如何为具有多个参数的画布函数创建循环？

Question

<canvas id="chart" width="700" height="550"></canvas>

<script>
  const canvas = document.getElementById('chart');
  const context = canvas.getContext('2d');

  /* Draw a line from (fromX, fromY) to (toX, toY) */
  function drawLine(fromX, fromY, toX, toY) {
    context.beginPath();
    context.moveTo(toX, toY);
    context.lineTo(fromX, fromY);
    context.stroke();
  }

  /* Draw a text (string) on (x, y) */
  function drawText(text, x, y) {
    context.fillStyle = 'black';
    context.fillText(text, x, y);
  }

  /* Draw a text and with a line to its right */
  function drawLineWithText(text, fromX, fromY, toX, toY) {
    drawText(text, fromX - 50, fromY + 3);
    drawLine(fromX, fromY, toX, toY);
  }

  for (var fromY = 50; fromY < 500; fromY += 50, toY = 50 toY < 500; toY += 50, fromX = 70, toX =700) {
    drawLineWithText(text, fromX, fromY, toX, toY);
  }
</script>

**强文本** 我实际上不知道如何为此做一个 for 语句，我试图玩弄，但只有在我手动完成时才设法使它工作，编写自己的参数示例； drawLineWithText(1000, 20, 30, 100, 30)

Answer 1

在这种情况下，我没有看到任何需要在 for 循环中使用多个变量的特殊情况...
如果我们分解循环中的内容，我们最终会得到：

/*
    fromY = 50; fromY < 500; fromY += 50, 
      toY = 50;   toY < 500;   toY += 50, 

    fromX = 70, 
      toX = 700
*/

fromY 和 toY 具有相同的模式，fromX 和 toX 只是硬编码值...

所以为了简化你的代码，我会这样做：

const canvas = document.getElementById('chart');
const context = canvas.getContext('2d');

function drawLine(fromX, fromY, toX, toY) {
  context.beginPath();
  context.moveTo(toX, toY);
  context.lineTo(fromX, fromY);
  context.stroke();
}

for (var i = 50; i < 500; i += 50) {
  drawLine(70, i, 700, i);
}

<canvas id="chart" width="700" height="550"></canvas>

---

您可以在一个循环中包含多个变量，这绝对是可能的，但您的示例并不是它的最佳用途，这里是多个变量的一个很好的示例。

const canvas = document.getElementById('chart');
const context = canvas.getContext('2d');

function drawLine(fromX, fromY, toX, toY) {
  context.beginPath();
  context.moveTo(toX, toY);
  context.lineTo(fromX, fromY);
  context.stroke();
}

var x, y
for (x=50, y=10;  x<200, y<100;  x*=1.3, y+=9) {
  drawLine(10, y/3, x, y);
}

<canvas id="chart" width="500" height="100"></canvas>

这里有一些关于 for 循环结构的好读物：
https://gomakethings.com/the-anatomy-of-a-for-loop-in-vanilla-js-and-when-you-would-want-to-use-it-instead-of-array.foreach/

...它分为三个部分，每个部分用分号 (;) 分隔：

• 在第一个分号之前，您可以声明或分配变量。
• 在第一个和第二个分号之间，您定义了一个在每个循环后检查的条件。只要此条件为真，循环就会继续运行。一旦条件为假，循环就会停止。
• 在第二个分号之后，您可以指定在每个循环之后运行的语句。

Answer 2

尝试将您的自定义参数收集到一个可以首先迭代的数组中。可能是这样的：

// establish your starts, ends, and steps
const text = 'Some text';

const fromXStart = 70;
const fromXEnd = 700;
const fromXStep = 50;

const fromYStart = 50;
const fromYEnd = 500;
const fromYStep = 50;

const toXStart = 700;
const toXEnd = 1000;
const toXStep = 50;

const toYStart = 50;
const toYEnd = 500;
const toYStep = 50;

// create an object to keep track of where we are at each iteration of the loop
const currentArgs = {
  fromX: fromXStart,
  fromY: fromYStart,
  toX: toXStart,
  toY: toYStart
}

// create a single array of args to loop over with our intial values
const args = [
  [text, currentArgs.fromX, currentArgs.fromY, currentArgs.toX, currentArgs.toY]
]

// create a check function to see if we're done looping (if all ends have been met)
const isDone = () => {
  const fromXIsDone = currentArgs.fromX >= fromXEnd;
  const fromYIsDone = currentArgs.fromY >= fromYEnd;
  const toXIsDone = currentArgs.toX >= toXEnd;
  const toYIsDone = currentArgs.toY >= toYEnd;
  return fromXIsDone && fromYIsDone && toXIsDone && toYIsDone;
}

// loop until done
while (!isDone()) {
  // use Math.min to ensure we don't go past the max
  currentArgs.fromX = Math.min(fromXEnd, currentArgs.fromX + fromXStep);
  currentArgs.fromY = Math.min(fromYEnd, currentArgs.fromY + fromYStep);
  currentArgs.toX = Math.min(toXEnd, currentArgs.toX + toXStep);
  currentArgs.toY = Math.min(toYEnd, currentArgs.toY + toYStep);
  args.push([
    text, currentArgs.fromX, currentArgs.fromY, currentArgs.toX, currentArgs.toY
  ])
}

// now loop over our args array and use the spread syntax (...) to spread the args as args
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
for (let i = 0; i < args.length; i++) {
  drawLineWithText(...args[i]);
}