fix(apple): contain measurement construction failures

CHANGELOG.md

@@ -16,6 +16,9 @@ The release run heads these entries with the version and opens a fresh
 
 ## Unreleased
 
+- Apple `Measure(string:)` is now failable. An invalid length returns `nil`
+  instead of letting a C++ exception escape into Swift.
+
 - Python, Java and Apple file/image/font reads report stream failures instead
   of returning partial data after a ZIP checksum or other read error.
 

apple/include/OdrCoreObjC/ODRStyle.h

@@ -96,9 +96,9 @@ NS_SWIFT_NAME(Measure)
 /// Magnitude and unit as odrcore writes them, e.g. `12pt`.
 @property(nonatomic, readonly, copy) NSString *stringValue;
 
-/// A css length as odrcore writes one, e.g. `3mm`. A magnitude with no unit
-/// reads as unitless.
-- (instancetype)initWithString:(NSString *)string;
+/// Parses a length such as `3mm` or a unitless magnitude; returns nil on
+/// failure.
+- (nullable instancetype)initWithString:(NSString *)string;
 - (instancetype)initWithMagnitude:(double)magnitude unit:(NSString *)unit;
 
 - (instancetype)init NS_UNAVAILABLE;

apple/src/ODRStyle.mm

@@ -121,12 +121,16 @@ - (NSString *)stringValue {
   return guarded_value([&] { return to_nsstring(_handle->to_string()); }, @"");
 }
 
-- (instancetype)initWithString:(NSString *)string {
+- (nullable instancetype)initWithString:(NSString *)string {
   if ((self = [super init]) == nil) {
     return nil;
   }
-  _handle = odr::Measure(to_string(string));
-  return self;
+  return guarded_value(
+      [&]() -> ODRMeasure * {
+        _handle = odr::Measure(to_string(string));
+        return self;
+      },
+      nil);
 }
 
 - (instancetype)initWithMagnitude:(double)magnitude unit:(NSString *)unit {

apple/tests/OdrCoreTests.swift

@@ -778,6 +778,19 @@ final class PdfAnnotationTests: XCTestCase {
   }
 }
 
+final class MeasureTests: XCTestCase {
+  func testConstructionReturnsNilOnFailure() throws {
+    let parsed = try XCTUnwrap(Measure(string: "2.5cm"))
+    XCTAssertEqual(parsed.magnitude, 2.5)
+    XCTAssertEqual(parsed.unit, "cm")
+    XCTAssertEqual(Measure(magnitude: 2.5, unit: "cm").stringValue, "2.5cm")
+    XCTAssertEqual(Measure(string: "12")?.unit, "")
+    for invalid in ["", "invalid", "1e999px"] {
+      XCTAssertNil(Measure(string: invalid), invalid)
+    }
+  }
+}
+
 final class TableAddressTests: XCTestCase {
   func testRoundTrips() throws {
     XCTAssertEqual(TableAddress.columnNumber(from: "C"), 2)