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class Pinch {
constructor(element) {
this.event_cache = [];
this.previous_diff = null;
this.onpinch = () => {};
this.onfinish = () => {};
element.onpointerdown = (e) => this.pointer_down_handler(e);
element.onpointermove = (e) => this.pointer_move_handler(e);
element.onpointerup = (e) => this.pointer_up_handler(e);
element.onpointercancel = (e) => this.pointer_up_handler(e);
element.onpointerout = (e) => this.pointer_up_handler(e);
element.onpointerleave = (e) => this.pointer_up_handler(e);
}
pointer_down_handler(event) {
this.event_cache.push(event);
}
pointer_move_handler(event) {
const index = this.event_cache.findIndex(
(cached) => cached.pointerId == event.pointerId,
);
this.event_cache[index] = event;
if (this.event_cache.length === 2) {
event.preventDefault();
const diff = Math.hypot(
this.event_cache[0].clientX - this.event_cache[1].clientX,
this.event_cache[0].clientY - this.event_cache[1].clientY,
);
if (this.previous_diff != null) {
const point = {
x: (this.event_cache[0].clientX + this.event_cache[1].clientX) / 2,
y: (this.event_cache[0].clientY + this.event_cache[1].clientY) / 2,
};
const delta = this.previous_diff - diff;
this.onpinch(delta, point);
}
this.previous_diff = diff;
}
}
pointer_up_handler(event) {
const index = this.event_cache.findIndex(
(cached) => cached.pointerId === event.pointerId,
);
this.event_cache.splice(index, 1);
if (this.event_cache.length < 2) {
this.previous_diff = null;
}
this.onfinish();
}
}
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