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）：

<pre 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();
    }
}</pre>