自从上文《看 Laravel 源代码了解 ServiceProvider 的加载》，我们知道 Application (or Container) 充当 Laravel 的容器，基本把所有 Laravel 核心的功能纳入这个容器里了。
我们今天来看看这个 Application / Container 到底是什么东西？
了解 Container 之前，我们需要先简单说说 Inversion of Control (控制反转) 的原理。
Inversion of Control
要知道什么是 Inversion of Control 之前，我们最好先了解一个原则：
依赖倒转原则 (Dependence Inversion Priciple, DIP)提倡：
- 高层模块不应该依赖底层模块。两个都应该依赖抽象
- 抽象不应该依赖细节，细节应该依赖抽象
- 针对接口编程，不要针对实现编程
在编程时，我们对代码进行模块化开发，它们之间避免不了有依赖，如模块 A 依赖模块 B，那么根据 DIP，模块 A 应该依赖模块 B 的接口，而不应该依赖模块 B 的实现。
下面我们举个例子。
我们需要一个 Logger 功能，将系统 log 输出到文件中。我们可以可以这么写：
class LogToFile {
public function execute($message) {
info('log the message to a file :'.$message);
}
}
在需要的地方直接调用：
class UseLogger {
protected $logger;
public function __construct(LogToFile $logger) {
$this->logger = $logger;
}
public function show() {
$user = 'yemeishu';
$this->logger->execute($user);
}
}
写个测试用例：
$useLogger = new UseLogger(new LogToFile());
$useLogger->show();
如果这时候我们需要将 log 输出到钉钉上，我们重新写一个 LogToDD 类：
class LogToDD {
public function execute($message) {
info('log the message to 钉钉 :'.$message);
}
}
这时候，我们还需要修改使用端 (UseLogger) 代码，让它引入 LogToDD 类：
class UseLogger {
protected $logger;
public function __construct(LogToDD $logger) {
$this->logger = $logger;
}
public function show() {
$user = 'yemeishu';
$this->logger->execute($user);
}
}
其实到这，你就能「嗅出」坏代码来了：
假如我使用端特别多，那就意味着每个地方我都要做引入修改。
根据 DIP 原则，我们应该面向接口开发。让使用端依赖接口，而不是实现。所以我们创建一个接口：
interface Logger {
public function execute($message);
}
然后让 LogToFile 和 LogToDD 作为 Logger 的实现：
class LogToFile implements Logger {
public function execute($message) {
info('log the message to a file :'.$message);
}
}
class LogToDD implements Logger {
public function execute($message) {
info('log the message to 钉钉 :'.$message);
}
}
这样我们在使用端时，直接引入 Logger 接口，让代码剥离具体实现。
class UseLogger {
protected $logger;
public function __construct(Logger $logger) {
$this->logger = $logger;
}
public function show() {
$user = 'yemeishu';
$this->logger->execute($user);
}
}
这样就可以保证，无论是使用文件保存，还是下发到钉钉上，都可以不用去改代码了，只需要再真正调用的时候，随着业务需要自行选择。如测试：
$useLogger1 = new UseLogger(new LogToFile());
$useLogger1->show();
$useLogger2 = new UseLogger(new LogToDD());
$useLogger2->show();
结果：
但这里有个问题，最后在实例化使用时，还是要「硬编码」的方式 new 我们的实现类 (LogToDD or LogToFile)。
那有没有办法更进一步把最后的 new LogToDD() 也控制反转了呢？
绑定的实现
这里我们把实现类绑定在 interface 或者标识 key 上，只要解析这个 interface 或者 key，就可以拿到我们的实现类。
我们来写一个简单的类来达到绑定和解析的功能：
class SimpleContainer {
// 用于存储所有绑定 key-value
protected static $container = [];
public static function bind($name, Callable $resolver) {
static::$container[$name] = $resolver;
}
public static function make($name) {
if(isset(static::$container[$name])){
$resolver = static::$container[$name] ;
return $resolver();
}
throw new Exception("Binding does not exist in container");
}
}
我们可以测试下：
SimpleContainer::bind(Logger::class, function () {
return new LogToFile();
});
$useLogger3 = new UseLogger(SimpleContainer::make(Logger::class));
$useLogger3->show();
只要有一处绑定了 Logger 和 LogToFile 的关系，就可以在任何需要调用的地方直接解析引用了。
也就意味着，通过这种方法，在所有编码的地方都是引入「interface」，而不是实现类。彻底实现 DPI 原则。
当我们把所有这种绑定聚集在一起，就构成了我们今天的主题内容：「Container」—— illuminate / container。
Laravel Container
在研究 Laravel Container 之前，我们根据上面的例子，使用 Container，看怎么方便实现。
$container = new Container();
$container->bind(Logger::class, LogToFile::class);
$useLogger4 = new UseLogger($container->make(Logger::class));
$useLogger4->show();
达到一样的效果
[2018-05-19 15:36:30] testing.INFO: log the message to a file :yemeishu
注：在 Laravel 开发时，我们会把这个绑定写在 APPServiceProvider 的 boot 或者 register 中，或者其他的 ServiceProvider 上也行。
结合上一篇文章《看 Laravel 源代码了解 ServiceProvider 的加载》和以上的原理的讲解。我们对 Container 的使用，已经不陌生了。
接下来就可以看看 Container 的源代码了。
Container 源码解析
从上文可以知道，Container 的作用主要有两个，一个是绑定，另个一个是解析。
绑定
我们看看主要有哪些绑定类型：
- 绑定一个单例
- 绑定实例
- 绑定接口到实现
- 绑定初始数据
- 情境绑定
- tag 标记绑定
下面我们根据这些类型进行分析：
1. 绑定一个单例
public function singleton($abstract, $concrete = null)
{
$this->bind($abstract, $concrete, true);
}
主要利用参数 $share = true 来标记此时绑定为一个单例。
2. 绑定实例
public function instance($abstract, $instance)
{
$this->removeAbstractAlias($abstract);
$isBound = $this->bound($abstract);
unset($this->aliases[$abstract]);
// We'll check to determine if this type has been bound before, and if it has
// we will fire the rebound callbacks registered with the container and it
// can be updated with consuming classes that have gotten resolved here.
$this->instances[$abstract] = $instance;
if ($isBound) {
$this->rebound($abstract);
}
return $instance;
}
绑定实例，主要是将 [$abstract, $instance]存储进数组 $instances 中。
3. tag 标记绑定
/**
* Assign a set of tags to a given binding.
*
* @param array|string $abstracts
* @param array|mixed ...$tags
* @return void
*/
public function tag($abstracts, $tags)
{
$tags = is_array($tags) ? $tags : array_slice(func_get_args(), 1);
foreach ($tags as $tag) {
if (! isset($this->tags[$tag])) {
$this->tags[$tag] = [];
}
foreach ((array) $abstracts as $abstract) {
$this->tags[$tag][] = $abstract;
}
}
}
这个挺好理解，主要是将 $abstracts 数组放在同一组标签下，最后可以通过 tag，解析这一组 $abstracts。
4. 绑定
public function bind($abstract, $concrete = null, $shared = false)
{
// If no concrete type was given, we will simply set the concrete type to the
// abstract type. After that, the concrete type to be registered as shared
// without being forced to state their classes in both of the parameters.
$this->dropStaleInstances($abstract);
// 如果传入的实现为空，则绑定 $concrete 自己
if (is_null($concrete)) {
$concrete = $abstract;
}
// If the factory is not a Closure, it means it is just a class name which is
// bound into this container to the abstract type and we will just wrap it
// up inside its own Closure to give us more convenience when extending.
// 目的是将 $concrete 转成闭包函数
if (! $concrete instanceof Closure) {
$concrete = $this->getClosure($abstract, $concrete);
}
// 存储到 $bindings 数组中，如果 $shared = true, 则表示绑定单例
$this->bindings[$abstract] = compact('concrete', 'shared');
// If the abstract type was already resolved in this container we'll fire the
// rebound listener so that any objects which have already gotten resolved
// can have their copy of the object updated via the listener callbacks.
if ($this->resolved($abstract)) {
$this->rebound($abstract);
}
}
解析
/**
* Resolve the given type from the container.
*
* @param string $abstract
* @param array $parameters
* @return mixed
*/
public function make($abstract, array $parameters = [])
{
return $this->resolve($abstract, $parameters);
}
/**
* Resolve the given type from the container.
*
* @param string $abstract
* @param array $parameters
* @return mixed
*/
protected function resolve($abstract, $parameters = [])
{
$abstract = $this->getAlias($abstract);
$needsContextualBuild = ! empty($parameters) || ! is_null(
$this->getContextualConcrete($abstract)
);
// If an instance of the type is currently being managed as a singleton we'll
// just return an existing instance instead of instantiating new instances
// so the developer can keep using the same objects instance every time.
if (isset($this->instances[$abstract]) && ! $needsContextualBuild) {
return $this->instances[$abstract];
}
$this->with[] = $parameters;
$concrete = $this->getConcrete($abstract);
// We're ready to instantiate an instance of the concrete type registered for
// the binding. This will instantiate the types, as well as resolve any of
// its "nested" dependencies recursively until all have gotten resolved.
if ($this->isBuildable($concrete, $abstract)) {
$object = $this->build($concrete);
} else {
$object = $this->make($concrete);
}
// If we defined any extenders for this type, we'll need to spin through them
// and apply them to the object being built. This allows for the extension
// of services, such as changing configuration or decorating the object.
foreach ($this->getExtenders($abstract) as $extender) {
$object = $extender($object, $this);
}
// If the requested type is registered as a singleton we'll want to cache off
// the instances in "memory" so we can return it later without creating an
// entirely new instance of an object on each subsequent request for it.
if ($this->isShared($abstract) && ! $needsContextualBuild) {
$this->instances[$abstract] = $object;
}
$this->fireResolvingCallbacks($abstract, $object);
// Before returning, we will also set the resolved flag to "true" and pop off
// the parameter overrides for this build. After those two things are done
// we will be ready to return back the fully constructed class instance.
$this->resolved[$abstract] = true;
array_pop($this->with);
return $object;
}
我们一步步来分析该「解析」函数：
$needsContextualBuild = ! empty($parameters) || ! is_null(
$this->getContextualConcrete($abstract)
);
该方法主要是区分，解析的对象是否有参数，如果有参数，还需要对参数做进一步的分析，因为传入的参数，也可能是依赖注入的，所以还需要对传入的参数进行解析；这个后面再分析。
// If an instance of the type is currently being managed as a singleton we'll
// just return an existing instance instead of instantiating new instances
// so the developer can keep using the same objects instance every time.
if (isset($this->instances[$abstract]) && ! $needsContextualBuild) {
return $this->instances[$abstract];
}
如果是绑定的单例，并且不需要上面的参数依赖。我们就可以直接返回 $this->instances[$abstract]。
$concrete = $this->getConcrete($abstract);
...
/**
* Get the concrete type for a given abstract.
*
* @param string $abstract
* @return mixed $concrete
*/
protected function getConcrete($abstract)
{
if (! is_null($concrete = $this->getContextualConcrete($abstract))) {
return $concrete;
}
// If we don't have a registered resolver or concrete for the type, we'll just
// assume each type is a concrete name and will attempt to resolve it as is
// since the container should be able to resolve concretes automatically.
if (isset($this->bindings[$abstract])) {
return $this->bindings[$abstract]['concrete'];
}
return $abstract;
}
这一步主要是先从绑定的上下文找，是不是可以找到绑定类；如果没有，则再从 $bindings[] 中找关联的实现类；最后还没有找到的话，就直接返回 $abstract 本身。
// We're ready to instantiate an instance of the concrete type registered for
// the binding. This will instantiate the types, as well as resolve any of
// its "nested" dependencies recursively until all have gotten resolved.
if ($this->isBuildable($concrete, $abstract)) {
$object = $this->build($concrete);
} else {
$object = $this->make($concrete);
}
...
/**
* Determine if the given concrete is buildable.
*
* @param mixed $concrete
* @param string $abstract
* @return bool
*/
protected function isBuildable($concrete, $abstract)
{
return $concrete === $abstract || $concrete instanceof Closure;
}
这个比较好理解，如果之前找到的 $concrete 返回的是 $abstract 值，或者 $concrete 是个闭包，则执行 $this->build($concrete)，否则，表示存在嵌套依赖的情况，则采用递归的方法执行 $this->make($concrete)，直到所有的都解析完为止。
$this->build($concrete)
/**
* Instantiate a concrete instance of the given type.
*
* @param string $concrete
* @return mixed
*
* @throws \Illuminate\Contracts\Container\BindingResolutionException
*/
public function build($concrete)
{
// If the concrete type is actually a Closure, we will just execute it and
// hand back the results of the functions, which allows functions to be
// used as resolvers for more fine-tuned resolution of these objects.
// 如果传入的是闭包，则直接执行闭包函数，返回结果
if ($concrete instanceof Closure) {
return $concrete($this, $this->getLastParameterOverride());
}
// 利用反射机制，解析该类。
$reflector = new ReflectionClass($concrete);
// If the type is not instantiable, the developer is attempting to resolve
// an abstract type such as an Interface of Abstract Class and there is
// no binding registered for the abstractions so we need to bail out.
if (! $reflector->isInstantiable()) {
return $this->notInstantiable($concrete);
}
$this->buildStack[] = $concrete;
// 获取构造函数
$constructor = $reflector->getConstructor();
// If there are no constructors, that means there are no dependencies then
// we can just resolve the instances of the objects right away, without
// resolving any other types or dependencies out of these containers.
// 如果没有构造函数，则表明没有传入参数，也就意味着不需要做对应的上下文依赖解析。
if (is_null($constructor)) {
// 将 build 过程的内容 pop，然后直接构造对象输出。
array_pop($this->buildStack);
return new $concrete;
}
// 获取构造函数的参数
$dependencies = $constructor->getParameters();
// Once we have all the constructor's parameters we can create each of the
// dependency instances and then use the reflection instances to make a
// new instance of this class, injecting the created dependencies in.
// 解析出所有上下文依赖对象，带入函数，构造对象输出
$instances = $this->resolveDependencies(
$dependencies
);
array_pop($this->buildStack);
return $reflector->newInstanceArgs($instances);
}
此方法分成两个分支：如果 $concrete instanceof Closure，则直接调用闭包函数，返回结果：$concrete()；另一种分支就是，传入的就是一个 $concrete === $abstract === 类名，通过反射方法，解析并 new 该类。
具体解释看上面的注释。 ReflectionClass 类的使用，具体参考：https://php.golaravel.com/class.reflectionclass.html
代码往下看：
// If we defined any extenders for this type, we'll need to spin through them
// and apply them to the object being built. This allows for the extension
// of services, such as changing configuration or decorating the object.
foreach ($this->getExtenders($abstract) as $extender) {
$object = $extender($object, $this);
}
// If the requested type is registered as a singleton we'll want to cache off
// the instances in "memory" so we can return it later without creating an
// entirely new instance of an object on each subsequent request for it.
if ($this->isShared($abstract) && ! $needsContextualBuild) {
$this->instances[$abstract] = $object;
}
$this->fireResolvingCallbacks($abstract, $object);
// Before returning, we will also set the resolved flag to "true" and pop off
// the parameter overrides for this build. After those two things are done
// we will be ready to return back the fully constructed class instance.
$this->resolved[$abstract] = true;
array_pop($this->with);
return $object;
这些就比较好理解了。主要判断是否存在扩展，则相应扩展功能；如果是绑定单例，则将解析的结果存到 $this->instances 数组中；最后做一些解析的「善后工作」。
最后我们再看看 tag 标签的解析：
/**
* Resolve all of the bindings for a given tag.
*
* @param string $tag
* @return array
*/
public function tagged($tag)
{
$results = [];
if (isset($this->tags[$tag])) {
foreach ($this->tags[$tag] as $abstract) {
$results[] = $this->make($abstract);
}
}
return $results;
}
现在看这个就更好理解了，如果传入的 tag 标签值存在 tags 数组中，则遍历所有 $abstract, 一一解析，将结果保存数组输出。
总结
虽然 Container 核心的内容我们了解了，但还有很多细节值得我们接着研究，如：$alias 相关的，事件相关的，扩展相关的。
最后收尾，推荐大家看看这个 Container：silexphp/Pimple
A small PHP 5.3 dependency injection container https://pimple.symfony.com
未完待续