I am trying to create a mobile application in which the user can draw several controls (a rectangle) on the canvas.
It works great if the canvas fits on the screen, but the controls do not draw well when I increase the height and width of the canvas outside the screen (scrolling appears when the canvas is larger than the size of the mobile screen). How to fix it in scroll mode?
Use the Chrome Inspect mobile view (iPhone 6/7/8) to verify this code. Use drag and drop to draw controls
<html>
<body>
<canvas id="canvas_transparent" height="3000" width="1000" style="border: 1px solid black;"></canvas>
</body>
</html>
<script type="text/javascript">
var canvas = document.getElementById("canvas_transparent");
var ctx = canvas.getContext("2d");
var endX;
var startX;
var endY;
var startY;
var positions = [];
canvas.addEventListener("touchstart", function(e) {
mousePos = getTouchPos(canvas, e);
startX = mousePos.x;
startY = mousePos.y;
var touch = e.touches[0];
var mouseEvent = new MouseEvent("mousedown", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchend", function(e) {
positions.push({
s_x: startX,
s_y: startY,
e_x: endX,
e_y: endY
});
drawAllControls();
console.log(positions);
var mouseEvent = new MouseEvent("mouseup", {});
canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchmove", function(e) {
var touch = e.touches[0];
endX = touch.clientX;
endY = touch.clientY;
drawSquare()
var mouseEvent = new MouseEvent("mousemove", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
function getTouchPos(canvasDom, touchEvent) {
var rect = canvasDom.getBoundingClientRect();
return {
x: touchEvent.touches[0].clientX - rect.left,
y: touchEvent.touches[0].clientY - rect.top
};
}
function drawSquare() {
var w = endX - startX;
var h = endY - startY;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.beginPath();
ctx.rect(startX + offsetX, startY + offsetY, width, height);
ctx.fillStyle = "#222222";
ctx.fill();
ctx.lineWidth = 2;
ctx.strokeStyle = 'black';
ctx.stroke();
}
function drawAllControls() {
for (var i = 0; i < positions.length; i++) {
var w = positions[i].e_x - positions[i].s_x;
var h = positions[i].e_y - positions[i].s_y;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);
ctx.beginPath();
ctx.rect(positions[i].s_x + offsetX, positions[i].s_y + offsetY, width, height);
ctx.fillStyle = "#222222";
ctx.fill();
ctx.lineWidth = 2;
ctx.strokeStyle = 'black';
ctx.stroke();
}
}
</script>
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