Clarify and simplify didChangeContent(in:delta:), add test

Sources/Neon/Highlighter.swift

@@ -59,13 +59,34 @@ extension Highlighter {
 }
 
 extension Highlighter {
+    /// Calculates any newly-visible text that is invalid
+    ///
+    /// You should invoke this method when the visible text
+    /// in your system changes.
     public func visibleContentDidChange() {
         let set = invalidSet.intersection(visibleSet)
 
         invalidate(.set(set))
     }
 
-    public func didChangeContent(in range: NSRange, delta: Int, limit: Int) {
+    /// Update internal state in response to an edit.
+    ///
+    /// This method must be invoked on every text change. The `range`
+    /// parameter must refer to the range of text that **was** changed.
+    /// Consider the example text `"abc"`.
+    ///
+    /// Inserting a "d" at the end:
+    ///
+    ///     range = NSRange(3..<3)
+    ///     delta = 1
+    ///
+    /// Deleting the middle "b":
+    ///
+    ///     range = NSRange(1..<2)
+    ///     delta = -1
+    public func didChangeContent(in range: NSRange, delta: Int) {
+        let limit = textLength - delta
+
         let mutation = RangeMutation(range: range, delta: delta, limit: limit)
 
         self.validSet = mutation.transform(set: validSet)

Sources/Neon/TextContainerSystemInterface.swift

@@ -45,10 +45,14 @@ public struct TextContainerSystemInterface {
 
 extension TextContainerSystemInterface: TextSystemInterface {
     public func clearStyle(in range: NSRange) {
+        assert(range.max <= length, "range is out of bounds, is the text state being updated correctly?")
+
         layoutManager?.setTemporaryAttributes([:], forCharacterRange: range)
     }
 
     public func applyStyle(to token: Token) {
+        assert(token.range.max <= length, "range is out of bounds, is the text state being updated correctly?")
+
         guard let attrs = attributeProvider(token) else { return }
 
         layoutManager?.setTemporaryAttributes(attrs, forCharacterRange: token.range)

Tests/NeonTests/HighlighterTests.swift

@@ -0,0 +1,40 @@
+import XCTest
+import Neon
+
+class MockInterface: TextSystemInterface {
+    var length: Int
+    var visibleRange: NSRange
+
+    init(length: Int = 0, visibleRange: NSRange = .zero) {
+        self.length = length
+        self.visibleRange = visibleRange
+    }
+
+    func clearStyle(in range: NSRange) {
+    }
+
+    func applyStyle(to token: Token) {
+    }
+}
+
+class HighlighterTests: XCTestCase {
+    func testEditAndVisibleRangeChange() throws {
+        let interface = MockInterface()
+
+        var requestedRange: NSRange? = nil
+        let provider: TokenProvider = { range, block in
+            requestedRange = range
+            block(.success(.noChange))
+        }
+
+        let highlighter = Highlighter(textInterface: interface, tokenProvider: provider)
+
+        interface.length = 10
+        highlighter.didChangeContent(in: NSRange(0..<0), delta: 10)
+
+        interface.visibleRange = NSRange(0..<10)
+        highlighter.visibleContentDidChange()
+
+        XCTAssertEqual(requestedRange, NSRange(0..<10))
+    }
+}