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https://github.com/johnno1962/SwiftTrace

Trace Swift and Objective-C method invocations
https://github.com/johnno1962/SwiftTrace

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Trace Swift and Objective-C method invocations

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# SwiftTrace



Trace Swift and Objective-C method invocations of non-final classes in an app bundle or framework.

Think [Xtrace](https://github.com/johnno1962/Xtrace) but for Swift and Objective-C. You can also 

add "aspects" to member functions of non-final Swift classes to have a closure called before or after

a function implementation executes which in turn can modify incoming arguments or the return value!

Apart from the logging functionality, with binary distribution of Swift frameworks on the horizon perhaps

this will be of use in the same way "Swizzling" was in days of yore.



![SwiftTrace Example](SwiftTrace.gif)



Note: none of these features will work on a class or method that is final or internal in 

a module compiled with whole module optimisation as the dispatch of the method

will be "direct" i.e. linked to a symbol at the call site rather than going through the

class' vtable. As such it is possible to trace calls to methods of a struct but only

if they are referenced through a protocol as they use a `witness table` which

can be patched.



SwiftTrace can be used with the Swift Package Manager or as a CocoaPod by

adding the following to your project's Podfile:



```Swift

    pod 'SwiftTrace'

 ```

Once the project has rebuilt, import SwiftTrace into the application's AppDelegate and add something like the following to the beginning of

it's didFinishLaunchingWithOptions method:



```Swift

    SwiftTrace.traceBundle(containing: type(of: self))

 ```

This traces all classes defined in the main application bundle.

To trace, for example, all classes in the RxSwift Pod add the following



```Swift

    SwiftTrace.traceBundle(containing: RxSwift.DisposeBase.self)

 ```

This gives output in the Xcode debug console such as that above.



To trace a system framework such as UIKit you can trace classes using a pattern:



```Swift

    SwiftTrace.traceClasses(matchingPattern:"^UI")

 ```

Individual classes can be traced using the underlying api:



```Swift

    SwiftTrace.trace(aClass: MyClass.self)

```

Or to trace all methods of instances of a particular class including those of their superclasses

use the following:



```Swift

    SwiftTrace.traceInstances(ofClass: aClass)

```

Or to trace only a particular instance use the following:



```Swift

    SwiftTrace.trace(anInstance: anObject)

```

If you have specified `"-Xlinker -interposable"` in your project's `"Other Linker Flags"`

it's possible to trace all methods in the application's main bundle at once which can be

useful for profiling `SwiftUI` using the following call:



```Swift

    SwiftTrace.traceMainBundleMethods()

```

It is possible to trace methods of a structs or other types if they are messaged through

protools as this would then be indirect via what is called a `witness table`. Tracing

protocols is available at the bundle level where the bundle being traced is specified

using a class instance. They can be further filtered by an optional regular expression. 

For example, the following:

```Swift

SwiftTrace.traceProtocolsInBundle(containing: AClassInTheBundle.self, matchingPattern: "regexp")

```

For example, to trace internal calls made in the `SwiftUI` framework you can use the following:

```Swift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    SwiftTrace.traceProtocolsInBundle(containing: UIHostingController.self)

    return true

}

```



Which traces are applied can be filtered using method name inclusion and exclusion regexps. 

```swift

    SwiftTrace.methodInclusionPattern = "TestClass"

    SwiftTrace.methodExclusionPattern = "init|"+SwiftTrace.defaultMethodExclusions

```

These methods must be called before you start the trace as they are applied during the "Swizzle" phase.

There is a default set of exclusions setup as a result of testing by tracing UIKit.

                      

    open class var defaultMethodExclusions: String {

        return """

            \\.getter| (?:retain|_tryRetain|release|_isDeallocating|.cxx_destruct|dealloc|description| debugDescription)]|initWithCoder|\

            ^\\+\\[(?:Reader_Base64|UI(?:NibStringIDTable|NibDecoder|CollectionViewData|WebTouchEventsGestureRecognizer)) |\

            ^.\\[(?:UIView|RemoteCapture) |UIDeviceWhiteColor initWithWhite:alpha:|UIButton _defaultBackgroundImageForType:andState:|\

            UIImage _initWithCompositedSymbolImageLayers:name:alignUsingBaselines:|\

            _UIWindowSceneDeviceOrientationSettingsDiffAction _updateDeviceOrientationWithSettingObserverContext:windowScene:transitionContext:|\

            UIColorEffect colorEffectSaturate:|UIWindow _windowWithContextId:|RxSwift.ScheduledDisposable.dispose| ns(?:li|is)_

            """

    }



If you want to further process output you can define your own custom tracing sub class:

```swift

    class MyTracer: SwiftTrace.Decorated {



        override func onEntry(stack: inout SwiftTrace.EntryStack) {

            print( ">> "+stack )

        }

    }

    

    SwiftTrace.swizzleFactory = MyTracer.self

 ```   

 As the amount of of data logged can quickly get out of hand you can control what is

 logged by combing traces with the optional `subLevels` parameter to the above functions.

 For example, the following puts a trace on all of UIKit but will only log calls to methods

 of the target instance and up to three levels of calls those method make:

 ```Swift

     SwiftTrace.traceBundle(containing: UIView.self)

     SwiftTrace.trace(anInstance: anObject, subLevels: 3)

 ```

 Or, the following will log methods of the application and calls to RxSwift they make:

 ```Swift

     SwiftTrace.traceBundle(containing: RxSwift.DisposeBase.self)

     SwiftTrace.traceMainBundle(subLevels: 3)

 ```

 If this seems arbitrary the rules are reasonably simple. When you add a trace with a

 non-zero subLevels parameter all previous traces are inhibited unless they are being

 made up to subLevels inside a method in the most recent trace or if they where filtered

 anyway by a class or instance (traceInstances(ofClass:) and trace(anInstance:)).



If you would like to extend SwiftTrace to be able to log one of your app's types

there are two steps. First, you may need to extend the type to conform to

SwiftTraceFloatArg if it contains only float only float types for example SwiftUI.EdgeInsets.

```Swift

extension SwiftUI.EdgeInsets: SwiftTraceFloatArg {}

```

Then, add a handler for the type using the following api:

```Swift

    SwiftTrace.addFormattedType(SwiftUI.EdgeInsets.self, prefix: "SwiftUI")

```

 Many of these API's are also available as a extension of NSObject which is useful

 when SwiftTrace is made available by dynamically loading bundle as in

 (InjectionIII)[https://github.com/johnno1962/InjectionIII].

 ```Swift

     SwiftTrace.traceBundle(containing: UIView.class)

     // becomes

     UIView.traceBundle()

     

     SwiftTrace.trace(inInstance: anObject)

     // becomes

     anObject.swiftTraceInstance()

 ```

 This is useful when SwiftTrace is made available by dynamically loading a bundle

 such as when using (InjectionIII)[https://github.com/johnno1962/InjectionIII]. Rather

 than having to include a CocoaPod, all you need to do is add SwiftTrace.h in the

 InjectionIII application's bundle to your bridging header and dynamically load the bundle.

 ```Swift

    Bundle(path: "/Applications/InjectionIII.app/Contents/Resources/iOSInjection.bundle")?.load()

 ```

 #### Benchmarking

 

 To benchmark an app or framework, trace it's methods then you can use one of the following:

 ```

    SwiftTrace.sortedElapsedTimes(onlyFirst: 10))

    SwiftTrace.sortedInvocationCounts(onlyFirst: 10))

 ```

#### Object lifetime tracking



You can track the allocations an deallocations of Swift and

Objective-C classes using the [SwiftTrace.LifetimeTracker](https://github.com/johnno1962/SwiftTrace/blob/main/SwiftTrace/SwiftLifetime.swift) class:



```Swift

SwiftTrace.swizzleFactory = SwiftTrace.LifetimeTracker.self

SwiftTrace.traceMainBundleMethods() == 0 {

    print("⚠️ Tracing Swift methods can only work if you have -Xlinker -interposable to your project's \"Other Linker Flags\"")

}

SwiftTrace.traceMainBundle()

```

Each time an object is allocated you will see a `.__allocating_init` message

followed by the result and the resulting count of live objects allocated 

since tracing was started. Each time an object is deallocated you will

see a `cxx_destruct` message followed by the number of objects

oustanding for that class.



If you would like to track the lifecycle of Swift structs, create a marker

class and add a property to the struct initialised to an instance of it.



```Swift

class Marker {}



struct MyView: SwiftUI.View {

    var marker = Marker()

}

```

This idea is based on the [LifetimeTracker](https://github.com/krzysztofzablocki/LifetimeTracker)

project by [Krzysztof Zabłocki](https://github.com/krzysztofzablocki).

#### Aspects



You can add an aspect to a particular method using the method's de-mangled name:

```swift

    print(SwiftTrace.addAspect(aClass: TestClass.self,

                      methodName: "SwiftTwaceApp.TestClass.x() -> ()",

                      onEntry: { (_, _) in print("ONE") },

                      onExit: { (_, _) in print("TWO") }))

 ```   

This will print "ONE" when method "x" of TextClass is called and "TWO when it has exited. The

two arguments are the Swizzle which is an object representing the "Swizzle" and the entry or 

exit stack. The full signature for the entry closure is:

```swift

       onEntry: { (swizzle: SwiftTrace.Swizzle, stack: inout SwiftTrace.EntryStack) in

 ```   

If you understand how [registers are allocated](https://github.com/apple/swift/blob/master/docs/ABI/RegisterUsage.md) to arguments it is possible to poke into the

stack to modify the incoming arguments and, for the exit aspect closure you can replace

the return value and on a good day log (and prevent) an error being thrown.



Replacing an input argument in the closure is relatively simple:

```swift

    stack.intArg1 = 99

    stack.floatArg3 = 77.3

 ```

Other types of argument a little more involved. They must be cast and String

takes up two integer registers.

```swift

    swizzle.rebind(&stack.intArg2).pointee = "Grief"

    swizzle.rebind(&stack.intArg4).pointee = TestClass()

 ```

In an exit aspect closure, setting the return type is easier as it is generic:

```swift

    stack.setReturn(value: "Phew")

 ```

When a function throws you can access NSError objects.

```swift

    print(swizzle.rebind(&stack.thrownError, to: NSError.self).pointee)

 ```

It is possible to set `stack.thrownError` to zero to cancel the throw but you will need to set

the return value.



If this seems complicated there is a property `swizzle.arguments` which can be used

`onEntry` which contains the arguments as an `Array` containing elements of type `Any`

which can be cast to the expected type. Element 0 is `self`.



#### Invocation interface



Now we have a trampoline infrastructure, it is possible to implement an invocation api for Swift:

```swift

    print("Result: "+SwiftTrace.invoke(target: b,

        methodName: "SwiftTwaceApp.TestClass.zzz(_: Swift.Int, f: Swift.Double, g: Swift.Float, h: Swift.String, f1: Swift.Double, g1: Swift.Float, h1: Swift.Double, f2: Swift.Double, g2: Swift.Float, h2: Swift.Double, e: Swift.Int, ff: Swift.Int, o: SwiftTwaceApp.TestClass) throws -> Swift.String",

        args: 777, 101.0, Float(102.0), "2-2", 103.0, Float(104.0), 105.0, 106.0, Float(107.0), 108.0, 888, 999, TestClass()))

 ```

In order to determine the mangled name of a method you can get the full list for a class 

using this function:

```swift

    print(SwiftTrace.methodNames(ofClass: TestClass.self))

 ```

There are limitations to this abbreviated interface in that it only supports Double, Float,

String, Int, Object, CGRect, CGSize and CGPoint arguments. For other struct types that

do not contain floating point values you can conform them to protocol `SwiftTraceArg`

to be able to pass them on the argument list or `SwiftTraceFloatArg` if they contain

only floats. These values and return values must fit into 32 bytes and not contain floats.



#### How it works

                      

A Swift `AnyClass` instance has a layout similar to an Objective-C class with some

additional data documented in the `ClassMetadataSwift` in SwiftMeta.swift. After this data

there is the vtable of pointers to the class and instance member functions of the class up to

the size of the class instance. SwiftTrace replaces these function pointers with a pointer

to a unique assembly language "trampoline" entry point which has destination function and

data pointers associated with it. Registers are saved and this function is called passing

the data pointer to log the method name. The method name is determined by de-mangling the

symbol name associated the function address of the implementing method. The registers are

then restored and control is passed to the original function implementing the method. 

 

Please file an issue if you encounter a project that doesn't work while tracing. It should

be far more reliable as it uses assembly language trampolines rather than Swizzling like

Xtrace did. Otherwise, the author can be contacted on Twitter [@Injection4Xcode](https://twitter.com/@Injection4Xcode). 



Thanks to Oliver Letterer for the [imp_implementationForwardingToSelector](https://github.com/OliverLetterer/imp_implementationForwardingToSelector) project adapted to set up the

trampolines, included under an MIT license.



The repo includes a very slightly modified version of the very handy

[https://github.com/facebook/fishhook](https://github.com/facebook/fishhook).

See the source and header files for their licensing details.



Thanks also  to [@twostraws](https://twitter.com/twostraws)'

[Unwrap](https://github.com/twostraws/Unwrap) and [@artsy](https://twitter.com/ArtsyOpenSource)'s

[eidolon](https://github.com/artsy/eidolon) used extensively during testing.



Enjoy!



$Date: 2022/01/22 $