Add diffToolBuilder for building complex diff command messages (pointfreeco#840)

* Add SnapshotTesting.diffToolBuilder for building complex diff command messages

It takes the existing and the failure snapshot as arguments, and thus complex commands which are not just prefixed with the existing diffTool can be created.

* Rename diffToolBuilder to diffToolCommand, replace diffTool with custom getter/setter

Author: js

README.md

@@ -210,9 +210,9 @@ targets: [
   - **`Codable` support**. Snapshot encodable data structures into their JSON and property list
     representations.
   - **Custom diff tool integration**. Configure failure messages to print diff commands for
-    [Kaleidoscope](https://kaleidoscope.app) (or your diff tool of choice).
+    [Kaleidoscope](https://kaleidoscope.app) or your diff tool of choice.
     ``` swift
-    SnapshotTesting.diffTool = "ksdiff"
+    SnapshotTesting.diffToolCommand = { "ksdiff \($0) \($1)" }
     ```
 
 [available-strategies]: https://swiftpackageindex.com/pointfreeco/swift-snapshot-testing/main/documentation/snapshottesting/snapshotting

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -1,12 +1,28 @@
 import XCTest
 
 /// Enhances failure messages with a command line diff tool expression that can be copied and pasted
-/// into a terminal.
+/// into a terminal. For more complex difftool needs, see `diffToolCommand`.
 ///
 /// ```swift
 /// diffTool = "ksdiff"
 /// ```
-public var diffTool: String? = nil
+public var diffTool: String? {
+    get { diffToolCommand?("", "").trimmingCharacters(in: .whitespaces) }
+    set {
+        diffToolCommand = newValue.map { value in
+            { [value, $0, $1].joined(separator: " ") }
+        }
+    }
+}
+
+/// Enhances failure messages with a diff tool expression created by the closure, such as an clickable
+/// URL or a complex command. The closure will receive the existing screenshot path and the failed
+/// screenshot path as arguments.
+///
+/// ```swift
+/// diffToolCommand = { "compare \"\($0)\" \"\($1)\" png: | open -f -a Preview.app" }
+/// ```
+public var diffToolCommand: ((String, String) -> String)?
 
 /// Whether or not to record all new references.
 public var isRecording: Bool = {
@@ -333,8 +349,7 @@ public func verifySnapshot<Value, Format>(
     }
 
     let diffMessage =
-      diffTool
-      .map { "\($0) \"\(snapshotFileUrl.path)\" \"\(failedSnapshotFileUrl.path)\"" }
+      diffToolCommand?(snapshotFileUrl.path, failedSnapshotFileUrl.path)
         ?? """
         @\(minus)
         "\(snapshotFileUrl.absoluteString)"