其實在我們很多應用中都看到當ViewPager滑到第一頁或者最後一頁的時候，如果再滑動的時候，就會有一個緩衝的過程，也就是回彈效果。之前在研究回彈效果的時候，也順便實現了ViewPager的回彈效果，其實也很簡單，一下是實現程式碼，註釋比較少：

package com.freesonfish.viewpager_2;

import android.content.Context;
import android.graphics.Rect;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.animation.TranslateAnimation;

public class MyViewPager extends ViewPager {

	private Rect mRect = new Rect();//用來記錄初始位置
	private int pagerCount = 3;
	private int currentItem = 0;
	private boolean handleDefault = true;
	private float preX = 0f;
	private static final float RATIO = 0.5f;//摩擦係數
	private static final float SCROLL_WIDTH = 30f;

	public MyViewPager(Context context) {
		super(context);
	}

	public MyViewPager(Context context, AttributeSet attrs) {
		super(context, attrs);
	}

	//設定總共有多少頁,請記得呼叫它
	public void setpagerCount(int pagerCount) {
		this.pagerCount = pagerCount;
	}

	//這是當前是第幾頁，請在onPageSelect方法中呼叫它。
	public void setCurrentIndex(int currentItem) {
		this.currentItem = currentItem;
	}
	
	@Override
	public boolean dispatchKeyEvent(KeyEvent event) {
		return super.dispatchKeyEvent(event);
	}

	@Override
	public boolean onInterceptTouchEvent(MotionEvent arg0) {
		if (arg0.getAction() == MotionEvent.ACTION_DOWN) {
			preX = arg0.getX();//記錄起點
		}
		return super.onInterceptTouchEvent(arg0);
	}

	@Override
	public boolean onTouchEvent(MotionEvent arg0) {
		switch (arg0.getAction()) {
		case MotionEvent.ACTION_UP:
			onTouchActionUp();
			break;
		case MotionEvent.ACTION_MOVE:
			//當時滑到第一項或者是最後一項的時候。
			if ((currentItem == 0 || currentItem == pagerCount - 1)) {
				float nowX = arg0.getX();
				float offset = nowX - preX;
				preX = nowX;
				if (currentItem == 0) {
					if (offset > SCROLL_WIDTH) {//手指滑動的距離大於設定值
						whetherConditionIsRight(offset);
					} else if (!handleDefault) {//這種情況是已經出現緩衝區域了，手指慢慢恢復的情況
						if (getLeft() + (int) (offset * RATIO) >= mRect.left) {
							layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom());
						}
					}
				} else {
					if (offset < -SCROLL_WIDTH) {
						whetherConditionIsRight(offset);
					} else if (!handleDefault) {
						if (getRight() + (int) (offset * RATIO) <= mRect.right) {
							layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom());
						}
					}
				}
			} else {
				handleDefault = true;
			}

			if (!handleDefault) {
				return true;
			}
			break;

		default:
			break;
		}
		return super.onTouchEvent(arg0);
	}

	private void whetherConditionIsRight(float offset) {
		if (mRect.isEmpty()) {
			mRect.set(getLeft(), getTop(), getRight(), getBottom());
		}
		handleDefault = false;
		layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom());
	}

	private void onTouchActionUp() {
		if (!mRect.isEmpty()) {
			recoveryPosition();
		}
	}

	private void recoveryPosition() {
		TranslateAnimation ta = null;
		ta = new TranslateAnimation(getLeft(), mRect.left, 0, 0);
		ta.setDuration(300);
		startAnimation(ta);
		layout(mRect.left, mRect.top, mRect.right, mRect.bottom);
		mRect.setEmpty();
		handleDefault = true;
	}

}