爲什麼.then（）在promise實際解決之前觸發？

我試圖從異步任務中返回承諾，以便我可以使用。然後（）調用使它們同步。不幸的是，我正在經歷一些真正奇怪的行爲。基本上，第二個then（）在第二個解決之前執行。有人可以解釋發生了什麼嗎？這裏是我的代碼：爲什麼.then（）在promise實際解決之前觸發？

function runReleaseLoop() { 
    console.log('Starting release loop'); 
    const themes = ['blue', 'red']; 
    return new Promise(resolve => { 
    buildReleases(themes, resolve); 
    }) 
} 

function buildReleases(themes, onComplete) { 
    console.log(themes); 
    if (themes.length === 0) { 
    onComplete(); 
    } else { 
    const theme = themes.pop(); 
    Promise.resolve() 
     .then(setTheme(theme)) 
     .then(someDelayedTask(theme)) 
     .then(buildReleases(themes, onComplete)); 
    } 
} 

function setTheme(theme) { 
    return new Promise(resolve => { 
    console.log(`Setting theme to ${theme}`); 
    resolve(); 
    }); 
} 

function someDelayedTask(theme) { 
    console.log(`Starting delayed task related to ${theme}`); 
    return new Promise(resolve => { 
    setTimeout(() => { 
     console.log(`Finished delayed task related to ${theme}`); 
     resolve(); 
    }, 3000) 
    }); 
} 

// Run the process 
runReleaseLoop() 
    .then(() => { 
    console.log('Release loop finished') 
    });

這裏是我的控制檯輸出：

"Starting release loop" 
["blue", "red"] 
"Setting theme to red" 
"Starting delayed task related to red" 
["blue"] 
"Setting theme to blue" 
"Starting delayed task related to blue" 
[] 
"Release loop finished" 
"Finished delayed task related to red" 
"Finished delayed task related to blue"

來源

2016-04-15 ordanj

因此，對於帶參數的，我需要返回，返回一個承諾函數的功能是什麼？ – ordanj

是的正確（哎呀刪除第一個評論大聲笑），但你也需要採取行動的承諾，返回，等 – Deryck

你調用這些功能的同步（setTheme，someDelayedTask，並buildReleases），然後通過他們的返回值（許諾）成then而不是傳遞函數本身進入然後：

Promise.resolve() 
    .then(setTheme(theme)) 
    .then(someDelayedTask(theme)) 
    .then(buildReleases(themes, onComplete));

這裏是一個解決方案：

Promise.resolve() 
    .then(_ => setTheme(theme)) 
    .then(_ => someDelayedTask(theme)) 
    .then(_ => buildReleases(themes, onComplete));

這裏是你的代碼的輸出與上述變化：

Starting release loop 
[ 'blue', 'red' ] 
Setting theme to red 
Starting delayed task related to red 
Finished delayed task related to red 
[ 'blue' ] 
Setting theme to blue 
Starting delayed task related to blue 
Finished delayed task related to blue 
[] 
Release loop finished

來源

2016-04-15 20:31:32 Paulpro

你也可以做這樣的事情： Promise.resolve（） .then（setTheme） .then（someDelayedTask） .then（buildReleases）; – northsideknight

的第一個問題是，你混有回調的承諾。第二個是你調用所有功能setTheme, someDelayedTask, buildReleases在一個時間：

function runReleaseLoop() { 
    console.log('Starting release loop'); 
    const themes = ['blue', 'red']; 
    return buildReleases(themes); 
} 

function buildReleases(themes) { 
    console.log(themes); 
    if (themes.length === 0) { 
    return Promise.resolve(); 
    } 

    const theme = themes.pop(); 
    return setTheme(theme) 
    .then(() => someDelayedTask(theme)) 
    .then(() => buildReleases(themes)); 
} 

function setTheme(theme) { 
    console.log(`Setting theme to ${theme}`); 
} 

function someDelayedTask(theme) { 
    console.log(`Starting delayed task related to ${theme}`); 
    return new Promise(resolve => { 
    setTimeout(() => { 
     console.log(`Finished delayed task related to ${theme}`); 
     resolve(); 
    }, 3000) 
    }); 
} 

// Run the process 
runReleaseLoop() 
    .then(() => console.log('Release loop finished'));

來源

2016-04-15 20:37:57 alexmac

爲什麼.then（）在promise實際解決之前觸發？

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