Bulk patching with stubs()

when() and its related functions are quite simple and straightforward.
However, it can be quite verbose when multiple functions needs to be patched.
For this reason, version 2.1 introduced a new API function to define multiple functions in bulk: stubs()
stubs()
Functions\stubs() accepts an array of functions to be defined.
The first way to use it is to pass function names as array item keys and the wanted return values as array values:
Functions\stubs(
    [
        'is_user_logged_in' => true,
        'current_user_can'  => false,
    ]
);
There are two special cases:
when the array item value is a callable, the function given as array item key is aliased to the given callback instead of returning the callback itself;
when the array item value is null, the function given as array item key will return the first argument received, just like when( $function_name )->justReturnArg() was used for it
Functions\stubs(
    [
        'is_user_logged_in'    => true,             // will return `true` as provided
        'wp_get_current_user' => function () {      // will return the WP_User mock
            return \Mockery::mock(\WP_User::class);
        },
        '__' => null,                               // will return the 1st argument received
    ]
);
Another way to use stubs, useful to stub many function with same return value, is to pass to a non-associative array of function names as first argument, and the wanted return value for all of them as second argument.
For example, the snippet below will create a stub that returns true for all the given functions:
Functions\stubs(
    [
        'is_user_logged_in',
        'current_user_can',
        'is_multisite',
        'is_admin',
    ],
    true
);
Please note that the default value for the second argument, being it optional, is null, and because using null as value means "return first received argument" it is possible to stub many functions that have to return first received argument, by passing their names as first argument to stubs() (and no second argument), like this:
Functions\stubs(
    [
        'esc_attr',
        'esc_html',
        '__',
        '_x',
        'esc_attr__',
        'esc_html__',
    ]
);
(Even if there's a simpler way to stub escaping and translation WP functions, more on this below).
It worth noting that the two ways of using stubs() can be mixed together, for example like this:
Functions\stubs(
    [
        // will both return 1st argument received, because `stubs` 2nd param defaults to `null`
        'esc_attr',
        'esc_html',
​
        // will all return what is given as array item value
        'is_user_logged_in'   => true,
        'current_user_can'    => false,
        'get_current_user_id' => 1,
    ]
);
Pre-defined stubs for escaping functions
To stub WordPress escaping functions is a very common usage for Functions\stubs.
This is why, since version 2.3, Brain Monkey introduced a new API function:
Functions\stubEscapeFunctions()
When called, it will create a stub for each of the following functions:
esc_js()
esc_sql()
esc_attr()
esc_html()
esc_textarea()
esc_url()
esc_url_raw()
esc_xml() (since 2.6)
By calling Functions\stubEscapeFunctions(), for all of the functions listed above a stub will be created that will do some very basic escaping on the received first argument before returning it.
It will not be the exact same escape mechanism that WordPress would apply, but "similar enough" for unit tests purpose and could still be helpful to discover some bugs.
Pre-defined stubs for translation functions
Another common usage for Functions\stubs, since its introduction, has been to stub translation functions.
Since version 2.3, this has became much easier thanks to the introduction of a new API function:
Functions\stubTranslationFunctions()
When called, it will create a stub for all the following functions:
__()
_e()
_ex()
_x()
_n() (since 2.6)
_nx() (since 2.6)
translate()
esc_html__()
esc_html_x()
esc_attr__() 
esc_attr_x() 
esc_html_e() 
esc_attr_e() 
The created stub will not attempt any translation, but will return (or echo) the first received argument.
Only for functions that both translate and escape (esc_html__(), esc_html_x()...) the same escaping mechanism used by the pre-defined escaping functions stubs (see above) is applied before returning first received argument.
Please note how Functions\stubTranslationFunctions() creates stubs for functions that echo translated text, something not easily doable with Functions\stubs() alone.
Gotcha for Functions\stubs
Functions that returns null
When using stubs(), passing null as the "value" of the function to stub, the return value of the stub will not be null, but the first received value.
To use stubs() to stub functions that return null it is possible to do something like this:
Functions\stubs( [ 'function_that_returns_null' => '__return_null' ] );
It works because __return_null is a WP function that Brain Monkey also defines since version 2.0.
Functions that returns callbacks
When using stubs, passing a callable as the "value" of the function to stub, the created stub will be an alias of the given callable, will not return it.
If one want to use stubs to stub a function that returns a callable, a way to do it would be something like this:
Functions\stubs(
   [
        'function_that_returns_a_callback' => function() { 
            return 'the_expected_returned_callback';
        }
   ]
);
but it is probably simpler to use the "usual" when + justReturn:
when('function_that_returns_a_callback')->justReturn('the_expected_returned_callback')

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stubs()

Pre-defined stubs for escaping functions

Pre-defined stubs for translation functions

Gotcha for Functions\stubs

Functions that returns null

Functions that returns callbacks

`stubs()`

Pre-defined stubs for escaping functions

Pre-defined stubs for translation functions

Gotcha for `Functions\stubs`

Functions that returns null

Functions that returns callbacks