9 files changed, 40 insertions, 0 deletions
diff --git a/platform/darwin/docs/guides/For Style Authors.md.ejs b/platform/darwin/docs/guides/For Style Authors.md.ejs
index 51fc10d874..ead3b81ce6 100644
--- a/platform/darwin/docs/guides/For Style Authors.md.ejs
+++ b/platform/darwin/docs/guides/For Style Authors.md.ejs
@@ -402,6 +402,7 @@ In style specification | Method, function, or predicate type | Format string syn
 `tan`                  | `mgl_tan:` | `mgl_tan(0)`
 `zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoomLevel`
 `heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
+`line-progress`        | `NSExpression.lineProgressVariableExpression` | `$lineProgress`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.
diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md
index 90f010e954..c70d76b581 100644
--- a/platform/darwin/docs/guides/Predicates and Expressions.md
+++ b/platform/darwin/docs/guides/Predicates and Expressions.md
@@ -290,6 +290,17 @@ The following variables are defined by this SDK for use with style layers:
       <code>NSExpression.zoomLevelVariableExpression</code> property.
    </td>
 </tr>
+<tr>
+   <td><code>$lineProgress</code></td>
+   <td>Number</td>
+   <td>
+      A number that indicates the relative distance along a line at a given
+      point along the line. This variable evaluates to 0 at the beginning of the
+      line and 1 at the end of the line. It can only be used with the
+      `MGLLineStyleLayer.lineGradient` property. It corresponds to the
+      <code>NSExpression.lineProgressVariableExpression</code> property.
+   </td>
+</tr>
 </tbody>
 </table>
 
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.h b/platform/darwin/src/NSExpression+MGLAdditions.h
index d13f0227b4..9ee7361448 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.h
+++ b/platform/darwin/src/NSExpression+MGLAdditions.h
@@ -63,6 +63,13 @@ FOUNDATION_EXTERN MGL_EXPORT const MGLExpressionInterpolationMode MGLExpressionI
 
 /**
  `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-line-progress"><code>line-progress</code></a>
+ expression operator in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *lineProgressVariableExpression;
+
+/**
+ `NSExpression` variable that corresponds to the
  <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#eexpressions-geometry-type"><code>geometry-type</code></a>
  expression operator in the Mapbox Style Specification.
  */
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.mm b/platform/darwin/src/NSExpression+MGLAdditions.mm
index cb7f30f65a..9c12c6beac 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.mm
+++ b/platform/darwin/src/NSExpression+MGLAdditions.mm
@@ -542,6 +542,10 @@ const MGLExpressionInterpolationMode MGLExpressionInterpolationModeCubicBezier =
     return [NSExpression expressionForVariable:@"heatmapDensity"];
 }
 
++ (NSExpression *)lineProgressVariableExpression {
+    return [NSExpression expressionForVariable:@"lineProgress"];
+}
+
 + (NSExpression *)geometryTypeVariableExpression {
     return [NSExpression expressionForVariable:@"geometryType"];
 }
@@ -814,6 +818,8 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             return NSExpression.zoomLevelVariableExpression;
         } else if ([op isEqualToString:@"heatmap-density"]) {
             return NSExpression.heatmapDensityVariableExpression;
+        } else if ([op isEqualToString:@"line-progress"]) {
+            return NSExpression.lineProgressVariableExpression;
         } else if ([op isEqualToString:@"geometry-type"]) {
             return NSExpression.geometryTypeVariableExpression;
         } else if ([op isEqualToString:@"id"]) {
@@ -911,6 +917,9 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             if ([self.variable isEqualToString:@"heatmapDensity"]) {
                 return @[@"heatmap-density"];
             }
+            if ([self.variable isEqualToString:@"lineProgress"]) {
+                return @[@"line-progress"];
+            }
             if ([self.variable isEqualToString:@"zoomLevel"]) {
                 return @[@"zoom"];
             }
diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index 1b0630cef3..8856cbf528 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -171,6 +171,14 @@ using namespace std::string_literals;
         XCTAssertEqualObjects([expression expressionValueWithObject:nil context:context], @1);
     }
     {
+        NSExpression *expression = [NSExpression expressionForVariable:@"lineProgress"];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, @[@"line-progress"]);
+        XCTAssertEqualObjects([NSExpression expressionWithFormat:@"$lineProgress"].mgl_jsonExpressionObject, @[@"line-progress"]);
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:@[@"line-progress"]], expression);
+        NSMutableDictionary *context = [@{@"lineProgress": @1} mutableCopy];
+        XCTAssertEqualObjects([expression expressionValueWithObject:nil context:context], @1);
+    }
+    {
         NSExpression *expression = [NSExpression expressionForVariable:@"geometryType"];
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, @[@"geometry-type"]);
         XCTAssertEqualObjects([NSExpression expressionWithFormat:@"$geometryType"].mgl_jsonExpressionObject, @[@"geometry-type"]);
diff --git a/platform/ios/CHANGELOG.md b/platform/ios/CHANGELOG.md
index b37087d189..47feef842e 100644
--- a/platform/ios/CHANGELOG.md
+++ b/platform/ios/CHANGELOG.md
@@ -8,6 +8,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 
 * Added the ability to style symbol layers labels with multiple fonts and text sizes via the `format` expression operator. ([#12624](https://github.com/mapbox/mapbox-gl-native/pull/12624))
 * Fixed a crash when using the `MGL_LET`, `MGL_MATCH`, `MGL_IF`, or `MGL_FUNCTION` functions without a colon inside an `NSExpression` or `NSPredicate` format string. ([#13189](https://github.com/mapbox/mapbox-gl-native/pull/13189))
+* Fixed a crash setting the `MGLLineStyleLayer.lineGradient` property to an expression containing the `$lineProgress` variable. Added an `NSExpression.lineProgressVariableExpression` class property that returns an expression for the `$lineProgress` variable. ([#13192](https://github.com/mapbox/mapbox-gl-native/pull/13192))
 
 ### Offline maps
 
diff --git a/platform/ios/docs/guides/For Style Authors.md b/platform/ios/docs/guides/For Style Authors.md
index b2ebd0de86..23766e696a 100644
--- a/platform/ios/docs/guides/For Style Authors.md
+++ b/platform/ios/docs/guides/For Style Authors.md
@@ -388,6 +388,7 @@ In style specification | Method, function, or predicate type | Format string syn
 `tan`                  | `mgl_tan:` | `mgl_tan(0)`
 `zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoomLevel`
 `heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
+`line-progress`        | `NSExpression.lineProgressVariableExpression` | `$lineProgress`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.
diff --git a/platform/macos/CHANGELOG.md b/platform/macos/CHANGELOG.md
index d9c70cb24a..b0179eb6fe 100644
--- a/platform/macos/CHANGELOG.md
+++ b/platform/macos/CHANGELOG.md
@@ -7,6 +7,7 @@
 * Added an `MGLSymbolStyleLayer.symbolZOrder` property for forcing point features in a symbol layer to be layered in the same order that they are specified in the layer’s associated source. ([#12783](https://github.com/mapbox/mapbox-gl-native/pull/12783))
 * Fixed a crash when a style layer `*-pattern` property evaluates to nil for a particular feature. ([#12896](https://github.com/mapbox/mapbox-gl-native/pull/12896))
 * Fixed a crash when using the `MGL_LET`, `MGL_MATCH`, `MGL_IF`, or `MGL_FUNCTION` functions without a colon inside an `NSExpression` or `NSPredicate` format string. ([#13189](https://github.com/mapbox/mapbox-gl-native/pull/13189))
+* Fixed a crash setting the `MGLLineStyleLayer.lineGradient` property to an expression containing the `$lineProgress` variable. Added an `NSExpression.lineProgressVariableExpression` class property that returns an expression for the `$lineProgress` variable. ([#13192](https://github.com/mapbox/mapbox-gl-native/pull/13192))
 * Fixed an issue where fill and line layers would occasionally flicker on zoom ([#12982](https://github.com/mapbox/mapbox-gl-native/pull/12982))
 
 ### Offline maps
diff --git a/platform/macos/docs/guides/For Style Authors.md b/platform/macos/docs/guides/For Style Authors.md
index 49b96fb48b..5c34c857b2 100644
--- a/platform/macos/docs/guides/For Style Authors.md
+++ b/platform/macos/docs/guides/For Style Authors.md
@@ -381,6 +381,7 @@ In style specification | Method, function, or predicate type | Format string syn
 `tan`                  | `mgl_tan:` | `mgl_tan(0)`
 `zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoomLevel`
 `heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
+`line-progress`        | `NSExpression.lineProgressVariableExpression` | `$lineProgress`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.