MVP-升级版

然而我们在用 基础版mvp 开发的时候, 会遇到这样一些问题:
一般来讲, Activity会持有Presenter的引用, Presenter也会持有Activity的引用, 这就导致了一个问题, 当Activity退出销毁后, 由于 P层仍持有Activity的引用, 导致Activity无法释放, 最终会引起内存泄漏! 怎么办呢? 网上也有许多解决方法, 把这个Activity用弱引用包裹一下 :

public abstract class BasePresenter {
    /**弱引用, 防止内存泄漏*/
    private WeakReference weakReference;

    /**
     * 关联V层和P层
     */
    public void attatchView(V v) {
        weakReference = new WeakReference<>(v);
    }

    /**
     * @return P层和V层是否关联.
     */
    public boolean isViewAttached() {
        return weakReference != null && weakReference.get() != null;
    }

    /**
     * 断开V层和P层
     * 在Acitivity的onDestory()中调用
     */
    public void detachView() {
        if (isViewAttached()) {
            weakReference.clear();
            weakReference = null;
        }
    }
    ...
}

问题二

当P层的逻辑处理完后, 我们就要调用V层来处理UI了, 怎么拿到V层的引用呢? 很简单, 定义一个方法:

    public V getView() {
        return isViewAttached() ? weakReference.get() : null;
    }

但是这个方法有个很让人不安的返回值, 它有可能返回null. 试想一下, 用户打开了一个页面(Activity), 这个页面的P层去网络请求, 也许网络比较卡, 用户没等结果返回, 就退出了该页面.此时网络请求仍在继续.直到好不容易有结果返回的时候, P层调用getView()方法去更新ui, activity弱引用已经释放掉了, getView()就会返回null, 就会发生喜闻乐见的空指针异常!
怎么办? 办法也很简单, 在getView()方法调用的时候, 加一层判断:

if(isViewAttached()){
    getView().xxxx
}

但是,在每一次调用 getView().xxx 时都要判断一下就很麻烦,这个时候 java的动态代理就可以在每次调用 getView().xxx 前都进行一下判断.
最好在BasePresenter类中都处理好了, 调用起来没有后顾之忧就爽多了.
在attatchView()的时候, 生成Activity的代理类, 在每个Activity方法被调用之前判空下 Activity, 如果Activity存在, 就让这个代理类执行更新ui的方法, 如果被销毁了, 就啥都不做. 废话不多说, show code:

public interface IView {
}

========================  Presenter层  ===================================
public abstract class BasePresenter {

    /**弱引用, 防止内存泄漏*/
    private WeakReference weakReference;
    private V mProxyView;

    /**
     * 关联V层和P层
     */
    public void attatchView(V v) {
        weakReference = new WeakReference<>(v);
        MvpViewHandler viewHandler = new MvpViewHandler(weakReference.get());
        mProxyView = (V) Proxy.newProxyInstance(v.getClass().getClassLoader(), v.getClass().getInterfaces(), viewHandler);
    }

    /**
     * @return P层和V层是否关联.
     */
    public boolean isViewAttached() {
        return weakReference != null && weakReference.get() != null;
    }

    /**
     * 断开V层和P层
     */
    public void detachView() {
        if (isViewAttached()) {
            weakReference.clear();
            weakReference = null;
        }
    }

    public V getView() {
        return mProxyView;
    }

    private class MvpViewHandler implements InvocationHandler {
        private IView mvpView;

        MvpViewHandler(IView mvpView) {
            this.mvpView = mvpView;
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            //如果V层没被销毁, 执行V层的方法.
            if (isViewAttached()) {
                return method.invoke(mvpView, args);
            }
            //P层不需要关注V层的返回值
            return null;
        }
    }
}

========================  View层  ===================================
public abstract class BaseActivity extends Activity implements IView{

    V mPresenter;

    @NonNull
    protected abstract  V newPresenter();

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mPresenter=newPresenter();
        mPresenter.attatchView(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mPresenter.detachView();
    }
}

封装以后,以后就可以这样用了

/**
 * 功能:View和Presenter 的协议
 */
public interface Contract {

    interface View extends IView{
        void updateText(String str);
    }

    interface Presenter {
        void changeText();
    }
}

========================  View层  ===================================
public class MyActivity extends BaseActivity implements Contract.View {

    TextView textView;
    Button btn;

    @NonNull
    @Override
    protected MyActivityPresenter newPresenter() {
        return new MyActivityPresenter();
    }

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my);
        textView=findViewById(R.id.tv);
        btn=findViewById(R.id.btnChangeTv);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mPresenter.changeText();
            }
        });
    }

    @Override
    public void updateText(String str) {
        textView.setText(str);
    }
}

========================  Presenter层  ===================================
public class MyActivityPresenter extends BasePresenter implements Contract.Presenter {

    @Override
    public void changeText() {
        String newText=Model.getNewString();
        //动态代理自动帮我们 判断 mView是否为null
        getView().updateText(newText);
    }
}

========================  Model层  ===================================
/**
 * 功能:Model层提供数据
 */
public class Model {

    public static String getNewString(){
        return  "New String...";
    }

}

感谢: Android项目采用Mvp模式开发的一些问题


还有另外的代理方式 ,学习成本很高(难看懂)
引入视图动态代理+一级缓存的MVP

public abstract class AbstractViewCacheProxy implements InvocationHandler {

    /* 如果是weakhashmap。Fragment destroy view就会回收数据了 */
    private final Map mViewCaches = new HashMap<>();
    private WeakReference mView;

    public T proxy(Class viewClass) {
        if (viewClass == null) {
            throw new NullPointerException("Proxy class is NULL, vmProxy is NULL!");
        }
        return (T) Proxy.newProxyInstance(viewClass.getClassLoader(), new Class[]{viewClass}, this);
    }

    void bind(T view) {
        if (view == null) {
            return;
        }
        unBind();
        mView = new WeakReference<>(view);

        for (Method method : mViewCaches.keySet()) {
            invokeMethod(view, method, mViewCaches.get(method));
            LogHelperUtil.i("AbstractViewCacheProxy-bind: ", method.getName());
        }

        view.bindProxyFinish();
    }

    void unBind() {
        if (mView != null) {
            mView.clear();
            mView = null;
        }
    }

    boolean isBind() {
        return mView != null && mView.get() != null;
    }

    void destroy() {
        unBind();
        mViewCaches.clear();
        onDestroy();
    }

    /* 请在此处释放和清理资源 */
    protected abstract void onDestroy();

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (isCacheMethod(method)) {
            mViewCaches.put(method, args);
        }

        if (mView != null && mView.get() != null) {
            return invokeMethod(mView.get(), method, args);
        }
        return null;
    }

    private boolean isCacheMethod(Method method) {
        CacheMethod cacheMethod = method.getAnnotation(CacheMethod.class);
        return cacheMethod != null && cacheMethod.isCached();
    }

    private Object invokeMethod(Object view, Method method, Object[] args) {
        if (view == null || method == null) {
            return null;
        }
        try {
            return method.invoke(view, args);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        return null;
    }
}

来自 : 浅谈Android中的MVP与动态代理

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