implement transition-group

src/platforms/web/runtime/components/transition-group.js

@@ -1 +1,49 @@
-export default {}
+import { warn } from 'core/util/index'
+
+// Because the vdom's children update algorithm is "unstable" - i.e.
+// it doesn't guarantee the relative positioning of removed elements,
+// we force transition-group to update its children into two passes:
+// in the first pass, we remove all nodes that need to be removed,
+// triggering their leaving transition; in the second pass, we insert/move
+// into the final disired state. This way in the second pass removed
+// nodes will remain where they should be.
+
+export default {
+  props: ['tag'],
+
+  beforeUpdate () {
+    this.__patch__(this._vnode, this.kept)
+    this._vnode = this.kept
+  },
+
+  render (h) {
+    const prevMap = this.prevChildrenMap
+    const children = this.$slots.default || []
+    const map = this.prevChildrenMap = {}
+    const kept = []
+
+    for (let i = 0; i < children.length; i++) {
+      const c = children[i]
+      if (c.tag) {
+        if (c.key == null) {
+          process.env.NODE_ENV !== 'production' && warn(
+            'transition-group children must be keyed.'
+          )
+          c.key = i
+        }
+        map[c.key] = c
+        ;(c.data || (c.data = {})).transition = { name: 'fade' }
+        if (prevMap && prevMap[c.key]) {
+          kept.push(c)
+        }
+      }
+    }
+
+    const tag = this.tag || this.$vnode.data.tag || 'span'
+    if (this._isMounted) {
+      this.kept = h(tag, null, kept)
+    }
+
+    return h(tag, null, children)
+  }
+}