探究为什么Glide 可以与 Activity 生命周期绑定

用过Glide同学都知道，Glide的最大不同之处就是可以与Activity 生命周期绑定，在Activity onDestroy时自动结束网络请求，释放资源。
那么为什么Glide 可以做到与Activity绑定生命周期，并且是怎么做到到呢？
我们通过一些代码来说明(代码均来自Glide源码)：

首先我们创建一个 LifeCycleListener

public interface LifeCycleListener {
    /**
     * Callback for when {@link android.app.Fragment#onStart()}} or {@link
     * android.app.Activity#onStart()} is called.
     */
    void onStart();

    /**
     * Callback for when {@link android.app.Fragment#onStop()}} or {@link
     * android.app.Activity#onStop()}} is called.
     */
    void onStop();

    /**
     * Callback for when {@link android.app.Fragment#onDestroy()}} or {@link
     * android.app.Activity#onDestroy()} is called.
     */
    void onDestroy();
}

创建 LifeCycleListener 管理类

public class ActivityFragmentLifeCycle {
    private LifeCycleListener lifecycleListener;

    public void addListener(@NonNull LifeCycleListener listener) {
        lifecycleListener = listener;
    }

    void onStart() {
        if (lifecycleListener != null)
            lifecycleListener.onStart();

    }

    void onStop() {
        if (lifecycleListener != null)
            lifecycleListener.onStop();

    }

    void onDestroy() {
        if (lifecycleListener != null)
            lifecycleListener.onDestroy();
    }
}

创建一个操作ActivityFragmentLifeCycle的 Fragment

public class SupportManagerFragment extends Fragment {
    private ActivityFragmentLifeCycle lifecycle;
    public SupportManagerFragment() {
        this(new ActivityFragmentLifeCycle());
    }

    @SuppressLint("ValidFragment")
    public SupportManagerFragment(@NonNull ActivityFragmentLifeCycle lifeCycle) {
        this.lifecycle = lifeCycle;
    }

    @NonNull
    ActivityFragmentLifeCycle getLifeCycle() {
        return lifecycle;
    }


    @Override
    public void onStart() {
        super.onStart();
        lifecycle.onStart();
    }

    @Override
    public void onStop() {
        super.onStop();
        lifecycle.onStop();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        lifecycle.onDestroy();
    }
}

这里就是关键类了，为什么要有个Fragment？因为他绑定Activity生命周期的方式就是给当前Activity加上Fragment通过这个Fragment 来管理生命周期。
下面就介绍如何创建Fragment依托于当前的Activity获得生命周期管理

获得Activity ，来创建Fragment管理生命周期

/**
 * Create by JokAr. on 2018/12/24.
 */
public class RequestManager implements LifeCycleListener {

    private static final String TAG = RequestManager.class.getSimpleName();

    /**
     * 绑定activity
     * @param context 传入的上下文
     */
    public void bind(Context context) {
        if (context == null) {
            throw new IllegalArgumentException("You cannot start a load on a null Context");
        } else if (Util.isOnMainThread() && !(context instanceof Application)) {
            if (context instanceof FragmentActivity) {
                bind((FragmentActivity) context);
            } else if (context instanceof Activity) {
                bind((AppCompatActivity) context);
            } else if (context instanceof ContextWrapper) {
                bind(((ContextWrapper) context).getBaseContext());
            }
        }
    }

    
    private void bind(AppCompatActivity activity) {
        FragmentManager fragmentManager = activity.getSupportFragmentManager();
        supportFragmentGet(fragmentManager, isActivityVisible(activity));
    }

    private void bind(FragmentActivity activity) {
        FragmentManager fragmentManager = activity.getSupportFragmentManager();
        supportFragmentGet(fragmentManager, isActivityVisible(activity));
    }

    /**
     * 创建Fragment 并依托于当前Activity(当然Glide做了缓存去重操作) 
     * @param fragmentManager
     * @param activityVisible
     */
    private void supportFragmentGet(FragmentManager fragmentManager, boolean activityVisible) {
        SupportManagerFragment current = new SupportManagerFragment();

        fragmentManager.beginTransaction()
                .add(current, "managerLife")
                .commitAllowingStateLoss();
        current.getLifeCycle().addListener(this);
        if (activityVisible) {
            current.getLifeCycle().onStart();
        }
    }

    private boolean isActivityVisible(Activity activity) {
        return !activity.isFinishing();
    }

    @Override
    public void onStart() {
        //do you buzz
        Log.i(TAG, "onStart: ");
    }

    /**
     * 当出现该函数时，可以暂停Glide网络图片的请求
     */
    @Override
    public void onStop() {
        //do you buzz
        Log.i(TAG, "onStop: ");
    }

    @Override
    public void onDestroy() {
        //do you buzz
        Log.i(TAG, "onDestroy: ");
    }
}

调用

RequestManager requestManager = new RequestManager();
requestManager.bind(this);

通过这样，我们就可以自己实现绑定Activity生命周期管理，也知道了Glide 是怎样绑定的