跟着源码学设计：Glide 框架及源码解析（一）

前言
近期研究了一下Glide的图片加载框架，在这里和大家分享一下。由于代码研读有限，难免有错误的地方，了解的童鞋还望指正。如果这篇文章对大家学习Glide有帮助，还望大家多多转载。学习小组QQ群: 193765960。

本篇是Glide框架及源码解析的第一篇，更多文章敬请关注后续文章。版权归作者所有，如有转发，请注明文章出处：https://xiaodanchen.github.io/

相关文章：

跟着源码学设计：Glide框架及源码解析（一）
跟着源码学设计：Glide框架及源码解析（二）
跟着源码学设计：Glide框架及源码解析（三）
跟着源码学设计：Glide框架及源码解析（四）
跟着源码学设计：Glide框架及源码解析（五）

1. Glide 简介

Glide是一个性能优良的第三方网络图片加载框架，在节省内存和快速流畅加载方面具有较好体现。究其内部机制，发现其优良性能得益于以下几点：

• 与使用环境生命周期相绑定：RequestManagerFragment & SupportRequestManagerFragment
• 内存的三级缓存池：LruMemoryResources, ActiveResources, BitmapPool
• 内存复用机制：BitmapPool

更多的关于Glide的介绍网上资料很多，在这里不再赘述，下文中将针对Glide的内部机制展开说明。

1.1 为什么要绑定生命周期（有什么优点）？

• 可以实现网络请求根据生命周期而暂停、执行、恢复、释放等
• 可以实现资源比如图片的自动释放
• 降低了内存的压力
• 降低了内存泄漏的风险

1.2 绑定原理

• 原理的知识基础：FragmentManager（简称fm）中的所有fragment（通过fm.add()添加进来）都与fm所处的context生命周期绑定。例如：我们的activity中的fragment的生命周期自动通过activity的fm和activity的生命周期绑定。
• Glide定义了RequestManagerFragment 和 SupportRequestManagerFragment两种fragment。该两类Fragment不具有任何的界面和其他功能，通过入口传入的context获取到的fm绑定生命周期到context上。
• Glide内部的生命周期绑定机制进一步通过基于xxxxRequestManagerFragment 的生命周期接口的回调实现。

2. Glide生命周期回调示意图

3. Glide生命周期绑定机制类图

3.1 RequestManagerRetriever(单例模式)

• 根据context获取fm;
• 获取xxxxRequestManagerFragment实例
• 获取RequestManager实例
• 相互绑定xxxxRequestManagerFragment和RequestManager
• xxxxRequestManagerFragment绑定到context生命周期

/**
 * 获取RequestManager实例
 */
 @TargetApi(Build.VERSION_CODES.HONEYCOMB)
 public RequestManager get(Activity activity) {
     if (Util.isOnBackgroundThread() 
         || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
         return get(activity.getApplicationContext());
     } else {
         assertNotDestroyed(activity);
         
         //获取fm
         android.app.FragmentManager fm = activity.getFragmentManager();
         return fragmentGet(activity, fm);
     }
 }
 
/**
 * Note: new RequestManager(context,lifecycle,RequestManagerTreeNode)中，绑定了lifecycle;
 * current.setRequestManager(requestManager):requestManager绑定到fragment
 */
 @TargetApi(Build.VERSION_CODES.HONEYCOMB)
 RequestManager fragmentGet(Context context, FragmentManager fm) {
     //获取fragment实例：与fm绑定生命周期
     RequestManagerFragment current = getRequestManagerFragment(fm);
     //获取之前绑定的RequestManager
     RequestManager requestManager = current.getRequestManager();
     if (requestManager == null) {
         requestManager = new RequestManager(context, current.getLifecycle(),current.getRequestManagerTreeNode());
         
         //requestManager绑定到fragment
         current.setRequestManager(requestManager);
     }
     return requestManager;
 }
  
/**
 * Note: fm.beginTransaction().add(current, FRAGMENT_TAG): fragment绑定context生命周期
 */
 @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
 RequestManagerFragment getRequestManagerFragment(final android.app.FragmentManager fm) {
     RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
     if (current == null) {
         current = pendingRequestManagerFragments.get(fm);
         if (current == null) {
             current = new RequestManagerFragment();
             pendingRequestManagerFragments.put(fm, current);
             //将fragment与fm绑定
             fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
             handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
         }
     }
     return current;
 }

3.2 xxxxRequestManagerFragment(绑定context的生命周期)

• 内部绑定一个RequestManager对象;
• 内部绑定一个ActivityFragmentLifecycle对象
• 在生命周期的回调中调用ActivityFragmentLifecycle的相应生命周期方法
• RequestManager通过注册到ActivityFragmentLifecycle的lifecycleListeners集合实现生命周期绑定

private final ActivityFragmentLifecycle lifecycle;
private RequestManager requestManager;
 
public RequestManagerFragment() {
	this(new ActivityFragmentLifecycle());
}
 
RequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
	this.lifecycle = lifecycle;
}
  
/**
* Sets the current {@link com.bumptech.glide.RequestManager}.
* @param requestManager The request manager to use.
*/
public void setRequestManager(RequestManager requestManager) {
	this.requestManager = requestManager;
}
 
/**
* 生命周期：Glide通过xxxxRequestManagerFragment的生命周期回调实现内部生命周期回调
*/
@Override
public void onStart() {
    super.onStart();
    lifecycle.onStart();
}
 
@Override
public void onStop() {
    super.onStop();
    lifecycle.onStop();
}
 
@Override
public void onDestroy() {
    super.onDestroy();
    lifecycle.onDestroy();
}

3.3 ActivityFragmentLifecycle

• 管理同一个xxxxRequestManagerFragment分支下的所有LifecycleListener（具有生命周期）
• 被xxxxRequestManagerFragment生命周期接口回调
• 遍历回调所有LifecycleListener生命周期接口

/**
 * A {@link com.bumptech.glide.manager.Lifecycle} implementation for tracking and notifying listeners of
 * {@link android.app.Fragment} and {@link android.app.Activity} lifecycle events.
 */
class ActivityFragmentLifecycle implements Lifecycle {
    private final Set<LifecycleListener> lifecycleListeners =
            Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
    private boolean isStarted;
    private boolean isDestroyed;
  
    @Override
    public void addListener(LifecycleListener listener) {
        lifecycleListeners.add(listener);

        if (isDestroyed) {
            listener.onDestroy();
        } else if (isStarted) {
            listener.onStart();
        } else {
            listener.onStop();
        }
    }
  
    void onStart() {
        isStarted = true;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStart();
        }
    }
  
    void onStop() {
        isStarted = false;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onStop();
        }
    }
  
    void onDestroy() {
        isDestroyed = true;
        for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
            lifecycleListener.onDestroy();
        }
    }
}

3.4 RequestManager

• RequestManager被绑定于xxxxRequestManagerFragment
• RequestManager实现了LifecycleListener接口
• RequestManager注册给xxxxRequestManagerFragment的ActivityFragmentLifecycle

public RequestManager(Context context, Lifecycle lifecycle, RequestManagerTreeNode treeNode) {
        this(context, lifecycle, treeNode, new RequestTracker(), new ConnectivityMonitorFactory());
    }
 
 RequestManager(Context context, final Lifecycle lifecycle, RequestManagerTreeNode treeNode,
 RequestTracker requestTracker, ConnectivityMonitorFactory factory) {
    this.context = context.getApplicationContext();
    this.lifecycle = lifecycle;
    this.treeNode = treeNode;
    this.requestTracker = requestTracker;
    this.glide = Glide.get(context);
    this.optionsApplier = new OptionsApplier();
 
    ConnectivityMonitor connectivityMonitor = factory.build(context,
  new RequestManagerConnectivityListener(requestTracker));
 
    if (Util.isOnBackgroundThread()) {
        new Handler(Looper.getMainLooper()).post(new Runnable() {
            @Override
            public void run() {
                lifecycle.addListener(RequestManager.this);
            }
        });
    } else {
  	    lifecycle.addListener(this);
    }
    lifecycle.addListener(connectivityMonitor);
}

4. Glide生命周期绑定机制时序图

（本篇是Glide框架及源码解析的第一篇，更多文章敬请关注后续文章。版权归作者所有，如有转发，请注明文章出处：原文链接）