Kotlin 委派(ReadOnlyProperty) 仿造viewModels 写一个viewBindings通过委派机制和Kotlin的函数扩展，仿造viewModels 写一个viewBindings功能，辅助创建ViewBinding,同时在onDestroyView的回收ViewBinding

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我是靠谱客的博主繁荣钻石，最近开发中收集的这篇文章主要介绍Kotlin 委派(ReadOnlyProperty) 仿造viewModels 写一个viewBindings通过委派机制和Kotlin的函数扩展，仿造viewModels 写一个viewBindings功能，辅助创建ViewBinding,同时在onDestroyView的回收ViewBinding，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

通过委派机制和Kotlin的函数扩展，仿造viewModels 写一个viewBindings功能，辅助创建ViewBinding,同时在onDestroyView的回收ViewBinding

先看下怎么使用

// 一个简陋的MVP
// DemoFragment 作为P的角色
// DemoViewModel 作为M的角色
// FragmentDemoBinding 作为View的角色
// 不需要继承BaseFragmentxxx，没有泛型约束，想用就用
class DemoFragment: Fragment(R.layout.fragment_demo) {

	//委托创建ViewModel
	private val viewModels by activityViewModels<DemoViewModel>()

	//仿造 activityViewModels()方法 委托创建ViewBinding
	// 会在Fragment生命周期 onDestroyView的时候销毁ViewBinding
	private val viewBindings by viewBindings<FragmentDemoBinding>()
    
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        //可以调用viewBindings 的方法了
        viewBindings...
    }

}

看下怎么实现的。

Fragment.viewBindings<ViewBinding>()方法的实现

/**
 * 对Fragment 扩展viewBindings方法
 * 懒加载创建：
 * 自动判断root View 已经创建了。
 * 那么使用ViewBinding::bind方法
 * 如果没有则使用ViewBinding::inflater方法
 *
 * 自动销毁：
 * 监听生命周期onDestroyView事件，
 * 把ViewBinding 置为 null
 * 防止内存泄漏
 */
@MainThread
inline fun <reified T : ViewBinding> Fragment.viewBindings(
    crossinline viewBindingFactory: (View?) -> ViewBindingFactory<T> = {
    //这做了下判断，如果当前Fragment已经有rootView了，那么就通过ViewBinding::bind的方法获取ViewBinding,反之则通过inflater方法实现
        if(it == null) DefaultInflateViewBindingFactory(layoutInflater, T::class) else DefaultBindViewBindingFactory(it, T::class)
    }
): ReadOnlyProperty<Fragment, T> = object : ReadOnlyProperty<Fragment, T> {

    private var binding: T? = null

    init {

        viewLifecycleOwnerLiveData.observe(this@viewBindings, Observer { viewLifecycleOwner ->
            viewLifecycleOwner.lifecycle.addObserver(object : DefaultLifecycleObserver {
                //监听Fragment onDestroyView
                override fun onDestroy(owner: LifecycleOwner) {
                    (binding as? ViewDataBinding)?.unbind()
                    binding = null
                    viewLifecycleOwner.lifecycle.removeObserver(this)
                }
            })
        })
    }

    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        binding?.let { return it }

        // 操作ViewBinding 必须在onCreateView之后，所以给一个错误提示
        val viewLifecycleOwner = try {
            thisRef.viewLifecycleOwner
        } catch (e: IllegalStateException) {
            error("Should not attempt to get bindings when Fragment views haven't been created yet. The fragment has not called onCreateView() at this point.")
        }
        //操作ViewBinding要在onDestroyView() super方法之前，不然提示错误
        if (!viewLifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.INITIALIZED)) {
            error("Should not attempt to get bindings when Fragment views are destroyed. The fragment has already called onDestroyView() at this point.")
        }

        return viewBindingFactory(thisRef.view).getViewBindings().also { viewBinding ->
            if (viewBinding is ViewDataBinding) {
                viewBinding.lifecycleOwner = viewLifecycleOwner
            }
            this.binding = viewBinding
        }
    }
}

ViewBindingFactory<ViewBinding> 是个什么东西

interface ViewBindingFactory<T: ViewBinding> {

    fun getViewBindings(): T
}

// 通过inflate反射方法获取ViewBinding
@Suppress("UNCHECKED_CAST")
class DefaultInflateViewBindingFactory<T: ViewBinding>(layoutInflater: LayoutInflater, clazz: KClass<T>): ViewBindingFactory<T> {

    private val vb: T = clazz.java.getMethod("inflate", LayoutInflater::class.java).invoke(null, layoutInflater) as T

    override fun getViewBindings(): T {
        return vb
    }
}

// 通过bind反射方法获取ViewBinding
@Suppress("UNCHECKED_CAST")
class DefaultBindViewBindingFactory<T: ViewBinding>(view: View, clazz: KClass<T>): ViewBindingFactory<T> {

    private val vb: T = clazz.java.getMethod("bind", View::class.java).invoke(null, view) as T

    override fun getViewBindings(): T {
        return vb
    }
}