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Android Jetpack组件Lifecycle

Android技术栈 人气:0

1.为什么需要Lifecycle

在应用开发中,处理Activity或者Fragment组件的生命周期相关代码是必不可免的; 官方文档中举了一个例子,这里简化一下,在Activity中写一个监听,在Activity的不同生命周期方法中调用这个监听

public class MainActivity extends AppCompatActivity {
    private MyListener myListener;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myListener = new MyListener(MainActivity.this);
    }
    @Override
    protected void onStart() {
        super.onStart();
        myListener.start();
    }
    @Override
    protected void onStop() {
        super.onStop();
        myListener.stop();
    }
}
class MyListener {
    public MyListener(Context context) {
    ...
    }
    void start() {
    ...
    }
    void stop() {
    ...
    }
}

再举个MVP中常见的情况,如下所示。

public class MainActivity extends AppCompatActivity {
    private MyPresenter myPresenter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myPresenter = new MyPresenter();
    }
    @Override
    protected void onResume() {
        super.onResume();
        myPresenter.onResume();
    }
    @Override
    protected void onPause() {
        super.onPause();
        myPresenter.onPause();
    }
}
class MyPresenter{
    void onResume() {
    ...
    }
    void onPause() {
    ...
    }
}

这两个例子的写法已经很普遍了,实现起来也不难; 但实际开发中,可能会有多个组件在Activity的生命周期中进行回调; 这样Activity的生命周期的方法中可能就需要放大量的代码,这就使得它们难以维护

还有一个问题是,如果我们在组件中做了耗时操作(比如在onStart方法),这种写法就无法保证组件在Activity或者Fragment停止之前完成启动

因此我们需要一个能管理Activity和Fragment的生命周期的库,这个库就是Lifecycle

2.如何使用Lifecycle

分别来介绍下依赖Lifecycle库和Lifecycle基本用法

2.1 依赖Lifecycle库

官网给出的依赖代码如下所示:

dependencies {
    def lifecycle_version = "2.0.0"

    // ViewModel and LiveData
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
    // alternatively - just ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version" // For Kotlin use lifecycle-viewmodel-ktx
    // alternatively - just LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    // alternatively - Lifecycles only (no ViewModel or LiveData). Some UI
    //     AndroidX libraries use this lightweight import for Lifecycle
    implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"

    annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" // For Kotlin use kapt instead of annotationProcessor
    // alternately - if using Java8, use the following instead of lifecycle-compiler
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

    // optional - ReactiveStreams support for LiveData
    implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version" // For Kotlin use lifecycle-reactivestreams-ktx

    // optional - Test helpers for LiveData
    testImplementation "androidx.arch.core:core-testing:$lifecycle_version"
}

官网用的是AndroidX,因为使用AndroidX,可能会产生一些迁移的问题,这里的举例就不使用AndroidX,而是使用lifecycleandroid.arch.lifecycle库,如下所示。

dependencies {
    def lifecycle_version = "1.1.1"

    // 包含ViewModel和LiveData
    implementation "android.arch.lifecycle:extensions:$lifecycle_version"
    // 仅仅包含ViewModel
    implementation "android.arch.lifecycle:viewmodel:$lifecycle_version" // For Kotlin use viewmodel-ktx
    // 仅仅包含LiveData
    implementation "android.arch.lifecycle:livedata:$lifecycle_version"
    // 仅仅包含Lifecycles
    implementation "android.arch.lifecycle:runtime:$lifecycle_version"

    annotationProcessor "android.arch.lifecycle:compiler:$lifecycle_version" // For Kotlin use kapt instead of annotationProcessor
    // 如果用Java8, 用于替代compiler
    implementation "android.arch.lifecycle:common-java8:$lifecycle_version"

    // 可选,ReactiveStreams对LiveData的支持
    implementation "android.arch.lifecycle:reactivestreams:$lifecycle_version"

    // 可选,LiveData的测试
    testImplementation "android.arch.core:core-testing:$lifecycle_version"
}

实际上我们不需要全部把这些代码全写进build.gralde进去(当然全写进去也不会有什么错),因为Gradle默认是支持依赖传递的, 我们直接添加如下依赖就可以满足日常的工作,如果缺少哪个库,再去单独添加就好了。

implementation "android.arch.lifecycle:extensions:1.1.1"

添加这一句代码就依赖了如下的库。

2.2 Lifecycle基本用法

先不谈Activity和Fragment中如何使用,先举一个Lifecycle的简单例子。

public class MyObserver implements LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void connectListener() {
        ...
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void disconnectListener() {
        ...
    }
}
myLifecycleOwner.getLifecycle().addObserver(new MyObserver());//1

新建一个MyObserver类,它实现了LifecycleObserver接口,说明MyObserver成为了一个Lifecycle的观察者。 然后在注释1处将MyObserver添加到LifecycleOwner中。LifecycleOwner是一个接口,其内部只有一个方法getLifecycle(),getLifecycle方法用于获取Lifecycle,这样就可以将MyObserver添加到Lifecycle中,当Lifecycle的生命周期发生变化时,MyObserver就会观察到,或者说是感知到。

如果使用是Java8 ,那么可以使用DefaultLifecycleObserver来替代LifecycleObserver:

class MyObserver implements DefaultLifecycleObserver {
     @Override
     public void onCreate(LifecycleOwner owner) {
         ...
     }
 }

除此之外,不要忘了在build.gradle添加 "androidx.lifecycle:common-java8:<version>"

3.Lifecycle应用举例

应用举例准备两个示例,一个是在Activity中使用,一个是在第一小节的MVP例子上进行改进。

3.1 Activity中使用

package com.example.lifecycledemo1;
import android.arch.lifecycle.Lifecycle;
import android.arch.lifecycle.LifecycleObserver;
import android.arch.lifecycle.OnLifecycleEvent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getLifecycle().addObserver(new MyObserver());//1
    }
    public class MyObserver implements LifecycleObserver{
        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        void onResume(){
            Log.d(TAG, "Lifecycle call onResume");
        }
        @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        void onPause(){
            Log.d(TAG, "Lifecycle call onPause");
        }
    }
    @Override
    protected void onResume() {
        super.onResume();
        Log.d(TAG, "onResume");
    }
    @Override
    protected void onPause() {
        super.onPause();
        Log.d(TAG, "onPause");
    }
}

先实现MyObserver,对ON_CREATE和ON_RESUME事件进行监听。因为在Android Support Library 26.1.0 及其之后的版本,Activity和Fragment已经默认实现了LifecycleOwner接口,所以在注释1处可以直接使用getLifecycle方法获取Lifecycle对象,这样MyObserver就可以观察MainActivity的生命周期变化了

LifecycleOwner可以理解为被观察者,MainActivity默认实现了LifecycleOwner接口,也就是说MainActivity是被观察者

运行程序,打印的log如下所示

D/MainActivity: onResume
D/MainActivity: Lifecycle call onResume
D/MainActivity: Lifecycle call onPause
D/MainActivity: onPause

只要在MainActivity的onCreate方法中添加MyObserver,那么MyObserver就可以观察到MainActivity的各个生命周期的变化

3.2 MVP中使用

改写第一小节MVP的例子,先实现MyPresenter,如下所示

public class MyPresenter implements IPresenter {
    private static final String TAG = "test";
    @Override
    public void onResume() {
        Log.d(TAG, "Lifecycle call onResume");
    }
    @Override
    public void onPause() {
        Log.d(TAG, "Lifecycle call onPause");
    }
}
interface IPresenter extends LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    void onResume();
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    void onPause();
}

IPresenter接口继承自LifecycleObserver接口,MyPresenter又实现了IPresenter接口,这样MyPresenter成为了一个观察者。 接在在MainActivity中加入MyPresenter:

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "test";
    private IPresenter mPresenter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mPresenter = new MyPresenter();
        getLifecycle().addObserver(mPresenter);
    }
    @Override
    protected void onResume() {
        super.onResume();
        Log.d(TAG, "onResume");
    }
    @Override
    protected void onPause() {
        super.onPause();
        Log.d(TAG, "onPause");
    }
}

MainActivity成为了被观察者,当它的生命周期发生变化时,MyPresenter就可以观察到,这样就不需要在MainActivity的多个生命周期方法中调用MyPresenter的方法了

打印的日志如下:

D/test: onResume
D/test: Lifecycle call onResume
D/test: Lifecycle call onPause
D/test: onPause

4.自定义LifecycleOwner

如果想实现自定义LifecycleOwner,可以使用LifecycleRegistry,它是Lifecycle的实现类。Android Support Library 26.1.0及其之后的版本,Activity和Fragment已经默认实现了LifecycleOwner接口,因此我们可以这么写:

import android.arch.lifecycle.Lifecycle;
import android.arch.lifecycle.LifecycleRegistry;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
public class MyActivity extends AppCompatActivity {
    private LifecycleRegistry lifecycleRegistry;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        lifecycleRegistry = new LifecycleRegistry(this);
        lifecycleRegistry.markState(Lifecycle.State.CREATED);
    }
    @Override
    public void onStart() {
        super.onStart();
        lifecycleRegistry.markState(Lifecycle.State.STARTED);
    }
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return lifecycleRegistry;
    }
}

通过新建LifecycleRegistry,为LifecycleRegistry设置Lifecycle的各种状态,并通过getLifecycle方法返回该LifecycleRegistry。

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