PolicyManager工廠模式與動態加載

PolicyManager工廠模式與動態加載

                                     Android FrameWork——PolicyManager

在學習Android跟蹤setContentView執行過程代碼的時遇到：

Attach(){

　　//着實不清楚PolicyManager到底如何創建的，哪一個對象。

　　mWindow = PolicyManager.makeNewWindow(this);

}

其中用到Class.forName動態加載，Java是最近才學的還真不曉得有這樣的用法；

一 類之間關系

於是就了解一下PolicyManager以及相關類實現方式；

首先看一下各個類之間的UML圖：

　　　　

PolicyManager：提供了靜態類方法接口，用於創建Window，LayoutInflate，

         WindowManagerPolicy類實例；屬於工廠方法；

IPolicy：提供抽象Policy創建產品接口；

Policy：具體IPolicy，實現創建產品接口；

二 代碼分析

1 IPolicy

         

/* 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();          }

 

2 Policy

         

public class Policy implements IPolicy { private static final String TAG = "PhonePolicy";
 　　//提供需要預先加載的 類
    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", };
 　　//靜態模塊
    static { // For performance reasons, preload some policy specific classes when // the policy gets loaded.
        for (String s : preload_classes) { try { 　　　　　　　　　　//動態加載指定的類 Class.forName(s); } catch (ClassNotFoundException ex) { Log.e(TAG, "Could not preload class for phone policy: " + s); } } }
 　　//創建具體對象的接口
    public PhoneWindow makeNewWindow(Context context) { return new PhoneWindow(context); } public PhoneLayoutInflater makeNewLayoutInflater(Context context) { return new PhoneLayoutInflater(context); } public PhoneWindowManager makeNewWindowManager() { return new PhoneWindowManager(); } }

 

3 PolicyManager

 

//訪問類的靜態成員的時候 會加載該類

public final class PolicyManager { private static final String POLICY_IMPL_CLASS_NAME =
        "com.android.internal.policy.impl.Policy";
 private static final IPolicy sPolicy; //加載該類時，所有靜態成員均被會加載
    static { try { 　　　　　　　//動態加載創建類實例 　　　　　　Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME); 　　　　　　//創建了sPolicy指向的實例 sPolicy = (IPolicy)policyClass.newInstance(); } catch (ClassNotFoundException ex) {} } //構造函數私有類型，不能創建該類實例
    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(); } }

 

這里要注意的就是：動態加載Class.forName：

Class.forName()返回一個類；

String str = 用戶輸入的字符串

Class classType = Class.forName(str);   //返回一個類

classType.newInstance();                           //創建一個classType 類型的對象

 

在上述代碼中：

Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);                  //加載Policy類

sPolicy = (IPolicy)policyClass.newInstance();  //創建Policy對象 給IPolicy類型sPolicy引用；

 

這樣動態加載加上使用工廠方法：降低耦合，提高代碼的靈活性和擴展性；