Add property.filter

CHANGELOG.md

@@ -1,6 +1,8 @@
 # master
 *Please add new entries at the top.*
 
+1. New property operator: `filter` (#586, kudos to @iv-mexx)
+
 # 3.1.0-rc.1
 1. Fixed a scenario of downstream interruptions being dropped. (#577, kudos to @andersio)
 

Sources/Property.swift

@@ -113,6 +113,20 @@ extension PropertyProtocol {
 		return lift { $0.map(keyPath) }
 	}
 
+	/// Passes only the values of the property that pass the given predicate
+	/// to a new property.
+	///
+	/// - parameters:
+	///   - initial: A `Property` always needs a `value`. The initial `value` is necessary in case the
+	///              predicate excludes the first (or all) `value`s of this `Property`
+	///   - predicate: A closure that accepts value and returns `Bool` denoting
+	///                whether current `value` of this `Property`  has passed the test.
+	///
+	/// - returns: A property that holds only values from `self` passing the given predicate.
+	public func filter(initial: Value, _ predicate: @escaping (Value) -> Bool) -> Property<Value> {
+		return Property(initial: initial, then: self.producer.filter(predicate))
+	}
+
 	/// Combines the current value and the subsequent values of two `Property`s in
 	/// the manner described by `Signal.combineLatest(with:)`.
 	///

Tests/ReactiveSwiftTests/PropertySpec.swift

@@ -750,6 +750,43 @@ class PropertySpec: QuickSpec {
 				}
 			}
 
+			describe("filter") {
+				it("should only receive values that pass the predicate")  {
+					let property = MutableProperty(1)
+					let filteredProperty = property
+						.filter(initial: 0) { $0 > 0 }
+					expect(filteredProperty.value) == 1
+
+					property.value = 0
+					expect(filteredProperty.value) == 1
+
+					property.value = 2
+					expect(filteredProperty.value) == 2
+
+					property.value = -5
+					expect(filteredProperty.value) == 2
+
+					property.value = 3
+					expect(filteredProperty.value) == 3
+				}
+
+				it("should behave correctly if the filter excludes the initial value")  {
+					let property = MutableProperty(1)
+					let filteredProperty = property
+						.filter(initial: 0) { $0 < 0 }
+					expect(filteredProperty.value) == 0
+
+					property.value = 2
+					expect(filteredProperty.value) == 0
+
+					property.value = -2
+					expect(filteredProperty.value) == -2
+
+					property.value = 0
+					expect(filteredProperty.value) == -2
+				}
+			}
+
 			describe("combineLatest") {
 				var property: MutableProperty<String>!
 				var otherProperty: MutableProperty<String>!