Xử lý vòng lặp async Javascript

Question

Tôi có một vòng lặp javascript cần một chút thời gian để xử lý. Tôi ước tôi có thể thu gọn nó nhưng nó phải xử lý một lượng lớn dữ liệu. Trong khi nó đang chạy trình duyệt trở nên không phản hồi tất nhiên. Tôi đã đọc cách tốt nhất để xử lý điều này trong javascript là sử dụng một vòng lặp không đồng bộ của một số loại. Bằng cách này, các cú click chuột, vv, có thể tiếp tục được xử lý trong quá trình xử lý vòng lặp. Có bất kỳ khung công tác không đồng bộ chuẩn nào sẽ hoạt động tốt cho điều này không? Hoặc ai đó có thể cung cấp một ví dụ đơn giản về cách mã này có thể được mã hóa? Cảm ơn!

Answer 1

Rất tiếc WebWorkers chưa khả dụng trên trình duyệt của mọi người. Tôi đã sử dụng "setTimeout (Func, 0);" lừa trong khoảng một năm. Dưới đây là một số nghiên cứu gần đây tôi đã viết lên để giải thích làm thế nào để tăng tốc độ nó lên một chút. Nếu bạn chỉ muốn câu trả lời, hãy chuyển sang Bước 4. Bước 1 2 và 3 giải thích lý do và cơ học;

// In Depth Analysis of the setTimeout(Func,0) trick. 

//////// setTimeout(Func,0) Step 1 //////////// 
// setTimeout and setInterval impose a minimum 
// time limit of about 2 to 10 milliseconds. 

    console.log("start"); 
    var workCounter=0; 
    var WorkHard = function() 
    { 
    if(workCounter>=2000) {console.log("done"); return;} 
    workCounter++; 
    setTimeout(WorkHard,0); 
    }; 

// this take about 9 seconds 
// that works out to be about 4.5ms per iteration 
// Now there is a subtle rule here that you can tweak 
// This minimum is counted from the time the setTimeout was executed. 
// THEREFORE: 

    console.log("start"); 
    var workCounter=0; 
    var WorkHard = function() 
    { 
    if(workCounter>=2000) {console.log("done"); return;} 
    setTimeout(WorkHard,0); 
    workCounter++; 
    }; 

// This code is slightly faster because we register the setTimeout 
// a line of code earlier. Actually, the speed difference is immesurable 
// in this case, but the concept is true. Step 2 shows a measurable example. 
/////////////////////////////////////////////// 


//////// setTimeout(Func,0) Step 2 //////////// 
// Here is a measurable example of the concept covered in Step 1. 

    var StartWork = function() 
    { 
    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var sum=0; 
    var WorkHard = function() 
    { 
     if(workCounter>=2000) 
     { 
     var ms = (new Date()).getTime() - startTime.getTime(); 
     console.log("done: sum=" + sum + " time=" + ms + "ms"); 
     return; 
     } 
     for(var i=0; i<1500000; i++) {sum++;} 
     workCounter++; 
     setTimeout(WorkHard,0); 
    }; 
    WorkHard(); 
    }; 

// This adds some difficulty to the work instead of just incrementing a number 
// This prints "done: sum=3000000000 time=18809ms". 
// So it took 18.8 seconds. 

    var StartWork = function() 
    { 
    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var sum=0; 
    var WorkHard = function() 
    { 
     if(workCounter>=2000) 
     { 
     var ms = (new Date()).getTime() - startTime.getTime(); 
     console.log("done: sum=" + sum + " time=" + ms + "ms"); 
     return; 
     } 
     setTimeout(WorkHard,0); 
     for(var i=0; i<1500000; i++) {sum++;} 
     workCounter++; 
    }; 
    WorkHard(); 
    }; 

// Now, as we planned, we move the setTimeout to before the difficult part 
// This prints: "done: sum=3000000000 time=12680ms" 
// So it took 12.6 seconds. With a little math, (18.8-12.6)/2000 = 3.1ms 
// We have effectively shaved off 3.1ms of the original 4.5ms of dead time. 
// Assuming some of that time may be attributed to function calls and variable 
// instantiations, we have eliminated the wait time imposed by setTimeout. 

// LESSON LEARNED: If you want to use the setTimeout(Func,0) trick with high 
// performance in mind, make sure your function takes more than 4.5ms, and set 
// the next timeout at the start of your function, instead of the end. 
/////////////////////////////////////////////// 


//////// setTimeout(Func,0) Step 3 //////////// 
// The results of Step 2 are very educational, but it doesn't really tell us how to apply the 
// concept to the real world. Step 2 says "make sure your function takes more than 4.5ms". 
// No one makes functions that take 4.5ms. Functions either take a few microseconds, 
// or several seconds, or several minutes. This magic 4.5ms is unattainable. 

// To solve the problem, we introduce the concept of "Burn Time". 
// Lets assume that you can break up your difficult function into pieces that take 
// a few milliseconds or less to complete. Then the concept of Burn Time says, 
// "crunch several of the individual pieces until we reach 4.5ms, then exit" 

// Step 1 shows a function that is asyncronous, but takes 9 seconds to run. In reality 
// we could have easilly incremented workCounter 2000 times in under a millisecond. 
// So, duh, that should not be made asyncronous, its horrible. But what if you don't know 
// how many times you need to increment the number, maybe you need to run the loop 20 times, 
// maybe you need to run the loop 2 billion times. 

    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    for(var i=0; i<2000000000; i++) // 2 billion 
    { 
    workCounter++; 
    } 
    var ms = (new Date()).getTime() - startTime.getTime(); 
    console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 

// prints: "done: workCounter=2000000000 time=7214ms" 
// So it took 7.2 seconds. Can we break this up into smaller pieces? Yes. 
// I know, this is a retarded example, bear with me. 

    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var each = function() 
    { 
    workCounter++; 
    }; 
    for(var i=0; i<20000000; i++) // 20 million 
    { 
    each(); 
    } 
    var ms = (new Date()).getTime() - startTime.getTime(); 
    console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 

// The easiest way is to break it up into 2 billion smaller pieces, each of which take 
// only several picoseconds to run. Ok, actually, I am reducing the number from 2 billion 
// to 20 million (100x less). Just adding a function call increases the complexity of the loop 
// 100 fold. Good lesson for some other topic. 
// prints: "done: workCounter=20000000 time=7648ms" 
// So it took 7.6 seconds, thats a good starting point. 
// Now, lets sprinkle in the async part with the burn concept 

    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var index=0; 
    var end = 20000000; 
    var each = function() 
    { 
    workCounter++; 
    }; 
    var Work = function() 
    { 
    var burnTimeout = new Date(); 
    burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future 
    while((new Date()) < burnTimeout) 
    { 
     if(index>=end) 
     { 
     var ms = (new Date()).getTime() - startTime.getTime(); 
     console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 
     return; 
     } 
     each(); 
     index++; 
    } 
    setTimeout(Work,0); 
    }; 

// prints "done: workCounter=20000000 time=107119ms" 
// Sweet Jesus, I increased my 7.6 second function to 107.1 seconds. 
// But it does prevent the browser from locking up, So i guess thats a plus. 
// Again, the actual objective here is just to increment workCounter, so the overhead of all 
// the async garbage is huge in comparison. 
// Anyway, Lets start by taking advice from Step 2 and move the setTimeout above the hard part. 

    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var index=0; 
    var end = 20000000; 
    var each = function() 
    { 
    workCounter++; 
    }; 
    var Work = function() 
    { 
    if(index>=end) {return;} 
    setTimeout(Work,0); 
    var burnTimeout = new Date(); 
    burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future 
    while((new Date()) < burnTimeout) 
    { 
     if(index>=end) 
     { 
     var ms = (new Date()).getTime() - startTime.getTime(); 
     console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 
     return; 
     } 
     each(); 
     index++; 
    } 
    }; 

// This means we also have to check index right away because the last iteration will have nothing to do 
// prints "done: workCounter=20000000 time=52892ms" 
// So, it took 52.8 seconds. Improvement, but way slower than the native 7.6 seconds. 
// The Burn Time is the number you tweak to get a nice balance between native loop speed 
// and browser responsiveness. Lets change it from 4.5ms to 50ms, because we don't really need faster 
// than 50ms gui response. 

    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var index=0; 
    var end = 20000000; 
    var each = function() 
    { 
    workCounter++; 
    }; 
    var Work = function() 
    { 
    if(index>=end) {return;} 
    setTimeout(Work,0); 
    var burnTimeout = new Date(); 
    burnTimeout.setTime(burnTimeout.getTime() + 50); // burnTimeout set to 50ms in the future 
    while((new Date()) < burnTimeout) 
    { 
     if(index>=end) 
     { 
     var ms = (new Date()).getTime() - startTime.getTime(); 
     console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 
     return; 
     } 
     each(); 
     index++; 
    } 
    }; 

// prints "done: workCounter=20000000 time=52272ms" 
// So it took 52.2 seconds. No real improvement here which proves that the imposed limits of setTimeout 
// have been eliminated as long as the burn time is anything over 4.5ms 
/////////////////////////////////////////////// 


//////// setTimeout(Func,0) Step 4 //////////// 
// The performance numbers from Step 3 seem pretty grim, but GUI responsiveness is often worth it. 
// Here is a short library that embodies these concepts and gives a descent interface. 

    var WilkesAsyncBurn = function() 
    { 
    var Now = function() {return (new Date());}; 
    var CreateFutureDate = function(milliseconds) 
    { 
     var t = Now(); 
     t.setTime(t.getTime() + milliseconds); 
     return t; 
    }; 
    var For = function(start, end, eachCallback, finalCallback, msBurnTime) 
    { 
     var i = start; 
     var Each = function() 
     { 
     if(i==-1) {return;} //always does one last each with nothing to do 
     setTimeout(Each,0); 
     var burnTimeout = CreateFutureDate(msBurnTime); 
     while(Now() < burnTimeout) 
     { 
      if(i>=end) {i=-1; finalCallback(); return;} 
      eachCallback(i); 
      i++; 
     } 
     }; 
     Each(); 
    }; 
    var ForEach = function(array, eachCallback, finalCallback, msBurnTime) 
    { 
     var i = 0; 
     var len = array.length; 
     var Each = function() 
     { 
     if(i==-1) {return;} 
     setTimeout(Each,0); 
     var burnTimeout = CreateFutureDate(msBurnTime); 
     while(Now() < burnTimeout) 
     { 
      if(i>=len) {i=-1; finalCallback(array); return;} 
      eachCallback(i, array[i]); 
      i++; 
     } 
     }; 
     Each(); 
    }; 

    var pub = {}; 
    pub.For = For;   //eachCallback(index); finalCallback(); 
    pub.ForEach = ForEach; //eachCallback(index,value); finalCallback(array); 
    WilkesAsyncBurn = pub; 
    }; 

/////////////////////////////////////////////// 


//////// setTimeout(Func,0) Step 5 //////////// 
// Here is an examples of how to use the library from Step 4. 

    WilkesAsyncBurn(); // Init the library 
    console.log("start"); 
    var startTime = new Date(); 
    var workCounter=0; 
    var FuncEach = function() 
    { 
    if(workCounter%1000==0) 
    { 
     var s = "<div></div>"; 
     var div = jQuery("*[class~=r1]"); 
     div.append(s); 
    } 
    workCounter++; 
    }; 
    var FuncFinal = function() 
    { 
    var ms = (new Date()).getTime() - startTime.getTime(); 
    console.log("done: workCounter=" + workCounter + " time=" + ms + "ms"); 
    }; 
    WilkesAsyncBurn.For(0,2000000,FuncEach,FuncFinal,50); 

// prints: "done: workCounter=20000000 time=149303ms" 
// Also appends a few thousand divs to the html page, about 20 at a time. 
// The browser is responsive the entire time, mission accomplished 

// LESSON LEARNED: If your code pieces are super tiny, like incrementing a number, or walking through 
// an array summing the numbers, then just putting it in an "each" function is going to kill you. 
// You can still use the concept here, but your "each" function should also have a for loop in it 
// where you burn a few hundred items manually. 
/////////////////////////////////////////////// 

Answer 2

Chỉ cần chia nhỏ công việc thành từng phần và xử lý từng đoạn một. Mã số here là một địa điểm bắt đầu tốt, nhưng sử dụng setImmediate hoặc setTimeout để gọi vòng lặp tiếp theo của vòng lặp.

Cách thích hợp cách để giải quyết vấn đề của bạn là sử dụng Web Workers, thực thi mã trên một chuỗi riêng biệt.

Answer 3

Bạn chỉ có thể quấn mỗi lần lặp của vòng lặp trong một setTimeout như vậy (xem jsfiddle):

$(document).ready(function(){ 
    var COUNT = 100000; 
    function process(item){ 
     var r = 0; 
     for(var i=0; i < item; i++){ 
      r += i; 
     } 
     return r; 
    } 
    for(var i=0; i < COUNT; i++){ 
     (function(item){ 
      setTimeout(function(){ 
       $('#log').html("Processing #" + item + " (" + process(item) + ")"); 
      }); 
     })(i); 
    } 
});