android首次运行滑动帮助实例（模仿微信）

package com.ericssonlabs; import android.content.Context; import android.util.AttributeSet; import android.util.Log; import android.view.MotionEvent; import android.view.VelocityTracker; import android.view.View; import android.view.ViewGroup; import android.widget.Scroller; public class ScrollLayout extends ViewGroup { private static final String TAG = "ScrollLayout"; private VelocityTracker mVelocityTracker; // 用于判断甩动手势 private static final int SNAP_VELOCITY = 600; private Scroller mScroller; private int mCurScreen; private int mDefaultScreen = 0; private float mLastMotionX; private OnViewChangeListener mOnViewChangeListener; public ScrollLayout(Context context) { super(context); init(context); } public ScrollLayout(Context context, AttributeSet attrs) { super(context, attrs); init(context); } public ScrollLayout(Context context, AttributeSet attrs, int defStyle) { super(context, attrs, defStyle); init(context); } private void init(Context context) { mCurScreen = mDefaultScreen; mScroller = new Scroller(context); } @Override protected void onLayout(boolean changed, int l, int t, int r, int b) { if (changed) { int childLeft = 0; final int childCount = getChildCount(); for (int i = 0; i < childCount; i++) { final View childView = getChildAt(i); if (childView.getVisibility() != View.GONE) { final int childWidth = childView.getMeasuredWidth(); childView.layout(childLeft, 0, childLeft + childWidth, childView.getMeasuredHeight()); childLeft += childWidth; } } } } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); final int width = MeasureSpec.getSize(widthMeasureSpec); final int count = getChildCount(); for (int i = 0; i < count; i++) { getChildAt(i).measure(widthMeasureSpec, heightMeasureSpec); } scrollTo(mCurScreen * width, 0); } public void snapToDestination() { final int screenWidth = getWidth(); final int destScreen = (getScrollX() + screenWidth / 2) / screenWidth; snapToScreen(destScreen); } public void snapToScreen(int whichScreen) { // get the valid layout page whichScreen = Math.max(0, Math.min(whichScreen, getChildCount() - 1)); if (getScrollX() != (whichScreen * getWidth())) { final int delta = whichScreen * getWidth() - getScrollX(); mScroller.startScroll(getScrollX(), 0, delta, 0, Math.abs(delta) * 2); mCurScreen = whichScreen; invalidate(); // Redraw the layout if (mOnViewChangeListener != null) { mOnViewChangeListener.OnViewChange(mCurScreen); } } } @Override public void computeScroll() { if (mScroller.computeScrollOffset()) { scrollTo(mScroller.getCurrX(), mScroller.getCurrY()); postInvalidate(); } } @Override public boolean onTouchEvent(MotionEvent event) { final int action = event.getAction(); final float x = event.getX(); switch (action) { case MotionEvent.ACTION_DOWN: if (mVelocityTracker == null) { mVelocityTracker = VelocityTracker.obtain(); mVelocityTracker.addMovement(event); } if (!mScroller.isFinished()) { mScroller.abortAnimation(); } mLastMotionX = x; break; case MotionEvent.ACTION_MOVE: int deltaX = (int) (mLastMotionX - x); if (IsCanMove(deltaX)) { if (mVelocityTracker != null) { mVelocityTracker.addMovement(event); } mLastMotionX = x; scrollBy(deltaX, 0); } break; case MotionEvent.ACTION_UP: int velocityX = 0; if (mVelocityTracker != null) { mVelocityTracker.addMovement(event); mVelocityTracker.computeCurrentVelocity(1000); velocityX = (int) mVelocityTracker.getXVelocity(); } if (velocityX > SNAP_VELOCITY && mCurScreen > 0) { // Fling enough to move left Log.e(TAG, "snap left"); snapToScreen(mCurScreen - 1); } else if (velocityX < -SNAP_VELOCITY && mCurScreen < getChildCount() - 1) { // Fling enough to move right Log.e(TAG, "snap right"); snapToScreen(mCurScreen + 1); } else { snapToDestination(); } if (mVelocityTracker != null) { mVelocityTracker.recycle(); mVelocityTracker = null; } break; } return true; } private boolean IsCanMove(int deltaX) { if (getScrollX() <= 0 && deltaX < 0) { return false; } if (getScrollX() >= (getChildCount() - 1) * getWidth() && deltaX > 0) { return false; } return true; } public void SetOnViewChangeListener(OnViewChangeListener listener) { mOnViewChangeListener = listener; } }