new: FIFO list impl

lib/data/model/server/time_seq.dart

@@ -1,11 +1,56 @@
-abstract class TimeSeq<T extends TimeSeqIface> {
-  List<T> pre;
-  List<T> now;
+import 'dart:collection';
+
+/// A FIFO queue with fixed capacity.
+abstract class TimeSeq<T extends List<TimeSeqIface>> extends ListBase<T> {
+  final int capacity;
+  late final List<T> _list;
+
+  /// Due to the design, at least two elements are required, otherwise [pre] /
+  /// [now] will throw.
+  TimeSeq(
+    T init1,
+    T init2, {
+    this.capacity = 30,
+  }) : _list = [init1, init2];
+
+  @override
+  void add(element) {
+    if (length == capacity) {
+      _list.removeAt(0);
+    }
+    _list.add(element);
+  }
+
+  @override
+  int get length => _list.length;
+
+  @override
+  set length(int newLength) {
+    throw UnimplementedError();
+  }
+
+  @override
+  T operator [](int index) {
+    return _list[index];
+  }
+
+  @override
+  void operator []=(int index, value) {
+    _list[index] = value;
+  }
+
+  T get pre {
+    return _list[length - 2];
+  }
+
+  T get now {
+    return _list[length - 1];
+  }
+
   void onUpdate();
 
-  void update(List<T> new_) {
-    pre = now;
-    now = new_;
+  void update(T new_) {
+    add(new_);
 
     if (pre.length != now.length) {
       pre.removeWhere((e) => now.any((el) => e.same(el)));
@@ -14,8 +59,6 @@ abstract class TimeSeq<T extends TimeSeqIface> {
 
     onUpdate();
   }
-
-  TimeSeq(this.pre, this.now);
 }
 
 abstract class TimeSeqIface<T> {