optimize Scroll scroll behavior

examples/routers/scroll.vue

@@ -1,5 +1,5 @@
 <template>
-    <Scroll :on-reach-edge="loadData" loading-text="L-o-a-d-i-n-g...">
+    <Scroll :on-reach-top="loadData" loading-text="L-o-a-d-i-n-g...">
         <section v-for="item in list">
             <div class="city">
                 <p>{{ item }}</p>

src/components/scroll/scroll.vue

@@ -31,6 +31,8 @@
         minimumStartDragOffset: 5, // minimum start drag offset
     };
 
+    const noop = () => Promise.resolve();
+
     export default {
         name: 'Scroll',
         mixins: [],
@@ -41,16 +43,13 @@
                 default: 300
             },
             onReachTop: {
-                type: Function,
+                type: Function
-                default: () => Promise.resolve()
             },
             onReachBottom: {
-                type: Function,
+                type: Function
-                default: () => Promise.resolve()
             },
             onReachEdge: {
-                type: Function,
+                type: Function
-                default: () => Promise.resolve()
             },
             loadingText: {
                 type: String,
@@ -134,8 +133,8 @@
                     }
                 }
 
-                const callbacks = [this.waitOneSecond(), this.onReachEdge(dir)];
+                const callbacks = [this.waitOneSecond(), this.onReachEdge ? this.onReachEdge(dir) : noop()];
-                callbacks.push(dir > 0 ? this.onReachTop() : this.onReachBottom());
+                callbacks.push(dir > 0 ? this.onReachTop ? this.onReachTop() : noop() : this.onReachBottom ? this.onReachBottom() : noop());
 
                 let tooSlow = setTimeout(() => {
                     this.reset();
@@ -183,6 +182,15 @@
             stretchEdge(direction) {
                 clearTimeout(this.rubberRollBackTimeout);
 
+                // check if set these props
+                if (!this.onReachEdge) {
+                    if (direction > 0) {
+                        if (!this.onReachTop) return;
+                    } else {
+                        if (!this.onReachBottom) return;
+                    }
+                }
+
                 // if the scroll is not strong enough, lets reset it
                 this.rubberRollBackTimeout = setTimeout(() => {
                     if (!this.isLoading) this.reset();