如何制作具有背景位置和百分比值的可拖动背景图像？

Question

我正在尝试使用background-position 和百分比值制作一个简单的可拖动背景。 到目前为止，我设法让拖动工作，但我似乎无法找到正确的计算方法让图像以相同的速度跟随光标（如果有意义的话）。

这是一个简单的例子（仅使用x 轴）：

const container = document.querySelector('div');
const containerSize = container.getBoundingClientRect();

let imagePosition = { x: 50, y: 50 };
let cursorPosBefore = { x: 0, y: 0 };
let imagePosBefore = null;
let imagePosAfter = imagePosition;

container.addEventListener('mousedown', function(event) {
  cursorPosBefore = { x: event.clientX, y: event.clientY };
  imagePosBefore = imagePosAfter; // Get current image position
});

container.addEventListener('mousemove', function(event) {
  if (event.buttons === 0) return;

  let newXPos = imagePosBefore.x + ((cursorPosBefore.x - event.clientX) * 100 / containerSize.width);
  newXPos = (newXPos < 0) ? 0 : (newXPos > 100) ? 100 : newXPos; // Stop at the end of the image
  
  imagePosAfter = { x: newXPos, y: imagePosition.y }; // Save position
  container.style.backgroundPosition = `${newXPos}% ${imagePosition.y}%`;
});

div {
  width: 400px;
  height: 400px;
  background-position: 50% 50%;
  background-size: cover;
  background-repeat: no-repeat;
  background-image: url('https://i.stack.imgur.com/5yqL8.png');
  cursor: move;
  border: 2px solid transparent;
}

div:active {
  border-color: red;
}

<div></div>

如果我单击背景上的一个白色十字并移动鼠标，则十字应始终保持在光标下方，直到我到达图像末端或容器末端。

这可能只是一个数学问题，但我有点困惑，因为百分比如何与background-position 一起工作。有什么想法吗？

Answer 1

我不知道公式到底是怎样的，但你也必须包括 css-background 的大小。

如果背景大小是容器 div 大小的 100%，您的公式将有效，但事实并非如此。您需要知道缩放级别（可以根据与 div 大小相关的大小来计算）。

以下是设置缩放级别的公式：

container.addEventListener('mousemove', function(event) {
    event.preventDefault();

    const zoomAffector = (currentZoomLevel - 100) / 100; // 100% zoom = image as big as container
    if (zoomAffector <= 0) return; // Cant drag a image that is zoomed out

    let newXPos = imagePosBefore.x + ((imagePosBefore.x - event.pageX) / zoomAffector * 100;);
    newXPos = (newXPos < 0) ? 0 : (newXPos > 100) ? 100 : newXPos;
  
    imagePosAfter = { x: newXPos, y: imagePosition.y };
    container.style.backgroundPosition = `${newXPos}% ${imagePosition.y}%`;
});

要获取 css-background 图像的大小，可以查看这个问题：Get the Size of a CSS Background Image Using JavaScript?

这是我对您的问题的尝试，使用链接问题中的一个答案来获取图像宽度（对于 y 轴也这样做，以完成）：

const container = document.querySelector('div');
const containerSize = container.getBoundingClientRect();

let imagePosition = { x: 50, y: 50 };
let cursorPosBefore = { x: 0, y: 0 };
let imagePosBefore = null;
let imagePosAfter = imagePosition;


var actualImage = new Image();
actualImage.src = $('#img').css('background-image').replace(/"/g,"").replace(/url\(|\)$/ig, "");
actualImage.onload = function() {
    const zoomX = this.width / containerSize.width - 1;
    const zoomY = this.height / containerSize.height - 1;
    
    container.addEventListener('mousedown', function(event) {
      cursorPosBefore = { x: event.clientX, y: event.clientY };
      imagePosBefore = imagePosAfter; // Get current image position
    });

    container.addEventListener('mousemove', function(event) {
        event.preventDefault();

        if (event.buttons === 0) return;

        let newXPos = imagePosBefore.x + ((cursorPosBefore.x - event.clientX) / containerSize.width * 100 / zoomX);
        newXPos = (newXPos < 0) ? 0 : (newXPos > 100) ? 100 : newXPos;
        let newYPos = imagePosBefore.y + ((cursorPosBefore.y - event.clientY) / containerSize.height * 100 / zoomY);
        newYPos = (newYPos < 0) ? 0 : (newYPos > 100) ? 100 : newYPos;

        imagePosAfter = { x: newXPos, y: newYPos };
        container.style.backgroundPosition = `${newXPos}% ${newYPos}%`;
    });
}

#img {
  width: 400px;
  height: 200px;
  background-position: 50% 50%;
  background-image: url('https://i.stack.imgur.com/5yqL8.png');
  cursor: move;
  border: 2px solid transparent;
}

#img:active {
  border-color: red;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="img"></div>

或者更干净一点：

const container = document.querySelector('div');
const containerSize = container.getBoundingClientRect();

let imagePosition = { x: 50, y: 50 };
let cursorPosBefore = { x: 0, y: 0 };
let imagePosBefore = null;
let imagePosAfter = imagePosition;

// Helpers
const minMax = (pos) => (pos < 0) ? 0 : (pos > 100) ? 100 : pos;
const setNewCenter = (x, y) => {
  imagePosAfter = { x: x, y: y }; 
  container.style.backgroundPosition = `${x}% ${y}%`;
};

const getImageZoom = () => {
  return new Promise((resolve, reject) => {
    let actualImage = new Image();
    actualImage.src = $('#img').css('background-image').replace(/"/g,"").replace(/url\(|\)$/ig, "");
    actualImage.onload = function() {
      resolve({
        x: zoomX = this.width / containerSize.width - 1,
        y: zoomY = this.height / containerSize.height - 1
      });
    }
  });
}
    
const addEventListeners = (zoomLevels) => {container.addEventListener('mousedown', function(event) {
      cursorPosBefore = { x: event.clientX, y: event.clientY };
      imagePosBefore = imagePosAfter; // Get current image position
    });

    container.addEventListener('mousemove', function(event) {
        event.preventDefault();

        if (event.buttons === 0) return;

        let newXPos = imagePosBefore.x + ((cursorPosBefore.x - event.clientX) / containerSize.width * 100 / zoomLevels.x);
        let newYPos = imagePosBefore.y + ((cursorPosBefore.y - event.clientY) / containerSize.height * 100 / zoomLevels.y);

        setNewCenter(minMax(newXPos), minMax(newYPos));
    });
};

getImageZoom().then(zoom => addEventListeners(zoom));
    

#img {
  width: 400px;
  height: 200px;
  background-position: 50% 50%;
  background-image: url('https://i.stack.imgur.com/5yqL8.png');
  cursor: move;
  border: 2px solid transparent;
}

#img:active {
  border-color: red;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="img"></div>

或回答您的后续问题：

const container = document.querySelector("div");
const containerSize = container.getBoundingClientRect();

let imagePosition = { x: 50, y: 50 };
let cursorPosBefore = { x: 0, y: 0 };
let imagePosBefore = null;
let imagePosAfter = imagePosition;

// Helpers
const minMax = (pos) => (pos < 0 ? 0 : pos > 100 ? 100 : pos);
const setNewCenter = (x, y) => {
  imagePosAfter = { x: x, y: y };
  container.style.backgroundPosition = `${x}% ${y}%`;
};

const getImageZoom = () => {
  return new Promise((resolve, reject) => {
    let actualImage = new Image();

    actualImage.src = $("#img")
      .css("background-image")
      .replace(/"/g, "")
      .replace(/url\(|\)$/gi, "");
    actualImage.onload = function () {
      const imgW = this.width,
        imgH = this.height,
        conW = containerSize.width,
        conH = containerSize.height,
        ratioW = imgW / conW,
        ratioH = imgH / conH;

      // Stretched to Height
      if (ratioH < ratioW) {
        resolve({
          x: imgW / (conW * ratioH) - 1,
          y: imgH / (conH * ratioH) - 1,
        });
      } else {
        // Stretched to Width
        resolve({
          x: imgW / (conW * ratioW) - 1,
          y: imgH / (conH * ratioW) - 1,
        });
      }
    };
  });
};

const addEventListeners = (zoomLevels) => {
  container.addEventListener("mousedown", function (event) {
    cursorPosBefore = { x: event.clientX, y: event.clientY };
    imagePosBefore = imagePosAfter; // Get current image position
  });

  container.addEventListener("mousemove", function (event) {
    event.preventDefault();

    if (event.buttons === 0) return;

    let newXPos =
      imagePosBefore.x +
      (((cursorPosBefore.x - event.clientX) / containerSize.width) * 100) /
        zoomLevels.x;
    let newYPos =
      imagePosBefore.y +
      (((cursorPosBefore.y - event.clientY) / containerSize.height) * 100) /
        zoomLevels.y;

    setNewCenter(minMax(newXPos), minMax(newYPos));
  });
};

getImageZoom().then((zoom) => addEventListeners(zoom));

#img {
  width: 400px;
  height: 200px;
  background-size: cover;
  background-position: 50% 50%;
  background-image: url('https://i.stack.imgur.com/5yqL8.png');
  cursor: move;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="img"></div>