityDb[i] = {
            cityName: result[0],
            Population: result[1],
        };

        console.log('Loop [' + i + ']' + cityDb[i].cityName);
    })

    console.log('Test promise loop: ' + cityDb[2]);
})

无法从NodeJS中的promise更新全局变量

1 个答案: