Android设计模式（二）－－策略模式

1、定义：

The Strategy Pattern defines a family of algorithms,encapsulates each one,and makes them interchangeable. Strategy lets the algorithm vary independently from
clients that use it.

定义了一系列的算法（这些算法实现了同样的工作。仅仅是实现不同），它能够已同样的方式调用全部的算法。降低算法类与算法之间的耦合。

2、目的：

将详细的算法抽象出来，把每一个算法独立出来，形成一系列的算法组，这个算法组里面的算法能够依据实际情况进行相互替换。

3、中心：

策略模式的中心，不在于怎样实现算法，而在于怎样组织和调用这些算法，即：解耦合。形成独立模块。增强程序拓展性。

写了一个简单的策略使用

首先，编写一个统一的算法接口

/**
 * 策略模式
 * 统一的算法接口
 * @author qubian
 * @data 2015年6月3日
 * @email naibbian@163.com
 *
 */
public interface StrategyPattern {

	/**
	 * 计算注数
	 */
	public int calcLottery(int num);

}

其次，编写每一个详细的实现

package com.example.demo;
/**
 * 策略模式
 * 详细的方法实现；
 * 比方说双色球
 * @author qubian
 * @data 2015年6月3日
 * @email naibbian@163.com
 *
 */
public class StrategyPatternImp_SSQ implements StrategyPattern {

	@Override
	public int calcLottery(int num) {
		return 0;
	}

}

package com.example.demo;

/**
 * 策略模式
 * 详细的方法实现；
 * 比方说大乐透
 * @author qubian
 * @data 2015年6月3日
 * @email naibbian@163.com
 *
 */
public class StrategyPatternImp_DLT implements StrategyPattern{

	@Override
	public int calcLottery(int num) {

		return 0;
	}

}

最后是策略的不同调用

package com.example.demo;

/**
 * 详细的使用
 * @author qubian
 * @data 2015年6月3日
 * @email naibbian@163.com
 *
 */
public class LotteryCount {

	private StrategyPattern strategyPattern;

	public enum LotteryEnum {
	        SSQ, DLT, QLC;
	}

	public int  getLotteryCount(LotteryEnum e,int num)
	{
		switch (e) {
		case SSQ:
			strategyPattern =  new StrategyPatternImp_SSQ();
			break;
		case DLT:
			strategyPattern =  new StrategyPatternImp_DLT();
			break;
		default:
			break;
		}

		return strategyPattern.calcLottery(num);
	}

}

策略模式 在Android Framework 中运用广泛。

比方说，我们常常使用的 BaseAdapter 实际也是策略模式；

我们编写的适配器继承自BaseAdapter，通过getview中实现不同的算法。实现不同的view的返回，

外部使用时也能够依据数据源。切换Adapter，这种使用事实上就是一种策略模式；

Adapter 就是一个最顶层的策略接口

public interface Adapter {
    /**
     * How many items are in the data set represented by this Adapter.
     *
     * @return Count of items.
     */
    int getCount();   

    /**
     * Get the data item associated with the specified position in the data set.
     *
     * @param position Position of the item whose data we want within the adapter's
     * data set.
     * @return The data at the specified position.
     */
    Object getItem(int position);

    /**
     * Get a View that displays the data at the specified position in the data set. You can either
     * create a View manually or inflate it from an XML layout file. When the View is inflated, the
     * parent View (GridView, ListView...) will apply default layout parameters unless you use
     * {@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)}
     * to specify a root view and to prevent attachment to the root.
     *
     * @param position The position of the item within the adapter's data set of the item whose view
     *        we want.
     * @param convertView The old view to reuse, if possible. Note: You should check that this view
     *        is non-null and of an appropriate type before using. If it is not possible to convert
     *        this view to display the correct data, this method can create a new view.
     *        Heterogeneous lists can specify their number of view types, so that this View is
     *        always of the right type (see {@link #getViewTypeCount()} and
     *        {@link #getItemViewType(int)}).
     * @param parent The parent that this view will eventually be attached to
     * @return A View corresponding to the data at the specified position.
     */
    View getView(int position, View convertView, ViewGroup parent);

    static final int IGNORE_ITEM_VIEW_TYPE = AdapterView.ITEM_VIEW_TYPE_IGNORE;

    int getItemViewType(int position);
    int getViewTypeCount();
    static final int NO_SELECTION = Integer.MIN_VALUE;
     boolean isEmpty();
}

再到BaseAdapter的抽象

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
    private final DataSetObservable mDataSetObservable = new DataSetObservable();

    public boolean hasStableIds() {
        return false;
    }
    /**
     * Notifies the attached observers that the underlying data has been changed
     * and any View reflecting the data set should refresh itself.
     */
    public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

    public View getDropDownView(int position, View convertView, ViewGroup parent) {
        return getView(position, convertView, parent);
    }

    public int getItemViewType(int position) {
        return 0;
    }

    public int getViewTypeCount() {
        return 1;
    }

    public boolean isEmpty() {
        return getCount() == 0;
    }
}