综合技术（三）：Android 的动态加载技术

1. 动态加载技术概述

• 动态加载技术又称插件化技术
• 插件化可以减轻应用的内存和 CPU 占用以及 apk 包体积大小，还可以实现热插拔，即不发布新版本的情况下更新某些模块
• 插件化技术必须要解决的三个基础性问题
  • 资源访问
  • Activity 生命周期管理
  • ClassLoader 的管理

2. 宿主和插件的概念

• 宿主是指普通的 apk，插件一般是指经过处理的 dex 或者 apk
• 主流的插件化框架中多采用经过特殊处理的 apk 来作为插件，处理方式往往和编译以及打包环节有关
• 很多插件化框架都需要用到代理 Activity 的概念，插件 Activity 的启动大多数是借助一个代理 Activity 来实现的

3. 插件化技术之资源访问

1. 加载 apk 中的资源

   protected void loadResources() {
   	try {
   		AssetManager assetManager = AssetManager.class.newInstance();
   		Method addAssetPath = assetManager.getClass().getMethod("addAssetPath", String.class);
   		addAssetPath.invoke(assetManger, mDexPath);
   		mAssetManger = assetManager;
   	} catch (Exception e) {
   		e.printStackTrace();
   	}
   	Resource superRes = super.getResources();
   	mResources = new Resources(mAssetManager, superRes.getDisplayMetrics(), superRes.getConfiguration());
   	mTheme = mResources.newTheme();
   	mTheme.setTo(super.getTheme());
   }

2. 在代理 Activity 中实现 getAssets() 和 getResources()

   @Override
   public AssetManager getAssets() {
   	return mAssetManager == null ? super.getAssets() : mAssetManager;
   }
   
   @Override
   public Resoure getResources() {
   	return mResources == null ? super.getResources() : mResources;
   }

4. 插件化技术之 Activity 生命周期的管理

• 反射方式

  @Override
  protected void onResume() {
  	super.onResume();
  	Method onResume = mActivityLifecircleMethods.get("onResume");
  	if (onResume != null) {
  		try {
  			onResume.invoke(mRemoteActivity, new Object[] {});
  		} catch (Exception e) {
  			e.printStackTrace();
  		}
  	}
  }
  
  @Override
  protected void onPause() {
  	Method onPause = mActivityLifecircleMethods.get("onPause");
  	if (onPause != null) {
  		try {
  			onPause.invoke(mRemoteActivity, new Object[] {});
  		} catch (Exception e) {
  			e.printStackTace();
  		}
  	}
  	super.onPause();
  }

  • 使用反射来管理插件 Activity 的生命周期有两个缺点：一是反射代码写起来比较复杂；二是过多使用反射会有一定的性能开销

• 接口方式

  // 声明 DLPlugin
  public interface DLPlugin {
  	public void onStart();
  	public void onRestart();
  	public void onActivityResult(int requestCode, int resultCode, Intent data);
  	public void onResume();
  	public void onPause();
  	public void onStop();
  	public void onDestroy();
  	public void onCreate(Bundle savedInstanceState);
  	public void setProxy(Activity proxyActivity, String dexPath);
  	public void onSaveInstanceState(Bundle outState);
  	public void onNewIntent(Intent intent);
  	public void onRestoreInstanceState(Bundle savedInstanceState);
  	public boolean onTouchEvent(MotionEvent event);
  	public boolean onKeyUp(int keyCode, KeyEvent event);
  	public void onWindowAttributesChanged(LayoutParams params);
  	public void onWindowFocusChanged(boolean hasFocus);
  	public void onBackPressed();
  	...
  }

  // 代理 Activity
  ...
  @Override
  protected void onStart() {
  	mRemoteActivity.onStart();
  	super.onStart();
  }
  
  @Override
  protected void onRestart() {
  	mRemoteActivity.onRestart();
  	super.onRestart();
  }
  
  @Override
  protected void onResume() {
  	mRemoteActivity.onResume();
  	super.onResume();
  }

5. 插件化技术之插件 ClassLoader 的管理

// 为了更好地对多插件进行支持，需要合理地去管理各个插件的 DexClassLoader
// 从而避免多个 ClassLoader 加载同一个类时多引发的类型转换错误
public class DLClassLoader extends DexClassLoader {
	private static final String TAG = "DLClassLoader";
	
	private static final HashMap<String, DLClassLoader> mPluginClassLoaders = new HashMap<String, DLClassLoader>();
	protected DLClassLoader(String dexPath, String optimizedDirectory, String libraryPath, ClassLoader parent) {
		super (dexPath, optimizedDirectory, libraryPath, parent);
	}
	
	/**
	 * return a available classloader which belongs to different apk
	 */
	public static DLClassLoader getClassLoader(String dexPath, Context context, ClassLoader parentLoader) {
		DLClassLoader dlClassLoader = mPluginClassLoaders.get(dexPath);
		if (dLClassLoader != null) {
			return dlClassLoader;
		}
		File dexOutputDir = context.getDir("dex", Context.MODE_PRIVATE);
		final String dexOutputPath = dexOutputDir.getAbsolutePath();
		dlClassLoader = new DLClassLoader(dexPath, dexOutputPath, null, parentLoader);
		mPluginClassLoaders.put(dexPath, dlClassLoader);
		
		return dLClassLoader;
	}
}