Tracer

Tracer is an experimental testing tool that lets you record & play back the behavior of arbitrary objects in Objective-C.

Let's say you have a dependency in your code, ThatThing. You call ThatThing, it calls you back, and behavior varies depending on user input or environmental factors.

@protocol ThatInterface <NSObject>
- (void)someCommand:(int)i;
- (void)someOtherCommand:(NSArray *)objects;
// ...
- (void)onError:(NSError *)error;
- (void)onOtherError:(NSError *)error;
@end

@interface ThatThing : NSObject <ThatInterface>
@end

Testing complex async behavior is hard, especially if you don't control the source of behavior.

Tracer lets you record behavior of ThatThing as a trace:

ThatThing *thing = [ThatThing new];
TRCRecorder *recorder = [TRCRecorder new];
[recorder startRecording:thing protocol:@protocol(ThatInterface)];
NSString *result = [thing someCommand:-100];
[recorder stopRecording:thing protocol:@protocol(ThatInterface) completion:^(TRCTrace *trace, NSError *error) {
    // save trace
}];

After recording completes, Tracer prints the trace to the console as JSON, so you can save behavior to a file.

2019-04-17 23:01:22.689124-0700 xctest[62038:4377601] -----BEGIN TRACE JSON-----
{
  "start_ms" : 1551678464427,
  "id" : "trace",
  "protocol" : "SomeProtocol",
  "calls" : [
    {
      "id" : "call",
      "start_ms" : 203,
      "method" : "someCommand:",
      "arguments" : [
        {
          "id" : "value",
          "type" : "int",
          "object_type" : "not_an_object",
          "object_value" : -100
        }
      ],
      "return_value" : {
        "id" : "value",
        "type" : "void",
      }
    }
  ]
}
-----END TRACE JSON-----

In your tests, instead of mocking the complex behavior of ThatThing, you can simply play recorded behavior:

ThatThing *thing = [ThatThing new];
TRCTrace *trace = [TRCTrace loadFromJSONFile:@"saved_trace"];
[TRCPlayer playTrace:trace onTarget:thing completion:^(NSError * _Nullable error) {

}];

Current limitations

Tracer doesn't support hooking async behavior (e.g. completion blocks)
You must provide a protocol to scope recording
Optional protocol methods won't be recorded
Naive introspection when recording unknown object types (Tracer simply records the object's description)
Playback is always on the main thread

sun6boys/tracer-objc

Tracer

Current limitations