Android Framework中的PolicyManager简介

PolicyManager类位于framework\base\core\java\com\android\internal\policy目录中的PolicyManager.java文件中。PolicyManager主要用于创建Window类、LayoutInflater类和WindowManagerPolicy类，它扮演着简单工厂模式中的工厂类角色，而抽象产品角色由IPolicy接口实现，具体产品角色由Policy类实现。它们的关系如下图所示：

从下面三个类的代码可以看出，都使用了hide注解，因此，这三个类都是不对外公开的API，只限于Framework内部使用。

抽象产品类IPolicy实现如下（IPolicy.java）：

 package com.android.internal.policy;

 import android.content.Context;
 import android.view.LayoutInflater;
 import android.view.Window;
 import android.view.WindowManagerPolicy;

 /**
  * {@hide}
  */

 /* The implementation of this interface must be called Policy and contained
  * within the com.android.internal.policy.impl package */
 public interface IPolicy {
     public Window makeNewWindow(Context context);

     public LayoutInflater makeNewLayoutInflater(Context context);

     public WindowManagerPolicy makeNewWindowManager();
 }

具体产品类Policy实现如下（Policy.java）：

 package com.android.internal.policy.impl;

 import android.content.Context;
 import android.util.Log;

 import com.android.internal.policy.IPolicy;
 import com.android.internal.policy.impl.PhoneLayoutInflater;
 import com.android.internal.policy.impl.PhoneWindow;
 import com.android.internal.policy.impl.PhoneWindowManager;

 /**
  * {@hide}
  */

 // Simple implementation of the policy interface that spawns the right
 // set of objects
 public class Policy implements IPolicy {
     private static final String TAG = "PhonePolicy";

     //需要在Policy类创建时预先加载的一些类
     private static final String[] preload_classes = {
         "com.android.internal.policy.impl.PhoneLayoutInflater",
         "com.android.internal.policy.impl.PhoneWindow",
         "com.android.internal.policy.impl.PhoneWindow$1",
         "com.android.internal.policy.impl.PhoneWindow$ContextMenuCallback",
         "com.android.internal.policy.impl.PhoneWindow$DecorView",
         "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState",
  "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState$SavedState",
     };

     static {
         // 出于性能考虑，在Policy类加载时提前加载会用到的相关类
         for (String s : preload_classes) {
             try {
         //加载指定类到Java虚拟机中，并执行类中的静态代码段
                 Class.forName(s);
             } catch (ClassNotFoundException ex) {
                 Log.e(TAG, "Could not preload class for phone policy: " + s);
             }
         }
     }

     public PhoneWindow makeNewWindow(Context context) {
         return new PhoneWindow(context);
     }

     public PhoneLayoutInflater makeNewLayoutInflater(Context context) {
         return new PhoneLayoutInflater(context);
     }

     //PhoneWindowManager实现了WindowManagerPolicy接口
     //这里返回值直接写成WindowManagerPolicy更合理些
     public PhoneWindowManager makeNewWindowManager() {
         return new PhoneWindowManager();
     }
 }

工厂类PolicyManager实现如下（PolicyManager.java）：

 class="java" name="code">package com.android.internal.policy;

 import android.content.Context;
 import android.view.LayoutInflater;
 import android.view.Window;
 import android.view.WindowManagerPolicy;

 import com.android.internal.policy.IPolicy;

 /**
  * {@hide}
  */

 public final class PolicyManager {
     private static final String POLICY_IMPL_CLASS_NAME =
         "com.android.internal.policy.impl.Policy";

     private static final IPolicy sPolicy;

     static {
        // 运行时动态装载IPolicy实现类
         try {
            //加载Policy类的同时会执行其中的静态代码段
             Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
             sPolicy = (IPolicy)policyClass.newInstance();
         } catch (ClassNotFoundException ex) {
             throw new RuntimeException(
                     POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
         } catch (InstantiationException ex) {
             throw new RuntimeException(
                     POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
         } catch (IllegalAccessException ex) {
             throw new RuntimeException(
                     POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
         }
     }

     // 构造函数私有，保证是单例类
     private PolicyManager() {}

     // The static methods to spawn new policy-specific objects
     public static Window makeNewWindow(Context context) {
         return sPolicy.makeNewWindow(context);
     }

     public static LayoutInflater makeNewLayoutInflater(Context context) {
         return sPolicy.makeNewLayoutInflater(context);
     }

     public static WindowManagerPolicy makeNewWindowManager() {
         return sPolicy.makeNewWindowManager();
     }
 }