Muack -- Yet Another Mocking Library
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README.md

Muack Build Status Coverage Status Join the chat at https://gitter.im/godfat/muack

by Lin Jen-Shin (godfat)

LINKS:

DESCRIPTION:

Muack -- A fast, small, yet powerful mocking library.

Inspired by RR, and it's 32x times faster (750s vs 23s) than RR for running Rib tests.

WHY?

Because RR has/had some bugs and it is too complex for me to fix it. Muack is much simpler and thus much faster and much more consistent.

REQUIREMENTS:

  • Tested with MRI (official CRuby), Rubinius and JRuby.

INSTALLATION:

gem install muack

SYNOPSIS:

Here's a quick example using Pork.

require 'pork/auto'
require 'muack'

describe 'Hello' do
  include Muack::API

  before{ Muack.reset  }
  after { Muack.verify }

  would 'say world!' do
    str = 'Hello'
    mock(str).say('!'){ |arg| "World#{arg}" }
    str.say('!').should.eq 'World!'
  end
end

Overview

There are 3 parts in Muack, which are:

Mocks are objects with injected methods which we could observe, and mocks modifiers are telling how we want to observe the mocks, and finally argument verifiers could help us observe the arguments passed to the injected methods.

Let's explain them one by one.

Mocks

There are also 4 different kinds of mocks in Muack, which are:

  • Mocks
  • Stubs
  • Spies
  • Coats

You could think of mocks are sort of stubs combined with spies. Here's the inequation: (we'll talk about coats later)

mock >= stub + spy

Stubs help us inject methods into the objects we want to observe. Spies help us observe the behaviours of the objects. As for mocks, they inject methods and observe the behaviours in realtime. They complain immediately if the behaviours were unexpected. In contrast, if we're not asking spies, stubs won't complain themselves.

Here's an example using a mock:

obj = Object.new
mock(obj).name{ 'obj' }
p obj.name     # 'obj'
p Muack.verify # true

Which is similar to using a stub with a spy:

obj = Object.new
stub(obj).name{ 'obj' }
p obj.name     # 'obj'
spy(obj).name
p Muack.verify # true

You might wonder, then why mocks or why stubs with spies? The advantage of using mocks is that, you only need to specify once. I guess this is quite obvious. However, sometimes we don't care if the injected methods are called or not, but sometimes we do care. With stubs and spies, we could always put stubs in the before/setup block, and only when we really care if they are called or not, we put spies to examine.

The other difference is that, spies could partially verify the corresponding stubs, but not necessarily completely as mocks. For example, we could stub two methods, but only verify one of them with a spy.

obj = Object.new
stub(obj).name{ 'obj' }
stub(obj).id  { 12345 }
p obj.name     # 'obj'
p obj.id       # 12345
spy(obj).name
p Muack.verify # true

This is similar as mixing mocks and stubs, as in the following example:

obj = Object.new
stub(obj).name{ 'obj' }
mock(obj).id  { 12345 }
p obj.name     # 'obj'
p obj.id       # 12345
p Muack.verify # true

However you should not mix mocks and stubs with the same method, or you might encounter some unexpected result. Jump to Caveat for more detail.

The other differences for stubs and spies, please check Pattern Matching for stubs and spies. In short, stubs and spies would do some kind of pattern matching, making the order of the same method irrelevant.

On the other hand, stubs aren't limited to testing. If we want to monkey patching something, stubs could be useful as we don't care how many times the injected methods are called. Jump to Muack as a mocky patching library section for more detail.

reset and verify

Calling Muack.reset is essentially resetting all mocks, returning all objects/classes back to their original states. In the very first example, we do this in a before block to make sure that we're in a clean state.

Calling Muack.verify is essentially verifying if all mocks and spies are satisfied, if so, it would return true; otherwise, raising an exception. Then, no matter verification passed or not, Muack would reset itself.

That means we don't really need to call Muack.reset in a before block if we're pretty sure that all test cases would call Muack.verify in the end, resetting everything.

On the other hand, we could also reset or verify a single object without affecting the others. This is helpful in the cases of mocking some very basic objects like Time, without causing too much side effect.

name = 'str'
stub(name).to_s{ 'hi'       }
stub(Time).new { Time.at(0) }
mock(Time).now { Time.new   }

p name.to_s          # 'hi'
p Time.now.to_i      # 0
p Time.new.to_i      # 0
p Muack.verify(Time) # true
p name.to_s          # 'hi'
p Time.now.to_i > 0  # true
p Time.new.to_i > 0  # true
Muack.reset(name)
p name.to_s          # 'str'
p Muack.verify       # true

Coats

Now we could talk about coats. It's a kind of mocks but it would wear out instead of raising an exception when it's called more than expected times. This is useful when we want to restore the original behaviour of a particular method at some point. The problem is that we can't simply call the original method because it's already mocked! We could workaround this by using Muack.verify or Muack.reset at some point, or let coats handle that.

Here's an example with coats:

coat(Time).now{ Time.at(0) }.times(2)

p Time.now.to_i == 0 # true
p Time.now.to_i == 0 # true
p Time.now.to_i  > 0 # true
p Muack.verify       # true

Without coats we might end up with:

mock(Time).now{ Time.at(0) }
mock(Time).now{ Muack.verify(Time); Time.at(0) }

p Time.now.to_i == 0 # true
p Time.now.to_i == 0 # true
p Time.now.to_i  > 0 # true
p Muack.verify       # true

Anonymous mode

Sometimes we just want to stub something without a concrete object in mind. By calling mock or stub without any argument, we're creating an anonymous mock/stub. This is because the default argument for mock and stub is just Object.new.

But how do we access the anonymously created object? We'll use the object method on the modifier to access it. Here's an example:

obj = mock.name{ 'obj' }.object
p obj.name     # 'obj'
p Muack.verify # true

This is exactly equivalent to this:

mock(obj = Object.new).name{ 'obj' }
p obj.name     # 'obj'
p Muack.verify # true

Also, if we want to mock over multiple methods, we could also take the advantage of block form of mock and stub method.

obj = mock{ |m|
  m.name{ 'obj' }
  m.id  { 12345 }
}.object
p obj.name     # 'obj'
p obj.id       # 12345
p Muack.verify # true

We can't omit the object method here because after defining the injected method, we'll get a modifier to describe the properties of the injected method. Jump to Mocks Modifiers for details.

Proxy mode

There are chances that we don't really want to change the underlying implementation for a given method, but we still want to make sure the named method is called, and that's what we're testing for.

In those cases, proxy mode would be quite helpful. To turn a mock or stub into proxy mode we simply do not provide any block to the injected method, but just name it. Here's an example:

str = 'str'
mock(str).reverse
p str.reverse  # 'rts'
p Muack.verify # true

Note that if reverse was not called exactly once, the mock would complain. We could also use stub + spy to do the same thing as well:

str = 'str'
stub(str).reverse
p str.reverse  # 'rts'
spy(str).reverse
p Muack.verify # true

You might also want to use peek_args and peek_return modifier along with proxies in order to slightly tweak the original implementation. Jump to Muack as a mocky patching library section for more detail.

Partial mode

Occasionally we would want to fake some of the values inside a hash, but we don't want to interfere with the other values in that hash, and we also don't want to modify it directly, or we'll need to make sure to restore it after the tests.

Partial mode is not really a mode, but a combination of using proxy mode and the pattern matching mechanism specialized in stubs. Suppose we want to stub ENV (which is not a hash but you get the idea), enabling some of the flags inside tests without really setting it, we'll do:

@user = ENV['USER']
p ENV['NDEBUG'] # nil

stub(ENV)[is_a(String)] #- NOTE: NEED TO DEFINE THIS PROXY FIRST
stub(ENV)['NDEBUG'].returns{ '1' } #- `returns` workaround Ruby syntax

p ENV['NDEBUG'] # '1'
p ENV['USER']   # @user
p Muack.verify  # true
p ENV['NDEBUG'] # nil

Note that in order to make this work, proxy should be defined first. Because stubs are searched in Last In First Out (LIFO) order, it would first check if the key is matching 'NDEBUG' in this case. If it's not matched, then search the next one. Eventually it would reach to the first stub, which we put is_a(String) there so it must match, and return the original value inside ENV.

If the order is reversed, then it would always return the original value, because the proxy would always match, and Muack would stop searching the next stub.

any_instance_of mode

We only talked about mocking a specific object, but never mentioned what if the objects we want to mock aren't at hand at the time we define mocks? In those cases, instead of trying to mock object creation and return the mock we defined, we might want to simply mock any instance of a particular class, since this would make the process much easier.

Here we could use a special "mock" called any_instance_of, which takes a class and returns a Muack::AnyInstanceOf which represents the instance of the class we just passed. Having this special representation, we could treat it as if a real instance and define regular mocks/stubs on it. It would then applies to any instance of the class we gave.

Example speaks:

array = any_instance_of(Array)
stub(array).name{ 'array' }
p [ ].name     # 'array'
p [0].name     # 'array'
p Muack.verify # true

And as most of the time we don't care about the representation after mocks were defined, we could use the block form:

any_instance_of(Array) do |array|
  stub(array).name{ 'array' }
  stub(array).id  { 1234567 }
end
p [ ].name   # 'array'
p [0].id     # 1234567
p Muack.verify # true

Note that if you need to access the real instance instead of the representation in the injected method, you might want to enable instance_exec mode. Please jump to instance_exec mode section for more detail.

Here's an quick example:

any_instance_of(Array) do |array|
  p array.class # Muack::AnyInstanceOf
  mock(array).name.returns(:instance_exec => true){ inspect }
end
p [0, 1].name   # '[0, 1]'
p Muack.verify  # true

Lastly, you could also use any_instance_of along with proxy mode, or any other combination you could think of:

any_instance_of(Array) do |array|
  stub(array).name{ 'array' }
  mock(array).max
end
p [ ].name     # 'array'
p [0].max      # 0
p Muack.verify # true

Though you should still not mix mocks and stubs with the same method, and as you could tell from the above example, Muack would not complain for every array without calling max once. This is because any_instance_of would count on all instances, instead of individual instances. Here we're actually telling Muack that max should be called exactly once amongst all instances of array, and it is indeed called exactly once amongst two instances here.

This might or might not be what we want. But think it twice, if we're mocking any instance of a very basic class in Ruby, testing against individual instances could be too strict since it's used everywhere!

Please check Caveat section for more details.

Mocks Modifiers

A modifier is something specifying a property of an injected method. By making a mock/stub/spy, it would return a modifier descriptor which we could then specify properties about the injected method.

Note that we could chain properties for a given modifier descriptor because all public methods for declaring a property would return the modifier descriptor itself. Let's see the specific usages for each properties with concrete examples.

times

By using mocks, we are saying that the injected method should be called exactly once. However the injected method might be called more than once, say, twice. We could specify this with times modifier:

obj = Object.new
mock(obj).name{ 'obj' }.times(2)
p obj.name     # 'obj'
p obj.name     # 'obj'
p Muack.verify # true

This is actually also semantically equivalent to making the mock twice:

obj = Object.new
mock(obj).name{ 'obj' }
mock(obj).name{ 'obj' }
p obj.name     # 'obj'
p obj.name     # 'obj'
p Muack.verify # true

Note that it does not make sense to specify times for stubs, because stubs don't care about times. Spies do, though. So this is also similar to below:

obj = Object.new
stub(obj).name{ 'obj' }
p obj.name     # 'obj'
p obj.name     # 'obj'
spy(obj).name.times(2)
p Muack.verify # true

Or without using times for spy:

obj = Object.new
stub(obj).name{ 'obj' }
p obj.name     # 'obj'
p obj.name     # 'obj'
spy(obj).name
spy(obj).name
p Muack.verify # true

The advantage of specifying mocks twice is that we could actually provide different results for each call. You could think of it as a stack. Here's a simple example:

obj = Object.new
mock(obj).name{ 0 }
mock(obj).name{ 1 }
mock(obj).name{ 2 }
p obj.name     # 0
p obj.name     # 1
p obj.name     # 2
p Muack.verify # true

We could also use the block form for convenience:

obj = Object.new
mock(obj) do |m|
  m.name{ 0 }
  m.name{ 1 }
  m.name{ 2 }
end
p obj.name     # 0
p obj.name     # 1
p obj.name     # 2
p Muack.verify # true

Note that this does not apply to stubs because stubs never run out. Instead, the latter stub would overwrite the previous one.

obj = Object.new
stub(obj) do |m|
  m.name{ 0 }
  m.name{ 1 }
  m.name{ 2 }
end
p obj.name     # 2
p obj.name     # 2
p obj.name     # 2
p Muack.verify # true

Note that if you do not want a given method be called at all, you could use times(0) to enforce this.

with_any_args

We haven't talked about verifying arguments. With with_any_args modifier, we're saying that we don't care about the arguments. If we're not specifying any arguments like above examples, we're saying there's no arguments at all.

Here we'll show an example for with_any_args. If you do want to verify some specific arguments, jump to Arguments Verifiers section.

obj = Object.new
mock(obj).name{ 'obj' }.with_any_args.times(4)
p obj.name       # 'obj'
p obj.name(1)    # 'obj'
p obj.name(nil)  # 'obj'
p obj.name(true) # 'obj'
p Muack.verify   # true

returns

For some methods, we can't really pass a block to specify the implementation. For example, we can't pass a block to [], which is a Ruby syntax limitation. To workaround it, we could use returns property:

obj = Object.new
mock(obj)[0].returns{ 0 }
p obj[0]       # 0
p Muack.verify # true

This is also useful when we want to put the implementation block in the last instead of the beginning. Here's an example:

obj = Object.new
mock(obj).name.times(2).with_any_args.returns{ 'obj' }
p obj.name     # 'obj'
p obj.name     # 'obj'
p Muack.verify # true

On the other hand, there's also another advantage of using returns than passing the block directly to the injected method. With returns, there's an additional option we could use by passing arguments to returns. We can't do this in regular injected method definition because those arguments are for verifying the actual arguments. Jump to Arguments Verifiers section for details.

The only option right now is :instance_exec.

instance_exec mode

By default, the block passed to the injected method is lexically/statically scoped. That means, the scope is bound to the current binding. This is the default because usually we don't need dynamic scopes, and we simply want to return a plain value, and this is much easier to understand, and it is the default for most programming languages, and it would definitely reduce surprises. If we really need to operate on the object, we have it, and we could touch the internal by calling instance_eval on the object.

However, things are a bit different if we're using any_instance_of. If we're using any_instance_of, then we don't have the instance at hand at the time we're defining the block, but only a Muack::AnyInstanceOf instance to represent the instance. There's no way we could really touch the object without instance_exec option.

This would also be extremely helpful if we're using Muack as a monkey patching library. We don't have to copy the original codes in order to monkey patching a class, we could simply inject what we really want to fix the internal stuffs in the broken libraries we're using. Jump to Muack as a mocky patching library section for more detail.

Here's an quick example:

any_instance_of(Array) do |array|
  p array.class # Muack::AnyInstanceOf
  mock(array).name.returns(:instance_exec => true){ inspect }
end
p [0, 1].name   # '[0, 1]'
p Muack.verify  # true

Note that this :instance_exec option also applies to other modifiers which accepts a block for its implementation, i.e. peek_args and peek_return.

peek_args

What if we don't really want to change an underlying implementation for a given method, but we just want to slightly change the arguments, or we might just want to take a look at the arguments? Here's an example using peek_args to modify the original arguments.

Note that here we use the proxy mode for the mock, because if we're defining our own behaviour, then we already have full control of the arguments. There's no points to use both. This also applies to peek_return.

str = 'ff'
mock(str).to_i.with_any_args.peek_args{ |radix| radix * 2 }
p str.to_i(8)  # 255
p Muack.verify # true

peek_args also supports :instance_exec mode. Here's an example:

any_instance_of(Array) do |array|
  stub(array).push.with_any_args.
    peek_args(:instance_exec => true){ |_| size }
end
a = []
p a.push.dup   # [0]
p a.push.dup   # [0, 1]
p a.push.dup   # [0, 1, 2]
p Muack.verify # true

We could also omit |_| if we don't care about the original argument in the above example.

peek_return

What if we don't really want to change an underlying implementation for a given method, but we just want to slightly change the return value, or we might just want to take a look at the return? Here's an example using peek_return to modify the original return value.

str = 'ff'
mock(str).to_i.with_any_args.peek_return{ |int| int * 2 }
p str.to_i(16) # 510
p Muack.verify # true

peek_return also supports :instance_exec mode. Here's an example:

any_instance_of(Array) do |array|
  stub(array).push.with_any_args.
    peek_return(:instance_exec => true){ |_| size }
end
a = []
p a.push(0)    # 1
p a.push(0)    # 2
p a.push(0)    # 3
p a            # [0, 0, 0]
p Muack.verify # true

We could also omit |_| if we don't care about the original return value in the above example.

Arguments Verifiers (Satisfying)

If we're not passing any arguments to the injected method we define, then basically we're saying that there's no arguments should be passed to the method. If we don't care about the arguments, then we should use with_any_args modifier. If we want the exact arguments, then we should just pass the arguments, which would be checked with == operator.

Here's an example:

obj = Object.new
mock(obj).say('Hi'){ |arg| arg }
p obj.say('Hi') # 'Hi'
p Muack.verify  # true

This also applies to multiple arguments:

obj = Object.new
mock(obj).say('Hello', 'World'){ |*args| args.join(', ') }
p obj.say('Hello', 'World') # 'Hello, World'
p Muack.verify  # true

We could also retrieve the block argument:

obj = Object.new
mock(obj).say{ |&block| block.call('Hi') }
obj.say{ |msg| p msg } # 'Hi'
p Muack.verify  # true

Pattern Matching for stubs and spies

Moreover, we could also have stubs on the same method for different arguments. We could think of this as a sort of pattern matching, and Muack would try to find the best matched stub for us.

obj = Object.new
stub(obj).find(0){ 0 }
stub(obj).find(1){ 1 }
p obj.find(1)  # 1
p obj.find(0)  # 0
p Muack.verify # true

If obj.find(2) is called and Muack cannot find a matched stub, it would raise a Muack::Unexpected and list the candidates for us. This also applies to spies.

However, What if we don't want to be so exact? Then we should use verifiers. We'll introduce each of them in next section. Note that verifiers are not recursive though. If you need complex arguments verification, you'll need to use satisfy verifier which you could give an arbitrary block to verify anything.

anything

anything is a wildcard arguments verifier. It matches anything. Although this actually verifies nothing, we could still think of this as an arity verifier. Since one anything is not two anythings.

obj = Object.new
mock(obj).say(anything){ |arg| arg }.times(2)
p obj.say(0)    # 0
p obj.say(true) # true
p Muack.verify  # true

is_a

is_a would check if the argument is a kind of the given class. Actually, it's calling kind_of? underneath.

obj = Object.new
mock(obj).say(is_a(String)){ |arg| arg }
p obj.say('something') # 'something'
p Muack.verify         # true

matching

matching would check the argument with match method. Usually this is used with regular expression, but anything which responds to match should work.

obj = Object.new
mock(obj).say(matching(/\w+/)){ |arg| arg }
p obj.say('Hi') # 'Hi'
p Muack.verify  # true

Note that please don't pass the regular expression directly without wrapping it with a match verifier, or how do we distinguish if we really want to make sure the argument is exactly the regular expression?

including

including would check if the actual argument includes the given value via include? method.

obj = Object.new
mock(obj).say(including(0)){ |arg| arg }
p obj.say([0,1]) # [0,1]
p Muack.verify   # true

within

within is the reverse version of including, verifying if the actual argument is included in the given value.

obj = Object.new
mock(obj).say(within([0, 1])){ |arg| arg }
p obj.say(0)   # 0
p Muack.verify # true

responding_to

responding_to would check if the actual argument would be responding to the given message, checked via respond_to?, also known as duck typing.

obj = Object.new
mock(obj).say(responding_to(:size)){ |arg| arg }
p obj.say([])  # []
p Muack.verify # true

Note that you could give multiple messages to responding_to.

obj = Object.new
mock(obj).say(responding_to(:size, :reverse)){ |arg| arg }
p obj.say([])  # []
p Muack.verify # true

where

where would check if the actual argument matches given specification.

obj = Object.new
mock(obj).say(where(:a => is_a(Fixnum))){ |arg| arg }
p obj.say(:a => 0) # {:a => 0}
p Muack.verify # true

Note that this could be recursive.

obj = Object.new
mock(obj).say(where(:a => {:b => [is_a(Fixnum)]})){ |arg| arg[:a] }
p obj.say(:a => {:b => [0]}) # {:b => [0]}
p Muack.verify # true

having

having would check if the actual argument is a superset of given specification.

obj = Object.new
mock(obj).say(having(:a => 0)){ |arg| arg }
p obj.say(:a => 0, :b => 1) # {:a => 0, :b => 1}
p Muack.verify # true

Note that this could be recursive.

obj = Object.new
mock(obj).say(having(:a => {:b => [is_a(Fixnum)]})){ |arg| arg[:c] }
p obj.say(:a => {:b => [1]}, :c => 2) # 2
p Muack.verify # true

allowing

allowing would check if the actual argument is a subset of given specification.

obj = Object.new
mock(obj).say(allowing(:a => 0, :b => [1])){ |arg| arg }
p obj.say(:a => 0) # {:a => 0}
p Muack.verify # true

Note that this could be recursive.

obj = Object.new
mock(obj).say(allowing(:a => {:b => is_a(Fixnum), :c => 1})){ |arg| arg[:a] }
p obj.say(:a => {:b => 2}) # {:b => 2}
p Muack.verify # true

satisfying

satisfying accepts a block to let you do arbitrary verification. nil and false are considered false, otherwise true, just like in regular if expression.

obj = Object.new
mock(obj).say(satisfying{ |arg| arg % 2 == 0 }){ |arg| arg }
p obj.say(0)   # 0
p Muack.verify # true

Disjunction (|)

If what we want is the actual argument be within either 0..1 or 3..4? We don't really have to use satisfy to build custom verifier, we could compose verifiers with disjunction operator (|).

obj = Object.new
mock(obj).say(within(0..1) | within(3..4)){ |arg| arg }.times(2)
p obj.say(0)   # 0
p obj.say(4)   # 4
p Muack.verify # true

Or boolean, you might say:

obj = Object.new
mock(obj).say(is_a(TrueClass) | is_a(FalseClass)){ |arg| arg }.times(2)
p obj.say(true)  # true
p obj.say(false) # false
p Muack.verify   # true

Conjunction (&)

If what we want is the actual argument not only a kind of something, but also responds to something. For example, an Enumerable requires the class implements each method. We could use conjunction for this.

obj = Object.new
mock(obj).say(is_a(Enumerable) & responding_to(:each)){}.times(3)
p obj.say( [] ) # nil
p obj.say( {} ) # nil
p obj.say(0..1) # nil
p Muack.verify  # true

Caveat

Mixing mocks and stubs

We could and probably would also want to mix mocks and stubs, for example, we might be concerned about some methods for a given object, but not the other methods.

obj = Object.new
stub(obj).name{ 'obj' }
mock(obj).id  { 12345 }
p obj.name     # 'obj'
p obj.name     # 'obj'
p obj.id       # 12345
p Muack.verify # true

However, it might act unexpectedly if we mock and stub on the same object for the same method. It would somehow act like the latter would always win! So if we define mock later for the same method, previously defined stub would never be called. On the other hand, if we define stub later for the same method, previously defined mock would always complain because it would never be called, either!

This does not mean previously defined mocks or stubs get overwritten, because it would still take effect. It's just that there's no way they could get called. So this is mostly not desired.

The ideal solution to this would be raising an error immediately, or really make it could be overwritten. However I didn't find a good way to handle this without rewriting the internal details. So I'll just leave it as it is, and hope no one would ever try to do this.

any_instance_of shares all calls for a given class

We might assume that mocks with any_instance_of would work exactly the same as regular mocks, but this is actually not the case. Regular mocks count on every individual instance, but all instances share the same count for any_instance_of.

With one instance:

any_instance_of(Array){ |array| mock(array).f{true}.times(2) }
a = []
p a.f          # true
p a.f          # true
p Muack.verify # true

With two instances:

any_instance_of(Array){ |array| mock(array).f{true}.times(2) }
p [].f         # true
p [].f         # true
p Muack.verify # true

So remember to count on all instances, but not individual ones.

Extra Topics

Muack as a mocky patching library

Consider you're using a broken library and you need an immediate fix without waiting for upstream to merge your patch, and release a new version.

You could fix it more elegantly by subclassing the original class, or try to include or extend a module to make the original class work correctly. But sometimes we just cannot do this because of the implementation. They might not be extensible at all. Consider if there's a method contains 1,000 lines... There's no way to change it in the middle of the method other than touching the lines directly, unless we have some line based AOP tools... which is not really practical.

In this case, we could fork it and maintain everything by ourselves, and merge from upstream occasionally. However we might only want to do this as the last resort since this could cost a lot.

Alternatively, we can copy the original code, and put it somewhere, and load it after the original code was loaded, so we have the patched and correct code running. This is also called monkey patching, patching like a monkey. Generally this is a bad idea, but sometimes we can only do this to workaround some broken libraries. For example, some libraries might not be maintained, or the authors refused to fix this due to other reasonable or unreasonable reason.

The most notable drawback of monkey patching is that, we're copying a lot of codes which could be changed upstream, and we might not be aware of that, and update our monkey patch accordingly. This could cause some incompatible issues.

That means, the fewer copied codes, the better. Muack could actually help in this case. I called this mocky patching. The advantage of using this technique is that, we have peek_args and peek_return which we could modify the arguments or return values in runtime, without changing any implementation of a particular method.

Here's a real world example with rails_admin. The problem in rails_admin is that, it assumes every associated records should have already been saved, thus having an id, and there's also a particular show page for it.

However, in our application, we could have associated records not yet saved in the database. rails_admin would try to retrieve routes for those unsaved records, and rails would raise RoutingError because rails_admin is passing no id for a show path.

The idea of this fix is simple. Just don't try to get the show page for records which are not yet saved, i.e. records without an id. However this is actually extremely hard to fix in rails_admin without monkey patching!

I'll skip all those details and my rants. In the end, I fixed this by trying to peek the arguments for a particular method, and if and only if the passed records are not yet saved in the database, we fake the arguments. Otherwise, we just bypass and fallback to the original implementation.

Here's the code:

Muack::API.stub(RailsAdmin::Config::Actions).find.with_any_args.
  peek_args do |*args|
    custom_key, bindings = args
    if bindings && bindings[:object] && bindings[:object].id.nil?
      [nil, {}] # There's no show page for unsaved records
    else
      args # Bypass arguments
    end
  end

If we don't do mocky patching but monkey patching, we'll end up with copying the entire method for RailsAdmin::Config::Actions.find, which then, we'll be responsible for updating this method if some of the original implementation changed.

Note that in mocky patching, we should always use stub and never call Muack.verify or Muack.reset, or that would defeat the purpose of mocky patching.

Muack as a development runtime static typing system

Ever consider a static type system in Ruby? You could actually see a lot of asserts inserted in the beginning of some methods in some libraries. For example, there are assert_valid_key_size, assert_kind_of, etc, in dm-core, and assert_valid_keys, assert_valid_transaction_action, and various random asserts in activerecord.

You could find them by searching against raise ArgumentError because rails is much less consistent and sometimes it's hard to find a pattern in rails. But you get the idea, those ArgumentError would much help us debug our code from misusing the API, and that's exactly the point of type system, or more specifically, static type system.

We could also use some static analysis tools to do something like this, for example, there's ruby-lint. However, as you might already know, since Ruby is so dynamic, static analysis tools cannot really do a great job if our code is quite dynamic. Of course we could write it more statically, and treat our static analysis tools better, but that might not be the spirit of Ruby somehow.

Alternatively, it would be great to do this static type checking dynamically... I mean, in the runtime rather than compile time. This means it would be much more accurate, just like those asserts in the above examples.

However, if we're doing those checks in a hot path, for example, right inside a loop looping over a million times, this would definitely slow things down if we're checking them in the runtime. Even if we put $DEBUG guards around those check, we're still suffering from checking the flag.

It would be great if we could actually just remove those checks in production, while turn it on when we're developing or debugging. Muack could actually fulfill this desire, as it could inject codes externally and seamlessly, and we could remove them anytime when we call Muack.reset, or, simply don't do any stubs in production config.

Consider we have two classes:

Food = Class.new
User = Class.new(Struct.new(:food))

And we could make sure User#food is always a kind of Food by putting this into a development config or so:

Muack::API.module_eval do
  any_instance_of(User) do |user|
    stub(user).food = is_a(Food)
  end
end

And then if we're trying to set a food other than a Food...

u, f = User.new, Food.new
u.food = f # ok
u.food = 1 # raise Muack::Unexpected

This could go wild and we could customize our own domain specific argument verifiers. For example, we could do this to check if the food is frozen:

Food = Class.new
User = Class.new(Struct.new(:food))

FoodFrozen = Class.new(Muack::Satisfying) do
  def match actual_arg
    actual_arg.frozen?
  end
end

Muack::API.module_eval do
  any_instance_of(User) do |user|
    stub(user).food = FoodFrozen.new
  end
end

u = User.new
p u.food = Food.new.freeze # ok
p u.food = Food.new        # raise Muack::Unexpected

Please check Arguments Verifiers (Satisfying) section for more argument verifiers details.

Why didn't mocks nor stubs check if the injected method exists before?

Long story short. I can't find a set of good APIs along with good implementation. My ideal APIs would be that for mocks and stubs, they do check if the injected methods exist before, and if we don't want that check, we use fake instead of mock or stub.

However, how do we specify if fake should act like mock or stub? Introducing yet another name would make the terms even more confusing (which are already fairly confusing!), and I don't want something like: fake.mock or mock.fake or fake_mock or mock_fake. Using an option would also raise the other questions.

What if we make mock.with_any_times work exactly like stub then? Then we could have fake.with_any_times and that would be the stub version of fake. This should greatly reduce the complexity and confusion. However this won't work well because stub is not just mock without times. They are different in:

  • Mocked methods are called in FIFO (queue) order
  • Stubbed methods are called in FILO (stack) order
  • Stubbed methods could do some pattern matching

Of course we could break them though, but do we really have to, just for this simple feature? Also, it could be pretty challenging to implement existing method checking for any_instance_of.

If you could find a good set of APIs while implementing it nicely, please do let me know. Compatibility is not an issue. We could always bump the major number to inform this incompatibility. I am open to breaking legacy. Or, I am happy to break legacy.

USERS:

CONTRIBUTORS:

  • Lin Jen-Shin (@godfat)

LICENSE:

Apache License 2.0

Copyright (c) 2013-2016, Lin Jen-Shin (godfat)

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.