[ios, macos] Update Style Functions guide (#11724)

2 files changed, 267 insertions, 154 deletions

diff --git a/platform/macos/docs/guides/Migrating to Expressions.md b/platform/macos/docs/guides/Migrating to Expressions.md
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+<!--
+  This file is generated.
+  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
+-->
+
+# Migrating from Style Functions to Expressions
+
+[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Developers can specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source.
+
+With Mapbox Maps SDK for macOS v0.7.0, style functions have been replaced with expressions. These provide even more tools for developers who want to style their maps dynamically. This guide outlines some tips for migrating from style functions to expressions, and offers an overview of some things that developers can do with expressions.
+
+An expression is represented at runtime by the `NSExpression` class. Expressions can be used to style paint and layout properties based on zoom level, data attributes, or a combination of the two.
+
+A constant expression can also be assigned to a style property. For example, the opacity of a fill style layer can be set to a constant value between 0 and 1.
+
+The documentation for each individual style layer property notes which non-constant expressions are enabled for that property. Style functions supported four interpolation modes: exponential, interval, categorical, and identity.
+
+This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php). Under each interpolation mode, the style function implementation will be shown, followed by the current syntax.
+
+For more information about how to work with GeoJSON data in our macOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide. To learn more about supported expressions, see our ["Predicates and Expressions"](predicates-and-expressions.html) guide. The "Predicates and Expressions" guide also outlines Mapbox custom functions that can be used to dynamically style a map.
+
+## Stops
+Stops are dictionary keys that are associated with layer attribute values. Constant values no longer need to be wrapped as style values when they are values in a stops dictionary.
+
+
+Style function syntax:
+
+```swift
+let stops = [
+    0: MGLStyleValue<UIColor>(rawValue: .yellow),
+    2.5: MGLStyleValue(rawValue: .orange),
+    5: MGLStyleValue(rawValue: .red),
+    7.5: MGLStyleValue(rawValue: .blue),
+    10: MGLStyleValue(rawValue: .white),
+]
+```
+
+Current syntax:
+```swift
+let stops: [NSNumber: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+```
+
+
+## Interpolation mode
+
+Style functions supported four interpolation modes: exponential/linear, interval, categorical, and identity. For more information about supported custom expressions, please see the "Predicates and Expressions" guide.
+
+### Linear
+
+`+[NSExpression(MGLAdditions) mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` takes the interpolation type as a parameter. If you previously used the default interpolation base, use the curve type `MGLExpressionInterpolationMode.linear`. See the [`mgl_interpolate:withCurveType:parameters:stops:`](predicates-and-expressions.html#code-mgl_interpolate-withcurvetype-parameters-stops-code) documentation for more details.
+
+The stops dictionary below, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
+
+Style function syntax:
+
+```swift
+let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
+let symbolSource = MGLSource(identifier: "source")
+let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbolSource)
+
+let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
+mapView.style?.addSource(source)
+
+let stops = [
+    0: MGLStyleValue<UIColor>(rawValue: .yellow),
+    2.5: MGLStyleValue(rawValue: .orange),
+    5: MGLStyleValue(rawValue: .red),
+    7.5: MGLStyleValue(rawValue: .blue),
+    10: MGLStyleValue(rawValue: .white),
+]
+
+let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+layer.circleColor = MGLStyleValue(interpolationMode: .exponential,
+                                  sourceStops: stops,
+                                  attributeName: "mag",
+                                  options: [.defaultValue: MGLStyleValue<UIColor>(rawValue: .green)])
+layer.circleRadius = MGLStyleValue(rawValue: 10)
+mapView.style?.insertLayer(layer, below: symbolLayer)
+```
+
+Current syntax:
+
+```swift
+let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
+let symbolSource = MGLSource(identifier: "source")
+let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbolSource)
+
+let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
+mapView.style?.addSource(source)
+
+let stops: [NSNumber: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+layer.circleColor = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(mag, 'linear', nil, %@)",
+                                 stops)
+layer.circleRadius = NSExpression(forConstantValue: 10)
+mapView.style?.insertLayer(layer, below: symbolLayer)
+```
+
+### Exponential
+
+If you previously used an interpolation base greater than `0` (other than `1`), you can use `MGLExpressionInterpolationMode.exponential` as the curve type for `+[NSExpression(MGLAdditions) mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` or `'exponential'` as the curve type for [`mgl_interpolate:withCurveType:parameters:stops:`](predicates-and-expressions.html#code-mgl_interpolate-withcurvetype-parameters-stops-code). The `parameters` argument takes that interpolation base. This interpolates between values exponentially, creating an accelerated ramp effect.
+
+Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom. In order to convert camera style functions, use `$zoomLevel` or `MGL_FUNCTION('zoomLevel')` as the attribute key.
+
+<img src="img/data-driven-styling/exponential-function.png" height=344/>
+<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
+
+The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The interpolation base is `1.5`.
+
+Style function syntax:
+
+```swift
+let stops = [
+    12: MGLStyleValue<NSNumber>(rawValue: 0.5),
+    14: MGLStyleValue(rawValue: 2),
+    18: MGLStyleValue(rawValue: 18),
+]
+
+layer.circleRadius = MGLStyleValue(interpolationMode: .exponential,
+                                   cameraStops: stops,
+                                   options: [.interpolationBase: 1.5])
+```
+
+Current syntax:
+
+```swift
+let stops = [
+    12: 0.5,
+    14: 2,
+    18: 18,
+]
+
+layer.circleRadius = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'exponential', 1.5, %@)",
+                                  stops)
+```
+
+### Interval
+
+Steps, or intervals, create a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the layout or paint value assigned to that key. You can use the `+[NSExpression(MGLAdditions) mgl_expressionForSteppingExpression:fromExpression:stops:]` method or the custom function [`mgl_step:from:stops:`](predicates-and-expressions.html#code-mgl_step-from-stops-code) for cases where you previously used interval interpolation mode. The first parameter takes the feature attribute name and the second parameter (`from:`) optionally takes the default or fallback value for that function. The final parameter takes a stops dictionary as an argument.
+
+When we use the stops dictionary given above with an `'mgl_step:from:stops:'`, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
+
+Style function syntax:
+
+```swift
+let stops = [
+    0: MGLStyleValue<UIColor>(rawValue: .yellow),
+    2.5: MGLStyleValue(rawValue: .orange),
+    5: MGLStyleValue(rawValue: .red),
+    7.5: MGLStyleValue(rawValue: .blue),
+    10: MGLStyleValue(rawValue: .white),
+]
+
+layer.circleColor = MGLStyleValue(interpolationMode: .interval,
+                                  sourceStops: stops,
+                                  attributeName: "mag",
+                                  options: [.defaultValue: MGLStyleValue<UIColor>(rawValue: .green)])
+````
+
+Current syntax:
+
+```swift
+let stops: [NSNumber: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+layer.circleColor = NSExpression(format: "mgl_step:from:stops:(mag, %@, %@)",
+                                 NSColor.green, stops)
+```
+
+### Categorical
+
+Categorical interpolation mode took a stops dictionary. If the value for a specified feature attribute name matched one in that stops dictionary, the style value for that attribute value would be used. Categorical style functions can now be replaced with `MGL_MATCH`.
+
+`MGL_MATCH` takes an initial condition, which in this case is an attribute key. This is followed by possible matches for that key and the value to assign to the layer property if there is a match. The final argument can be a default style value that is to be used if none of the specified values match.
+
+There are three main types of events in the USGS dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
+
+Style function syntax:
+
+```swift
+let categoricalStops = [
+    "earthquake": MGLStyleValue<UIColor>(rawValue: .orange),
+    "explosion": MGLStyleValue(rawValue: .red),
+    "quarry blast": MGLStyleValue(rawValue: .yellow),
+]
+
+layer.circleColor = MGLStyleValue(interpolationMode: .categorical,
+                                  sourceStops: categoricalStops,
+                                  attributeName: "type",
+                                  options: [.defaultValue: MGLStyleValue<UIColor>(rawValue: .blue)])
+```
+
+Current syntax:
+```swift
+let defaultColor = NSColor.blue
+layer.circleColor = NSExpression(
+format: "MGL_MATCH(type, 'earthquake', %@, 'explosion', %@, 'quarry blast', %@, %@)",
+    NSColor.orange, NSColor.red, NSColor.yellow, defaultColor)
+```
+
+If your use case does not require a default value, you can either apply a predicate to your layer prior to styling it, or use the format string `"valueForKeyPath:"`.
+
+### Identity
+
+Identity interpolation mode used the attribute’s value as the style layer property value. In this example, you might set the `circleRadius` to the earthquake’s magnitude. In order to use a feature attribute value to style a layer property, set the property value to `[NSExpression expressionForKeyPath:]`, which take the feature attribute name as an argument.
+
+Style function syntax:
+
+```swift
+layer.circleRadius = MGLStyleValue(interpolationMode: .identity,
+                                   sourceStops: nil,
+                                   attributeName: "mag",
+                                   options: [.defaultValue: MGLStyleValue<NSNumber>(rawValue: 0)])
+```
+
+Current syntax:
+```swift
+layer.circleRadius = NSExpression(forKeyPath: "mag")
+```
+
+![identity mode](img/data-driven-styling/identity.png)
+
+Some built-in functions can be applied to attribute values to style layer property values. To set the circle radius to three times the earthquake’s magnitude, create a `multiply:by:` function that takes the attribute value and the multiplier as arguments, or use a format string.
+
+```swift
+layer.circleRadius = NSExpression(forFunction: "multiply:by:", arguments: [NSExpression(forKeyPath: "mag"), 3])
+```
+
+![multiply magnitude](img/data-driven-styling/multiply.png)
+
+You can also cast attribute values in order to use them. One example is to cast an integer as an `NSString` and use it as a text value.
+
+```swift
+let magnitudeLayer = MGLSymbolStyleLayer(identifier: "mag-layer", source: source)
+magnitudeLayer.text = NSExpression(format: "CAST(mag, 'NSString')")
+mapView.style?.addLayer(magnitudeLayer)
+```
+
+![cast a value](img/data-driven-styling/cast.png)
+
+### Constant Values
+
+For constant values that do not necessarily change based on camera or attribute values, use `[NSExpression expressionForConstantValue:]` (previously `[MGLStyleValue valueWithRawValue:]`).
+
+## Resources
+
+* [USGS Earthquake Feed](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
+* [For Style Authors](for-style-authors.html)
+* [Predicates and Expressions](predicates-and-expressions.html)
diff --git a/platform/macos/docs/guides/Using Style Functions at Runtime.md b/platform/macos/docs/guides/Using Style Functions at Runtime.md
deleted file mode 100644
index e87cd00185..0000000000
--- a/platform/macos/docs/guides/Using Style Functions at Runtime.md
+++ /dev/null
@@ -1,154 +0,0 @@
-<!--
-  This file is generated.
-  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
--->
-
-# Using Style Functions at Runtime
-
-[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Much of the runtime styling API allows you to specify _style functions_ instead of constant values. A style function allows you to specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source.
-
-Style functions spare you the inconvenience of manually calculating intermediate values between different zoom levels or creating a multitude of style layers to handle homogeneous features in the map content. For example, if your content source indicates the prices of hotels in an area, you can color-code the hotels by price, relying on a style function to smoothly interpolate among desired colors without having to specify the color for each exact price.
-
-_Data-driven styling_ specifically refers to the use of style functions to vary the map’s appearance based on data in a content source.
-
-You can also specify style functions in a style JSON file, to be applied automatically when the map loads. See the [Mapbox Style Specification](https://www.mapbox.com/mapbox-gl-js/style-spec/#types-function) for details.
-
-![available bikes](img/data-driven-styling/citibikes.png) ![subway lines](img/data-driven-styling/polylineExample.png)
-
-This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php) and data-driven styling to style a map based on attributes. For more information about how to work with GeoJSON data in our iOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide.
-
-A style function is represented at runtime by the `MGLStyleFunction` class. There are three subclasses of `MGLStyleFunction`:
-
-* `MGLCameraStyleFunction` is a style value that changes with zoom level. For example, you can make the radius of a circle increase according to zoom level.
-* `MGLSourceStyleFunction` is a style value that changes with the attributes of a feature. For example, you can adjust the radius of a circle based on the magnitude of an earthquake.
-* `MGLCompositeStyleFunction` is a style value that changes with both zoom level and attribute values. For example, you can add a circle layer where each circle has a radius based on both zoom level and the magnitude of an earthquake.
-
-The documentation for each individual style layer property notes which style functions are enabled for that property.
-
-## Stops
-
-Stops are dictionary keys that are associated with layer attribute values. With feature attribute values as stops, you can use a dictionary with a zoom level for a key and an expression or constant value for the value. For example, you can use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors yellow, orange, and red as the values. Alternatively, attribute values can be the keys.
-
-```swift
-let stops: [Float: NSColor] = [
-    0: .yellow,
-    2.5: .orange,
-    5: .red,
-    7.5: .blue,
-    10: .white,
-]
-```
-
-## Interpolation mode
-
-The effect a key has on the style value is determined by the interpolation mode. There are four interpolation modes that can be used with a source style function: exponential, interval, categorical, and identity. You can also use exponential and interval interpolation modes with a camera style function.
-
-### Linear
-
-`MGLInterpolationModeExponential` interpolates linearly or exponentially between style function stop values. By default, the `MGLStyleFunction` options parameter `MGLStyleFunctionOptionInterpolationBase` equals `1`, which represents linear interpolation and doesn’t need to be included in the options dictionary.
-
-The stops dictionary below, for example, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
-
-```swift
-let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
-let symbolSource = MGLSource(identifier: "source")
-let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbolSource)
-
-let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
-mapView.style?.addSource(source)
-
-let stops: [Float: NSColor] = [
-    0: .yellow,
-    2.5: .orange,
-    5: .red,
-    7.5: .blue,
-    10: .white,
-]
-
-let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
-layer.circleColor = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(mag, 'linear', nil, %@)",
-                                 stops)
-layer.circleRadius = NSExpression(forConstantValue: 10)
-mapView.style?.insertLayer(layer, below: symbolLayer)
-```
-
-![exponential mode](img/data-driven-styling/exponential.png)
-
-### Exponential
-
-By combining `MGLInterpolationModeExponential` with an `MGLStyleFunctionOptionInterpolationBase` greater than `0` (other than `1`), you can interpolate between values exponentially, create an accelerated ramp effect.
-
-Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom.
-
-<img src="img/data-driven-styling/exponential-function.png" height=344/>
-<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
-
-The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The `MGLStyleFunctionOptionInterpolationBase` is `1.5`.
-
-```swift
-let stops = [
-    12: 0.5,
-    14: 2,
-    18: 18,
-]
-
-layer.circleRadius = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'exponential', 1.5, %@)",
-                                  stops)
-```
-
-### Interval
-
-`MGLInterpolationModeInterval` creates a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the style value assigned to that key.
-
-When we use the stops dictionary given above with an interval interpolation mode, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
-
-```swift
-let stops: [Float: NSColor] = [
-    0: .yellow,
-    2.5: .orange,
-    5: .red,
-    7.5: .blue,
-    10: .white,
-]
-
-layer.circleColor = NSExpression(format: "mgl_step:from:stops:(mag, %@, %@)",
-                                 NSColor.green, stops)
-```
-
-![interval mode](img/data-driven-styling/interval.png)
-
-### Categorical
-
-At each stop, `MGLInterpolationModeCategorical` produces an output value equal to the function input. We’re going to use a different stops dictionary than we did for the previous two modes.
-
-There are three main types of events in the dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
-
-```swift
-let colors: [String: NSColor] = [
-    "earthquake": .orange,
-    "explosion": .red,
-    "quarry blast": .yellow,
-]
-let defaultColor = NSColor.blue
-
-layer.circleColor = NSExpression(
-    format: "TERNARY(FUNCTION(%@, 'valueForKeyPath:', type) != nil, FUNCTION(%@, 'valueForKeyPath:', type), %@)",
-    colors, colors, defaultColor)
-```
-
-![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
-
-### Identity
-
-`MGLInterpolationModeIdentity` uses the attribute’s value as the style value. For example, you can set the `circleRadius` to the earthquake’s magnitude. Since the attribute value itself will be used as the style value, `sourceStops` should be set to `nil`.
-
-```swift
-layer.circleRadius = NSExpression(forKeyPath: "mag")
-```
-
-![identity mode](img/data-driven-styling/identity.png)
-
-##Resources
-
-* [USGS](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
-* [For Style Authors](for-style-authors.html)