Dagger2下的ViewModel

ゞ浴缸里的玫瑰 2023-01-17 09:21 95阅读 0赞

# 前言 #

本文需要一定的储备知识,可参阅博主其他文章:  
[Dagger2 生成类初探][Dagger2]  
[viewModel源码分析][viewModel]

假设我们想我们的`ViewModel`注入一些`Dagger2`所提供的依赖怎么办?直接给`ViewModel`构造函数注入?  
如下代码

//MyViewModel.java
    public class MyViewModel extends ViewModel {
        
        @Inject
        @Named("ActivityStr")
        String applicationMsg;
    
        @Inject
        public MyViewModel() {
        
    
        }
    
    }
    //MainActivity.kt
    class MainActivity : AppCompatActivity() {
        
    
        @Inject
        @Named("ActivityStr")
        lateinit var applicationMsg: String
        //直接注入
        @Inject
        lateinit var viewModel: MyViewModel
    
        override fun onCreate(savedInstanceState: Bundle?) {
        
            AndroidInjection.inject(this);
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
    
         
        }
    }

上面的代码没有编译问题,并且`viewModel`也会被注入`ActivityStr`字符串,但是`viewmodel`会丢失状态!!  
上面的代码在旋转屏幕时,`viewmodel`会被重建,因为`viewmodel`没有放入`activity.viewModelStore`.

如何将`viewmoel`放入`activity.viewModelStore`?仅能通过`ViewModelProvider`构造的`viewmodel`才会放入.

*  如下代码:

override fun onCreate(savedInstanceState: Bundle?) {
        
            AndroidInjection.inject(this);
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
    
    
            val viewModelProvider =
                ViewModelProvider(viewModelStore, ViewModelProvider.NewInstanceFactory())
    		
    		//用ViewModelProvider创建的viewmodel会放入viewModelStore.从而保证回收时viewmodel不会重建.
            val viewmodel = viewModelProvider.get(MyViewModel::class.java)
        }

所以我们可以通过自定义一个`ViewModelProvider.Factory`给`viewModelProvider`,然后通过自定义`Factory`获取`dagger`生成的`viewmodel`返回即可.

//MyViewModelFactory.java
    public class MyViewModelFactory implements ViewModelProvider.Factory {
        
    
    
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        
            return null;
        }
    }

问题又来了`MyViewModelFactory`又怎么拿到对应的`dagger`生成`viewmodel`?  
在这种情况下仅能将自己也交付给`dagger`

@Qualifier
    @Documented
    @Retention(RUNTIME)
    public @interface ViewModuleScope {
        
    
    }
    
    @Singleton
    public class MyViewModelFactory implements ViewModelProvider.Factory {
        
    
        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        
            //从集合中返回一个
            return (T) viewModelsMap.get(modelClass).get();
        }
    
    
        Map<Class<?>, Provider<ViewModel>> viewModelsMap;
    
    //    /**
    //     * 构造函数要求dagger2传入一个map.
    //     * key为viewmodel类名
    //     */
        @Inject
        public MyViewModelFactory(
                @ViewModuleScope
                Map<Class<?>, Provider<ViewModel>> viewModelsMap) {
        
            this.viewModelsMap = viewModelsMap;
        }
    
    
    }

问题又来了`dagger`怎么构造`Map<Class<? extends ViewModel>, Provider<ViewModel>>`集合对象?

我们只需要使用`Dagger2`的多重绑定即可

@Module
    public abstract class ViewModules {
        
        @Binds
        abstract ViewModelProvider.Factory bindViewModelFactory(MyViewModelFactory factory);
    
        @Binds
        @IntoMap
        @ViewModuleScope
        @ClassKey(MyViewModel.class)
        abstract ViewModel provideViewModel(MyViewModel myViewModel);
    
        @Binds
        @IntoMap
        @ViewModuleScope
        @ClassKey(MyViewModel2.class)
        abstract ViewModel provideViewModel2(MyViewModel2 myViewModel);
    
    }

实际使用

class MainActivity : AppCompatActivity() {
        
    
      
        @Inject
        lateinit var factory: ViewModelProvider.Factory
    
        override fun onCreate(savedInstanceState: Bundle?) {
        
            AndroidInjection.inject(this);
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
    
            val viewModelProvider = ViewModelProvider(this, factory)
            val myViewModel = viewModelProvider[MyViewModel::class.java]
            Log.e("MainActivity","viewModel ${myViewModel.hashCode()}")
        }
    }

输出:

E/MainActivity: viewModel 197683938
    -----------------旋转屏幕----------
     E/MainActivity: viewModel 197683938

另外关于网上流传的自定义`MapKey`,和上面原理一样就不在过多说明.不过需要注意添加下面的依赖

compileOnly "com.google.auto.value:auto-value-annotations:1.8.1"
     kapt "com.google.auto.value:auto-value:1.8.1"

[Dagger2]: https://fanmingyi.blog.csdn.net/article/details/116189368
[viewModel]: https://fanmingyi.blog.csdn.net/article/details/116137385