PHP中匿名函数的参数

Question

I am new to the anonymous functions world.

  $this->app->singleton(VideoServiceInterface::class, function($app) {
      statement1;
      statement2;
      .........       
  });

I came across the above code snippet somewhere. I didn't really understand where the $app parameter came from and how did the coder pass it to the anonymous function?

Answer 1

well, first , you need to think about anonymous function as a gate to execute some statements in another context .

it's a kind of reversing the function declaration -so to speak-.

for instance , here is the traditional way to declare/call a function :

// Declare a function .
function foo($parameter)
{
    // here we are executing some statements

}

// Calling the function
echo foo();

in the anonymous function case , we are calling the function somewhere , and moving the responsibility of declaring the function to the client user .

for example, you are writing a new package , and in a specific peace of it , you don't want to execute your package as a concrete object , giving the client user more power to declare and execute some statement to suit his needs .

function foo($parameter, $callback)
{
    echo $parameter . "\n";

    // here we are calling the function
    // leaving the user free to declare it
    // to suit his needs
    $callback($parameter);
}

// here the declaration of the function
echo foo('Yello!', function ($parameter) {
    echo substr($parameter, 0, 3);
});

in your example , if you browsed the source code of $this->app->singleton method which is belongs to app object , you will found a function -which is often called a callback - called there somewhere.

Answer 2

$app is just an argument to access what is passed to the function, you could use $a or $b or whatever just like a normal user-defined function:

  $this->app->singleton(VideoServiceInterface::class, function($variable) {
      //do something with $variable
  });

The singleton() method accepts an argument of type callable which is a string function name or an anonymous function.

The singleton() method will pass something to this function that the function can use as $app in your example or $variable above.