refactor: 使用flutter对原先的web项目重构，正在进行中

tmp/js/particles.js

@@ -0,0 +1,44 @@
+// 粒子系统
+import { rand, randInt } from './util.js';
+
+export class Particles {
+  constructor() { this.list = []; }
+  clear() { this.list.length = 0; }
+
+  spawn(x, y, n, opt = {}) {
+    const {
+      colors = ['#fff', '#ff5c7a', '#ffc63a'],
+      speed = 3, life = 40, size = 3, grav = 0.12, spread = Math.PI * 2, dir = 0,
+    } = opt;
+    for (let i = 0; i < n; i++) {
+      const a = dir + (Math.random() - 0.5) * spread;
+      const s = rand(speed * 0.35, speed);
+      this.list.push({
+        x, y, vx: Math.cos(a) * s, vy: Math.sin(a) * s,
+        life: life * rand(0.6, 1.25), max: life,
+        c: colors[randInt(0, colors.length - 1)],
+        s: size * rand(0.6, 1.3), g: grav,
+      });
+    }
+  }
+
+  burst(x, y, opt = {}) { this.spawn(x, y, opt.n || 18, opt); }
+
+  update() {
+    for (let i = this.list.length - 1; i >= 0; i--) {
+      const p = this.list[i];
+      p.x += p.vx; p.y += p.vy; p.vy += p.g;
+      p.vx *= 0.985;
+      if (--p.life <= 0) this.list.splice(i, 1);
+    }
+  }
+
+  draw(ctx) {
+    for (const p of this.list) {
+      ctx.globalAlpha = Math.min(1, p.life / (p.max * 0.5));
+      ctx.fillStyle = p.c;
+      ctx.fillRect(p.x | 0, p.y | 0, p.s | 0, p.s | 0);
+    }
+    ctx.globalAlpha = 1;
+  }
+}