在这个问题的答案中，有一个画布中物体移动的例子。
如何在中心显示画布中的文本，以便画布的内容可以像以前一样移动，但文本保持静止？
我放置ctx.font = "48px serif"; ctx.fillText(text, canvas.width / 2, 100);在正方形的输出之后，但由于某种原因，文本附加到它们并移动。
const ctx = canvas.getContext("2d");
canvas.width = 500;
canvas.height = 500;
const rand = (m = 255, M = m + (m = 0)) => (Math.random() * (M - m) + m) | 0;
const objects = [];
for (let i = 0; i < 100; i++) {
objects.push({x: rand(canvas.width), y: rand(canvas.height),w: rand(40),h: rand(40), col: `rgb(${rand()},${rand()},${rand()})`});
}
requestAnimationFrame(drawCanvas);
const view = (() => {
const matrix = [1, 0, 0, 1, 0, 0]; // current view transform
var m = matrix; // alias
var scale = 1; // current scale
var ctx; // reference to the 2D context
const pos = { x: 0, y: 0 }; // current position of origin
var dirty = true;
const API = {
set context(_ctx) { ctx = _ctx; dirty = true },
apply() {
if (dirty) { this.update() }
ctx.setTransform(m[0], m[1], m[2], m[3], m[4], m[5])
},
get scale() { return scale },
get position() { return pos },
isDirty() { return dirty },
update() {
dirty = false;
m[3] = m[0] = scale;
m[2] = m[1] = 0;
m[4] = pos.x;
m[5] = pos.y;
},
pan(amount) {
if (dirty) { this.update() }
pos.x += amount.x;
pos.y += amount.y;
dirty = true;
},
scaleAt(at, amount) { // at in screen coords
if (dirty) { this.update() }
scale *= amount;
pos.x = at.x - (at.x - pos.x) * amount;
pos.y = at.y - (at.y - pos.y) * amount;
dirty = true;
},
};
return API;
})();
view.context = ctx;
function drawCanvas() {
if (view.isDirty()) {
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.clearRect(0, 0, canvas.width, canvas.height);
view.apply(); // set the 2D context transform to the view
for (i = 0; i < objects.length; i++) {
var obj = objects[i];
ctx.fillStyle = obj.col;
ctx.fillRect(obj.x, obj.y, obj.h, obj.h);
}
ctx.font = "48px serif";
ctx.fillText("Hello World!", canvas.width / 2, 50);
}
requestAnimationFrame(drawCanvas);
}
canvas.addEventListener("mousemove", mouseEvent, {passive: true});
canvas.addEventListener("mousedown", mouseEvent, {passive: true});
canvas.addEventListener("mouseup", mouseEvent, {passive: true});
canvas.addEventListener("mouseout", mouseEvent, {passive: true});
canvas.addEventListener("wheel", mouseWheelEvent, {passive: false});
const mouse = {x: 0, y: 0, oldX: 0, oldY: 0, button: false};
function mouseEvent(event) {
if (event.type === "mousedown") { mouse.button = true }
if (event.type === "mouseup" || event.type === "mouseout") { mouse.button = false }
mouse.oldX = mouse.x;
mouse.oldY = mouse.y;
mouse.x = event.offsetX;
mouse.y = event.offsetY
if(mouse.button) { // pan
view.pan({x: mouse.x - mouse.oldX, y: mouse.y - mouse.oldY});
}
}
function mouseWheelEvent(event) {
var x = event.offsetX;
var y = event.offsetY;
if (event.deltaY < 0) { view.scaleAt({x, y}, 1.1) }
else { view.scaleAt({x, y}, 1 / 1.1) }
event.preventDefault();
}body {
background: gainsboro;
margin: 0;
}
canvas {
background: white;
box-shadow: 1px 1px 1px rgba(0, 0, 0, .2);
}<canvas id="canvas"></canvas>画布中需要文本，因为我将画布的所有内容保存到图像中。
