策略模式--从源码TabHost中看策略模式
什么是策略模式：针对一组算法，将每一个算法封装到具有共同接口的独立的类中，从而使得它们可以相互替换。
应当由客户端自己决定在什么情况下使用什么具体策略模式。
策略模式不适合于处理同时嵌套多于一个算法的情形。
针对的设计原则：
封装变化的。
多用组合，少用继承。
针对接口编程，不针对实现编程。
优缺点：
优点：
1. 提供了管理相关的算法族的办法。
2. 提供了可以替换继承关系的办法。
3. 可以避免使用多重条件转移语句。
缺点：
1. 客户端必须知道所有的策略类，并自行决定使用哪一个策略类。
2. 策略模式造成很多的策略类。
下面我们结合下源码中的TabHost的实现，来介绍下源码中如何使用策略模式
如果你在本地有下载源码的话，你可以在源码中找到这个类，在我的电脑中，它的位置如下：
F:\android-sdk\sources\android-15\android\widget\TabHost.java
这里引用林家男孩博客中的一张图：
http://dl.iteye.com/upload/attachment/0075/8663/5c1f6861-d1ef-318e-9562-8fce4e6afc6f.jpg
这张图很好地说明了TabHost.java中的模式结构。
首先TabHost继承自FrameLayout,TabHost由两部分构成，一个是TabWidget,一个是TabSpace.
TabWidget用来指定和监听TabHost切换的，不是我们今天要讨论的范围。
TabSpec就是我们今天策略模式的主角了，下面对它进行分析。
TabHost可以看做是由一个个独立的用于点击来切换显示面板的Indicator和所对应的显示内容的面板Content组成。
也就是说一个Indicator对应一个Content，这在源码中对应着的是TabSpec这个类，换句话说：每个TabSpec对应着一个Indicator和相应的显示内容的面板Content。这两个地方也是经常需要变动的。因为可能需要定制各种各样的TabHost,比如，使用自定义的View来作为Indicator，当然显示内容的面板也需要是可以被定制的。
那么这时候我们就需要将变化的东西抽象出来。以便它可以被很方便地替换。
举个例子来说：
我要定制一个TabSpec，需要满足两个基本条件：
1.一个Indicator
2.一个Content
这个时候我有可能仅仅想使用文字作为Indicator，那么我用LabelIndicatorStrategy来满足我的要求，假如这个时候，我想换成图片加文字的方式来作为Indicator，怎么办呢？很简单，我可以换成LabelAndIconIndicatorStrategy类来满足要求。显示面板Content也是一样道理。
当然，实现这些需要编程技巧和java的特性，针对接口编程，多态等。这个可以去查找相关资料。
下面直接上源码：
TabSpec：
/** * A tab has a tab indicator, content, and a tag that is used to keep * track of it.This builder helps choose among these options. * * For the tab indicator, your choices are: * 1) set a label * 2) set a label and an icon * * For the tab content, your choices are: * 1) the id of a {@link View} * 2) a {@link TabContentFactory} that creates the {@link View} content. * 3) an {@link Intent} that launches an {@link android.app.Activity}. */ public class TabSpec { private String mTag; private IndicatorStrategy mIndicatorStrategy; private ContentStrategy mContentStrategy; private TabSpec(String tag) { mTag = tag; } /** * Specify a label as the tab indicator. */ public TabSpec setIndicator(CharSequence label) { mIndicatorStrategy = new LabelIndicatorStrategy(label); return this; } /** * Specify a label and icon as the tab indicator. */ public TabSpec setIndicator(CharSequence label, Drawable icon) { mIndicatorStrategy = new LabelAndIconIndicatorStrategy(label, icon); return this; } /** * Specify a view as the tab indicator. */ public TabSpec setIndicator(View view) { mIndicatorStrategy = new ViewIndicatorStrategy(view); return this; } /** * Specify the id of the view that should be used as the content * of the tab. */ public TabSpec setContent(int viewId) { mContentStrategy = new ViewIdContentStrategy(viewId); return this; } /** * Specify a {@link android.widget.TabHost.TabContentFactory} to use to * create the content of the tab. */ public TabSpec setContent(TabContentFactory contentFactory) { mContentStrategy = new FactoryContentStrategy(mTag, contentFactory); return this; } /** * Specify an intent to use to launch an activity as the tab content. */ public TabSpec setContent(Intent intent) { mContentStrategy = new IntentContentStrategy(mTag, intent); return this; } public String getTag() { return mTag; } } Indicator接口：
IndicatorStrategy
/** * Specifies what you do to create a tab indicator. */ private static interface IndicatorStrategy { /** * Return the view for the indicator. */ View createIndicatorView(); }
Content接口：
ContentStrategy
/** * Specifies what you do to manage the tab content. */ private static interface ContentStrategy { /** * Return the content view.The view should may be cached locally. */ View getContentView(); /** * Perhaps do something when the tab associated with this content has * been closed (i.e make it invisible, or remove it). */ void tabClosed(); } Indicator接口实现类：
LabelIndicatorStrategy
LabelAndIconIndicatorStrategy
ViewIndicatorStrategy
/** * How to create a tab indicator that just has a label. */ private class LabelIndicatorStrategy implements IndicatorStrategy { private final CharSequence mLabel; private LabelIndicatorStrategy(CharSequence label) { mLabel = label; } public View createIndicatorView() { final Context context = getContext(); LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); View tabIndicator = inflater.inflate(mTabLayoutId, mTabWidget, // tab widget is the parent false); // no inflate params final TextView tv = (TextView) tabIndicator.findViewById(R.id.title); tv.setText(mLabel); if (context.getApplicationInfo().targetSdkVersion <= Build.VERSION_CODES.DONUT) { // Donut apps get old color scheme tabIndicator.setBackgroundResource(R.drawable.tab_indicator_v4); tv.setTextColor(context.getResources().getColorStateList(R.color.tab_indicator_text_v4)); } return tabIndicator; } } /** * How we create a tab indicator that has a label and an icon */ private class LabelAndIconIndicatorStrategy implements IndicatorStrategy { private final CharSequence mLabel; private final Drawable mIcon; private LabelAndIconIndicatorStrategy(CharSequence label, Drawable icon) { mLabel = label; mIcon = icon; } public View createIndicatorView() { final Context context = getContext(); LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); View tabIndicator = inflater.inflate(mTabLayoutId, mTabWidget, // tab widget is the parent false); // no inflate params final TextView tv = (TextView) tabIndicator.findViewById(R.id.title); final ImageView iconView = (ImageView) tabIndicator.findViewById(R.id.icon); // when icon is gone by default, we're in exclusive mode final boolean exclusive = iconView.getVisibility() == View.GONE; final boolean bindIcon = !exclusive || TextUtils.isEmpty(mLabel); tv.setText(mLabel); if (bindIcon && mIcon != null) { iconView.setImageDrawable(mIcon); iconView.setVisibility(VISIBLE); } if (context.getApplicationInfo().targetSdkVersion <= Build.VERSION_CODES.DONUT) { // Donut apps get old color scheme tabIndicator.setBackgroundResource(R.drawable.tab_indicator_v4); tv.setTextColor(context.getResources().getColorStateList(R.color.tab_indicator_text_v4)); } return tabIndicator; } } /** * How to create a tab indicator by specifying a view. */ private class ViewIndicatorStrategy implements IndicatorStrategy { private final View mView; private ViewIndicatorStrategy(View view) { mView = view; } public View createIndicatorView() { return mView; } }
Content接口实现类：
ViewIdContentStrategy
FactoryContentStrategy
IntentContentStrategy
/** * How to create the tab content via a view id. */ private class ViewIdContentStrategy implements ContentStrategy { private final View mView; private ViewIdContentStrategy(int viewId) { mView = mTabContent.findViewById(viewId); if (mView != null) { mView.setVisibility(View.GONE); } else { throw new RuntimeException("Could not create tab content because " + "could not find view with id " + viewId); } } public View getContentView() { mView.setVisibility(View.VISIBLE); return mView; } public void tabClosed() { mView.setVisibility(View.GONE); } } /** * How tab content is managed using {@link TabContentFactory}. */ private class FactoryContentStrategy implements ContentStrategy { private View mTabContent; private final CharSequence mTag; private TabContentFactory mFactory; public FactoryContentStrategy(CharSequence tag, TabContentFactory factory) { mTag = tag; mFactory = factory; } public View getContentView() { if (mTabContent == null) { mTabContent = mFactory.createTabContent(mTag.toString()); } mTabContent.setVisibility(View.VISIBLE); return mTabContent; } public void tabClosed() { mTabContent.setVisibility(View.GONE); } } /** * How tab content is managed via an {@link Intent}: the content view is the * decorview of the launched activity. */ private class IntentContentStrategy implements ContentStrategy { private final String mTag; private final Intent mIntent; private View mLaunchedView; private IntentContentStrategy(String tag, Intent intent) { mTag = tag; mIntent = intent; } public View getContentView() { if (mLocalActivityManager == null) { throw new IllegalStateException("Did you forget to call 'public void setup(LocalActivityManager activityGroup)'?"); } final Window w = mLocalActivityManager.startActivity( mTag, mIntent); final View wd = w != null ? w.getDecorView() : null; if (mLaunchedView != wd && mLaunchedView != null) { if (mLaunchedView.getParent() != null) { mTabContent.removeView(mLaunchedView); } } mLaunchedView = wd; // XXX Set FOCUS_AFTER_DESCENDANTS on embedded activities for now so they can get // focus if none of their children have it. They need focus to be able to // display menu items. // // Replace this with something better when Bug 628886 is fixed... // if (mLaunchedView != null) { mLaunchedView.setVisibility(View.VISIBLE); mLaunchedView.setFocusableInTouchMode(true); ((ViewGroup) mLaunchedView).setDescendantFocusability( FOCUS_AFTER_DESCENDANTS); } return mLaunchedView; } public void tabClosed() { if (mLaunchedView != null) { mLaunchedView.setVisibility(View.GONE); } } } 很容易看出
针对TabSpec来说，将接口定义成内部变量：
private IndicatorStrategy mIndicatorStrategy;private ContentStrategy mContentStrategy; 再通过setter方法很容易地控制它的Indicator和Content。
比如：
/** * Specify a label as the tab indicator. */ public TabSpec setIndicator(CharSequence label) { mIndicatorStrategy = new LabelIndicatorStrategy(label); return this; }这就是策略模式。
为了达到动态地改变类的行为的目的，我们需要将变化的部分，从不变的部分中抽象出来。由于需要实现动态替换，就需要利用面向对象的多态特性。我们需要利用接口，也就是针对接口编程。也就是说，相关的算法，需要具有同样的一个接口。本例中
LabelIndicatorStrategy，LabelAndIconIndicatorStrategy，ViewIndicatorStrategy是相关的一组算法，它们实现了Indicator接口。
ViewIdContentStrategy，FactoryContentStrategy，IntentContentStrategy是相关的一组算法，它们实现了Content接口。
你就可以很方便地通过多态（接口调用）来实现算法的替换。
这样一来，算法的变化就独立于使用算法的客户了，客户自己决定在什么情况下使用什么具体策略模式。
以上仅是个人见解，如果有不足的地方希望指出，共同进步，感谢一下参考链接。
参考链接：
http://bj007.blog.51cto.com/1701577/649063
http://bj007.blog.51cto.com/1701577/643572
http://blog.csdn.net/icrs23/article/details/1913871
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