Add some test case scene of motion streak and particle system. (cocos…

…#331)

assets/cases/ui/24.motion-streak.meta

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+{
+  "ver": "1.1.0",
+  "importer": "directory",
+  "imported": true,
+  "uuid": "fae49215-671d-4595-9669-93d73514da30",
+  "files": [],
+  "subMetas": {},
+  "userData": {
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+  }
+}

assets/cases/ui/24.motion-streak/motion-streak-ctrl.ts

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+// Learn TypeScript:
+//  - https://docs.cocos.com/creator/manual/en/scripting/typescript.html
+// Learn Attribute:
+//  - https://docs.cocos.com/creator/manual/en/scripting/reference/attributes.html
+// Learn life-cycle callbacks:
+//  - https://docs.cocos.com/creator/manual/en/scripting/life-cycle-callbacks.html
+
+import { _decorator, Component, Node, MotionStreak, Texture2D, Animation } from 'cc';
+const { ccclass, type } = _decorator;
+
+@ccclass('MotionStreakCtrl')
+export class MotionStreakCtrl extends Component {
+    @type(MotionStreak)
+    motionStreak: MotionStreak | null = null;
+
+    @type(Texture2D)
+    newTexture: Texture2D | null = null;
+
+    @type(Animation)
+    animationCom: Animation | null = null;
+
+    private _changed: boolean = true;
+    private _oldTexture: Texture2D | null = null;
+
+    onLoad () {
+        this._changed = true;
+        this._oldTexture = this.motionStreak!.texture;
+    }
+
+    onClick () {
+        if (this._changed) {
+            this.setMotionStreak(2, 3, 20, this.newTexture!);
+        }
+        else {
+            this.setMotionStreak(0.5, 1, 30, this._oldTexture!);
+        }
+        this._changed = !this._changed;
+    }
+
+    setMotionStreak (fadeTime: number, minSeg: number, stroke: number, texture: Texture2D) {
+        this.motionStreak!.fadeTime = fadeTime;
+        this.motionStreak!.minSeg = minSeg;
+        this.motionStreak!.stroke = stroke;
+        this.motionStreak!.texture = texture;
+    }
+
+    lateUpdate () {
+        if (!this.animationCom!.getState('move_around').isPlaying) {
+            this.animationCom!.play();
+        }
+    }
+
+    onDisable () {
+        this.animationCom!.stop();
+    }
+}

assets/cases/ui/24.motion-streak/motion-streak-ctrl.ts.meta

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+{
+  "ver": "4.0.2",
+  "importer": "typescript",
+  "imported": true,
+  "uuid": "22ac104c-eecb-401b-8181-0e8d15c29eea",
+  "files": [
+    ".js",
+    ".trans"
+  ],
+  "subMetas": {},
+  "userData": {
+    "importAsPlugin": false,
+    "moduleId": "project:///assets/cases/ui/24.motion-streak/motion-streak-ctrl.js",
+    "importerSettings": 3
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+}