Xamarin-macios: CGAffineTransform.Scale does not work like Swift's .scaledBy(x:y:)

[lobrien]

Steps to Reproduce

(Playground and Workbook attached)

1. In Xcode, create a new Playground
2. import CoreGraphics
3. Execute:

let xform = CGAffineTransform(translationX: 1, y: 2).scaledBy(x: 3, y: 4)

debugPrint(xform)

1. The result is __C.CGAffineTransform(a: 3.0, b: 0.0, c: 0.0, d: 4.0, tx: 1.0, ty: 2.0)

2. Open Xamarin Workbooks

3. Import CoreGraphics
4. Execute what seems to be the equivalent code:

var xlatAndScale = CGAffineTransform.MakeTranslation(1, 2);
xlatAndScale.Scale(3, 4);
xlatAndScale

Expected Behavior

"Like Swift" behavior would produce xx:3.0 yx:0.0 xy:0.0 yy:4.0 x0:1.0 y0:2.0

Actual Behavior

"xx:3.0 yx:0.0 xy:0.0 yy:4.0 x0:3.0 y0:8.0"

It's clear what we're doing, but if this is what we want to do, I'd suggest another method that is `MakeTranslatedAndScaled(CGPoint translation, CGFloat scaleX, CGFloat scaleY) that just sets the values directly.

An added complexity is that the Swift code :

let xformR = CGAffineTransform(translationX: 1, y: 2).scaledBy(x: 3, y: 4).rotated(by: 3.1417 / 4)

produces C.CGAffineTransform(a: 2.121263413764814, b: 2.828503029102253, c: -2.12137727182669, d: 2.828351218353085, tx: 1.0, ty: 2.0)

Workaround

Set the scale first and then set the translation:

var scaleAndXlat = CGAffineTransform.MakeScale(3, 4);
scaleAndXlat.Translate(1, 2);
scaleAndXlat

Results in: "xx:3.0 yx:0.0 xy:0.0 yy:4.0 x0:1.0 y0:2.0"

Environment

=== Visual Studio Community 2017 for Mac (Preview) ===

Version 7.6 Preview (7.6 build 1773)
Installation UUID: 6b94f136-026d-4a5a-bf6d-af2c0d8dc019
Runtime:
    Mono 5.12.0.273 (2018-02/f59eac4c0f1) (64-bit)
    GTK+ 2.24.23 (Raleigh theme)
    Xamarin.Mac 4.4.1.178 (master / eeaeb7e6)

    Package version: 512000273

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
    2.1.1
    2.0.5
    2.0.3
    2.0.0
SDK: /usr/local/share/dotnet/sdk/2.1.301/Sdks
SDK Versions:
    2.1.301
    2.1.4
    2.0.3
    2.0.0
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Xamarin.Android ===

Version: 8.4.0.1 (Visual Studio Community)
Android SDK: /Users/larryobrien/Library/Android/sdk
    Supported Android versions:
        4.1 (API level 16)
        5.0 (API level 21)
        6.0 (API level 23)
        7.0 (API level 24)
        7.1 (API level 25)
        8.0 (API level 26)
        8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 25.0.3
SDK Build Tools Version: 27.0.3

Java SDK: /usr
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Xamarin Inspector ===

Version: 1.4.0
Hash: b3f92f9
Branch: master
Build date: Fri, 19 Jan 2018 22:00:34 GMT
Client compatibility: 1

=== Apple Developer Tools ===

Xcode 10.0 (14313.2)
Build 10L213o

=== Xamarin.Mac ===

Version: 4.99.3.344 (Visual Studio Community)
Hash: 0b726519
Branch: 
Build date: 2018-08-09 10:37:58-0400

=== Xamarin.iOS ===

Version: 11.99.3.224 (Visual Studio Community)
Hash: 0b726519
Branch: HEAD
Build date: 2018-08-09 10:37:57-0400

=== Build Information ===

Release ID: 706001773
Git revision: e5958ff9015312c81fc3b1b6b6413585575ca869
Build date: 2018-06-19 12:19:27+00
Build branch: release-7.6
Xamarin extensions: 6910f36b43e7d559ff40ae268e73ae26c30a2a6b

=== Operating System ===

Mac OS X 10.13.6
Darwin 17.7.0 Darwin Kernel Version 17.7.0
    Thu Jun 21 22:53:14 PDT 2018
    root:xnu-4570.71.2~1/RELEASE_X86_64 x86_64

=== Enabled user installed extensions ===

AddinMaker 1.4.2
Larry's F# Templates 2.0.8
MicroFramework 1.0.3
Internet of Things (IoT) development (Preview) 7.5.3

Example Project (If Possible)

AffineTransforms.zip

[VincentDondain]

I could confirm the different results given by the C# and Swift test cases. Investigating this a bit further.

[VincentDondain]

Ah ok our scale is incorrect (not doing the same as the native version at least).

The native scaleBy code has a comment that indicates what it's doing: t' = [ sx 0 0 sy 0 0 ] * t

We do t' = t * [ sx 0 0 sy 0 0 ]

See CGAffineTransform.Scale where t is a (here) and [ sx 0 0 sy 0 0 ] or MakeScale is b (passed as argument here).

Matrices are non-commutative so our code isn't equivalent to the native operation.

[VincentDondain]

We need:

C# public void Scale (nfloat sx, nfloat sy) { this = Multiply (MakeScale (sx, sy), this); }

[rolfbjarne]

My suggestion is to add a new function called ScaleBy that mimics Swift's behavior.

public void ScaleBy (nfloat sx, nfloat sy)
{
    this = Multiply (MakeScale (sx, sy), this);
}

And additional document it:

• CGAffineTransform.Scale: "This function does t' = t * [ sx 0 0 sy 0 0 ], which is not equivalent to Swift's scaleBy:. Use CGAffineTransform.ScaleBy in that case"
• CGAffineTransform.ScaleBy: "This function does t' = [ sx 0 0 sy 0 0 ] * t, which is equivalent to Swift's scaleBy:.

[VincentDondain]

I like that idea. Otherwise Sebastien suggested we'd copy System.Drawing. So if we look at https://docs.microsoft.com/en-us/dotnet/api/system.drawing.drawing2d.matrixorder?view=netframework-4.7.2.

We'd do something like:

```C#
public enum MultiplyOrder {
Prepend = 0,
Append = 1,
}

public void Scale (nfloat sx, nfloat sy) {
Scale (sx, sy, MultiplyOrder.Append);
}

public void Scale (nfloat sx, nfloat sy, MultiplyOrder order)
{
switch (order) {
case MultiplyOrder.Append: // The new operation is applied after the old operation.
this = Multiply (this, MakeScale (sx, sy)); // t' = t * [ sx 0 0 sy 0 0 ]
break;
case MultiplyOrder.Prepend: // The new operation is applied before the old operation.
default:
this = Multiply (MakeScale (sx, sy), this); // t' = [ sx 0 0 sy 0 0 ] * t – Swift equivalent
break;
}
}
```

[migueldeicaza]

I like the System.Drawing approach.

[spouliot]

@VincentDondain yes, that's how we should expose both versions. However better use MatrixOrder (like .NET) since we're sharing the same API _pattern_

We could also add [Advice] on the existing API that mention the default order.

[rolfbjarne]

Using an enum (MatrixOrder) looks good to me too, although in the implementation I'd throw ArgumentOutOfRangeException in the default case (instead of treating it as .Prepend).