Add dropFirst in Accumulation (#486)

Sources/Verge/Store/StoreDriverType+Accumulator.swift

@@ -129,6 +129,9 @@ public struct AccumulationSinkIfChanged<Source, Target: Equatable>: Accumulation
   private var latestValue: Target?
   private var previousValue: Target?
 
+  private var counter: Int = 0
+  private var countToEmit: Int = 0
+
   private var handler: ((consuming Target) -> Void)?
 
   init(
@@ -137,6 +140,11 @@ public struct AccumulationSinkIfChanged<Source, Target: Equatable>: Accumulation
     self.selector = selector
   }
 
+  public consuming func dropFirst(_ k: Int = 1) -> Self {
+    countToEmit = k
+    return self
+  }
+
   /**
    the closure will be released after consumed.
    */
@@ -155,6 +163,7 @@ public struct AccumulationSinkIfChanged<Source, Target: Equatable>: Accumulation
   public consuming func receive(other: consuming AccumulationSinkIfChanged<Source, Target>) -> Self {
 
     self.previousValue = other.latestValue
+    self.counter = other.counter
 
     return self
   }
@@ -166,7 +175,10 @@ public struct AccumulationSinkIfChanged<Source, Target: Equatable>: Accumulation
     }
 
     if latestValue != previousValue {
-      handler(latestValue!)
+      if counter >= countToEmit {
+        handler(latestValue!)
+      }
+      counter += 1
     }
 
     self.handler = nil

Tests/VergeTests/AccumulationTests.swift

@@ -64,6 +64,36 @@ final class AccumulationTests: XCTestCase {
     let _ = sub
   }
 
+  func test_drop() {
+
+    let store = DemoStore()
+
+    let expForCount = expectation(description: "count")
+    expForCount.expectedFulfillmentCount = 1
+
+    let sub = store.accumulate(queue: .mainIsolated()) { [weak self] in
+
+      $0.ifChanged(\.count)
+        .dropFirst(2)
+        .do { value in
+          expForCount.fulfill()
+        }
+
+    }
+
+    store.commit {
+      $0.count += 1
+    }
+
+    store.commit {
+      $0.count += 1
+    }
+
+    wait(for: [expForCount], timeout: 1)
+
+    let _ = sub
+  }
+
   func test_background() {
 
     let store = DemoStore()