ctory) {
assert factory != null;
this.factory = factory;
}
@SuppressWarnings("unchecked") // cast only happens when result comes from the factory
@Override
public T get() {
// double-check idiom from EJ2: Item 71
Object result = instance;
if (result == UNINITIALIZED) {
synchronized (this) {
result = instance;
if (result == UNINITIALIZED) {
instance = result = factory.get();
}
}
}
return (T) result;
}
/** Returns a new scoped provider for the given factory. */
public static <T> Provider<T> create(Factory<T> factory) {
if (factory == null) {
throw new NullPointerException();
}
return new ScopedProvider<T>(factory);
}
}
我们看下源码,这里的create是创建了一个ScopeProvider的对象,并将工厂传入,当这个ScopeProvider去get的时候,内部有个单例来维持这个对象,这就是为什么我们自定义注解是个单例的秘密。
接下来看看神器的注解是怎么起作用的。
public final class BaseActivity_MembersInjector implements MembersInjector<BaseActivity> {
private final MembersInjector<FragmentActivity> supertypeInjector;
private final Provider<Context> contextProvider;
public BaseActivity_MembersInjector(MembersInjector<FragmentActivity> supertypeInjector, Provider<Context> contextProvider) {
assert supertypeInjector != null;
this.supertypeInjector = supertypeInjector;
assert contextProvider != null;
this.contextProvider = contextProvider;
}
public void injectMembers(BaseActivity instance) {
if(instance == null) {
throw new NullPointerException("Cannot inject members into a null reference");
} else {
this.supertypeInjector.injectMembers(instance);
instance.context = (Context)this.contextProvider.get();
}
}
public static MembersInjector<BaseActivity> create(MembersInjector<FragmentActivity> supertypeInjector, Provider<Context> contextProvider) {
return new BaseActivity_MembersInjector(supertypeInjector, contextProvider);
}
}
在我们注入这个对象后,injectMembers方法中,写明了BaseActivity中的context对象是从ContextProvider的get方法中得到,根据代码也可以看到,这部分依赖是从ApplicationComponent中的provideContext方法取得。
再看看Lazy Load:
public final class DoubleCheckLazy<T> implements Lazy<T> {
private static final Object UNINITIALIZED = new Object();
private final Provider<T> provider;
private volatile Object instance = UNINITIALIZED;
private DoubleCheckLazy(Provider<T> provider) {
assert provider != null;
this.provider = provider;
}
@SuppressWarnings("unchecked") // cast only happens when result comes from the factory
@Override
public T get() {
// to suppress it.
Object result = instance;
if (result == UNINITIALIZED) {
synchronized (this) {
result = instance;
if (result == UNINITIALIZED) {
instance = result = provider.get();
}
}
}
return (T) result;
}
public static <T> Lazy<T> create(Provider<T> provider) {
if (provider == null) {
throw new NullPointerException();
}
return new DoubleCheckLazy<T>(provider);
}
}
toasterLazy加载的时候是使用DoubleCheckLazy.create(this.toasterLazyProvider)来进行初始化。而内部是个单例,只有在get时才会初始化。
至此,Dagger2的主要源码就差不多理解了。
二.使用分析
Dagger2很优雅,优雅到你可以特别轻松的调试它,以为他和你手写的代码几乎一模一样,又优雅到几乎所有的错误都从编译器暴露出。
与RoboGuice的反射不同,生成代码必然会导致方法数的增加。但是,我们可以看到生成的代码数量并不多,而且在实际应用过程中也可以看出,确实影响不大,这个可以大家在以后的使用中慢慢体会。
性能上Dagger2会优于RoboGuice,尤其是天生支持懒加载,但是在易用性上,RoboGuice更容易上手和理解,并且针对Android做了很多通用依赖,为项目开发