Android设计模式系列(11)--SDK源码之策略模式

package android.animation;
 
/**
 * A time interpolator defines the rate of change of an animation. This allows animations
 * to have non-linear motion, such as acceleration and deceleration.
 */
public interface Interpolator {
 
    /**
     * Maps a value representing the elapsed fraction of an animation to a value that represents
     * the interpolated fraction. This interpolated value is then multiplied by the change in
     * value of an animation to derive the animated value at the current elapsed animation time.
     *
     * @param input A value between 0 and 1.0 indicating our current point
     *        in the animation where 0 represents the start and 1.0 represents
     *        the end
     * @return The interpolation value. This value can be more than 1.0 for
     *         interpolators which overshoot their targets, or less than 0 for
     *         interpolators that undershoot their targets.
     */
    float getInterpolation(float input);
}

package android.view.animation;
 
import android.content.Context;
import android.content.res.TypedArray;
import android.util.AttributeSet;
 
/**
 * An interpolator where the rate of change starts out slowly and
 * and then accelerates.
 *
 */
public class AccelerateInterpolator implements Interpolator {
    private final float mFactor;
    private final double mDoubleFactor;
 
    public AccelerateInterpolator() {
        mFactor = 1.0f;
        mDoubleFactor = 2.0;
    }
 
    /**
     * Constructor
     *
     * @param factor Degree to which the animation should be eased. Seting
     *        factor to 1.0f produces a y=x^2 parabola. Increasing factor above
     *        1.0f  exaggerates the ease-in effect (i.e., it starts even
     *        slower and ends evens faster)
     */
    public AccelerateInterpolator(float factor) {
        mFactor = factor;
        mDoubleFactor = 2 * mFactor;
    }
 
    public AccelerateInterpolator(Context context, AttributeSet attrs) {
        TypedArray a =
            context.obtainStyledAttributes(attrs, com.android.internal.R.styleable.AccelerateInterpolator);
 
        mFactor = a.getFloat(com.android.internal.R.styleable.AccelerateInterpolator_factor, 1.0f);
        mDoubleFactor = 2 * mFactor;
 
        a.recycle();
    }
 
    public float getInterpolation(float input) {
        if (mFactor == 1.0f) {
            return input * input;
        } else {
            return (float)Math.pow(input, mDoubleFactor);
        }
    }
}

public abstract class Animation implements Cloneable {
    Interpolator mInterpolator;
    // 通过set方法注入    
    public void setInterpolator(Interpolator i) {
         mInterpolator = i;
     }
 
    public boolean getTransformation(long currentTime, Transformation outTransformation) {
        // ... ...
        // 具体调用
        final float interpolatedTime = mInterpolator.getInterpolation(normalizedTime);
        applyTransformation(interpolatedTime, outTransformation);
       // ... ...
    }
 
     // 缺省实现，是个小技巧，顺便提下，这个不是重点
     protected void ensureInterpolator() {
         if (mInterpolator == null) {
             mInterpolator = new AccelerateDecelerateInterpolator();
         }
     }
 
}