1.1.2. Constructor Injection

When we use the Container to instantiate a new object, we often need to inject (i.e., set) constructor parameter values in various ways.

We can define default values for constructor parameters using the $di->params array on the Container before locking it.

Let's look at a class that takes some constructor parameters:

namespace Vendor\Package;

class Example
{
    protected $foo;
    protected $bar;
    public function __construct($foo, $bar)
    {
        $this->foo = $foo;
        $this->bar = $bar;
    }
}

If we were to try to create an object using $di->newInstance('Vendor\Package\Example'), the instantiation would fail. The $foo and $bar params are required, and the Container does not know what to use for that value.

To remedy this, we tell the Container what values to use for each Vendor\Package\Example constructor parameter by name using the $di->params array:

$di->params['Vendor\Package\Example']['foo'] = 'foo_value';
$di->params['Vendor\Package\Example']['bar'] = 'bar_value';

We can also specify by position:

$di->params['Vendor\Package\Example'][0] = 'foo_value';
$di->params['Vendor\Package\Example'][1] = 'bar_value';

Once all the params are set, we create an object with $di->newInstance('Vendor\Package\Example'), the instantiation will work correctly. Each time we create an instance through the Container, it will apply the $di->params values for the matching class.

N.b.: If you try to access $params after calling newInstance() (or after locking the Container using the lock() method) the Container will throw an exception. This is to prevent modifying the params after objects have been created. Thus, be sure to set up all params for all objects before creating an object.