以下是从系统源码中抽出的ScrollView的代码，基本的功能都有，去掉了一些其他不重要的功能的代码：
import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewParent;
import android.view.animation.AnimationUtils;
import android.widget.EdgeEffect;
import android.widget.FrameLayout;
import android.widget.OverScroller;
public class MyScrollView extends FrameLayout {
private static final int ANIMATED_SCROLL_GAP = 250;
private long mLastScroll;
private OverScroller mScroller;
private EdgeEffect mEdgeGlowTop;
private EdgeEffect mEdgeGlowBottom;
private int mLastMotionY;
private boolean mIsBeingDragged = false;
private VelocityTracker mVelocityTracker;
private int mTouchSlop;
private int mMinimumVelocity;
private int mMaximumVelocity;
private int mOverscrollDistance;
private int mOverflingDistance;
private int mActivePointerId = INVALID_POINTER;
private static final int INVALID_POINTER = -1;
public MyScrollView(Context context) {
this(context, null);
}
public MyScrollView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyScrollView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
initScrollView();
}
private void initScrollView() {
mScroller = new OverScroller(getContext());
setFocusable(true);
setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
setWillNotDraw(false);
final ViewConfiguration configuration = ViewConfiguration.get(getContext());
mTouchSlop = configuration.getScaledTouchSlop();
mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
mOverscrollDistance = configuration.getScaledOverscrollDistance();
mOverflingDistance = configuration.getScaledOverflingDistance();
}
private boolean inChild(int x, int y) {
if (getChildCount() > 0) {
final int scrollY = getScrollY();
final View child = getChildAt(0);
return !(y < child.getTop() - scrollY
|| y >= child.getBottom() - scrollY
|| x < child.getLeft()
|| x >= child.getRight());
}
return false;
}
private void initOrResetVelocityTracker() {
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
} else {
mVelocityTracker.clear();
}
}
private void initVelocityTrackerIfNotExists() {
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
}
private void recycleVelocityTracker() {
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
final int action = ev.getAction();
if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
return true;
}
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_MOVE: {
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
break;
}
final int pointerIndex = ev.findPointerIndex(activePointerId);
if (pointerIndex == -1) {
break;
}
final int y = (int) ev.getY(pointerIndex);
final int yDiff = Math.abs(y - mLastMotionY);
if (yDiff > mTouchSlop) {
mIsBeingDragged = true;
mLastMotionY = y;
initVelocityTrackerIfNotExists();
mVelocityTracker.addMovement(ev);
}
break;
}
case MotionEvent.ACTION_DOWN: {
final int y = (int) ev.getY();
if (!inChild((int) ev.getX(), (int) y)) {
mIsBeingDragged = false;
recycleVelocityTracker();
break;
}
mLastMotionY = y;
mActivePointerId = ev.getPointerId(0);
initOrResetVelocityTracker();
mVelocityTracker.addMovement(ev);
mIsBeingDragged = !mScroller.isFinished();
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
mIsBeingDragged = false;
mActivePointerId = INVALID_POINTER;
recycleVelocityTracker();
break;
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}
return mIsBeingDragged;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
initVelocityTrackerIfNotExists();
mVelocityTracker.addMovement(ev);
final int action = ev.getAction();
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
if (getChildCount() == 0) {
return false;
}
if ((mIsBeingDragged = !mScroller.isFinished())) {
final ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
}
/*
* If being flinged and user touches, stop the fling. isFinished
* will be false if being flinged.
*/
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
// Remember where the motion event started
mLastMotionY = (int) ev.getY();
mActivePointerId = ev.getPointerId(0);
break;
}
case MotionEvent.ACTION_MOVE:
final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
if (activePointerIndex == -1) {
break;
}
final int y = (int) ev.getY(activePointerIndex);
int deltaY = mLastMotionY - y;
if (!mIsBeingDragged && Math.abs(deltaY) > mTouchSlop) {
final ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
mIsBeingDragged = true;
if (deltaY > 0) {
deltaY -= mTouchSlop;
} else {
deltaY += mTouchSlop;
}
}
if (mIsBeingDragged) {
// Scroll to follow the motion event
mLastMotionY = y;
final int oldX = getScrollX();
final int oldY = getScrollY();
final int range = getScrollRange();
final int overscrollMode = getOverScrollMode();
final boolean canOverscroll = overscrollMode == OVER_SCROLL_ALWAYS ||
(overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0);
if (overScrollBy(0, deltaY, 0, getScrollY(),
0, range, 0, mOverscrollDistance, true)) {
// Break our velocity if we hit a scroll barrier.
mVelocityTracker.clear();
}
if (canOverscroll) {
final int pulledToY = oldY + deltaY;
if (pulledToY < 0) {
mEdgeGlowTop.onPull((float) deltaY / getHeight());
if (!mEdgeGlowBottom.isFinished()) {
mEdgeGlowBottom.onRelease();
}
} else if (pulledToY > range) {
mEdgeGlowBottom.onPull((float) deltaY / getHeight());
if (!mEdgeGlowTop.isFinished()) {
mEdgeGlowTop.onRelease();
}
}
if (mEdgeGlowTop != null
&& (!mEdgeGlowTop.isFinished() || !mEdgeGlowBottom.isFinished())) {
postInvalidate();
}
}
}
break;
case MotionEvent.ACTION_UP:
if (mIsBeingDragged) {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
if (getChildCount() > 0) {
if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
fling(-initialVelocity);
} else {
if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0,
getScrollRange())) {
postInvalidate();
}
}
}
mActivePointerId = INVALID_POINTER;
endDrag();
}
break;
case MotionEvent.ACTION_CANCEL:
if (mIsBeingDragged && getChildCount() > 0) {
if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) {
postInvalidate();
}
mActivePointerId = INVALID_POINTER;
endDrag();
}
break;
case MotionEvent.ACTION_POINTER_DOWN: {
final int index = ev.getActionIndex();
mLastMotionY = (int) ev.getY(index);
mActivePointerId = ev.getPointerId(index);
break;
}
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
mLastMotionY = (int) ev.getY(ev.findPointerIndex(mActivePointerId));
break;
}
return true;
}
private void onSecondaryPointerUp(MotionEvent ev) {
final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK) >>
MotionEvent.ACTION_POINTER_INDEX_SHIFT;
final int pointerId = ev.getPointerId(pointerIndex);
if (pointerId == mActivePointerId) {
// This was our active pointer going up. Choose a new
// active pointer and adjust accordingly.
final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
mLastMotionY = (int) ev.getY(newPointerIndex);
mActivePointerId = ev.getPointerId(newPointerIndex);
if (mVelocityTracker != null) {
mVelocityTracker.clear();
}
}
}
@Override
protected void onOverScrolled(int scrollX, int scrollY,
boolean clampedX, boolean clampedY) {
// Treat animating scrolls differently; see #computeScroll() for why.
super.scrollTo(scrollX, scrollY);
awakenScrollBars();
}
private int getScrollRange() {
int scrollRange = 0;
if (getChildCount() > 0) {
View child = getChildAt(0);
scrollRange = Math.max(0,
child.getHeight() - (getHeight() - getPaddingBottom() - getPaddingTop()));
}
return scrollRange;
}
/**
* * Like {@link View#scrollBy}, but scroll smoothly instead of immediately.
* *
* * @param dx the number of pixels to scroll by on the X axis
* * @param dy the number of pixels to scroll by on the Y axis
*
*/
public final void smoothScrollBy(int dx, int dy) {
if (getChildCount() == 0) {
// Nothing to do.
return;
}
long duration = AnimationUtils.currentAnimationTimeMillis() - mLastScroll;
if (duration > ANIMATED_SCROLL_GAP) {
final int height = getHeight() - getPaddingBottom() - getPaddingTop();
final int bottom = getChildAt(0).getHeight();
final int maxY = Math.max(0, bottom - height);
final int scrollY = getScrollY();
dy = Math.max(0, Math.min(scrollY + dy, maxY)) - scrollY;
mScroller.startScroll(getScrollX(), scrollY, 0, dy);
postInvalidate();
} else {
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
scrollBy(dx, dy);
}
mLastScroll = AnimationUtils.currentAnimationTimeMillis();
}
/**
* * Like {@link #scrollTo}, but scroll smoothly instead of immediately.
* *
* * @param x the position where to scroll on the X axis
* * @param y the position where to scroll on the Y axis
*
*/
public final void smoothScrollTo(int x, int y) {
smoothScrollBy(x - getScrollX(), y - getScrollY());
}
/**
* * <p>The scroll range of a scroll view is the overall height of all of its
* * children.</p>
*
*/
@Override
protected int computeVerticalScrollRange() {
final int count = getChildCount();
final int contentHeight = getHeight() - getPaddingBottom() - getPaddingTop();
if (count == 0) {
return contentHeight;
}
int scrollRange = getChildAt(0).getBottom();
final int scrollY = getScrollY();
final int overscrollBottom = Math.max(0, scrollRange - contentHeight);
if (scrollY < 0) {
scrollRange -= scrollY;
} else if (scrollY > overscrollBottom) {
scrollRange += scrollY - overscrollBottom;
}
return scrollRange;
}
@Override
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
// This is called at drawing time by ViewGroup. We don't want to
// re-show the scrollbars at this point, which scrollTo will do,
// so we replicate most of scrollTo here.
//
// It's a little odd to call onScrollChanged from inside the drawing.
//
// It is, except when you remember that computeScroll() is used to
// animate scrolling. So unless we want to defer the onScrollChanged()
// until the end of the animated scrolling, we don't really have a
// choice here.
//
// I agree. The alternative, which I think would be worse, is to post
// something and tell the subclasses later. This is bad because there
// will be a window where mScrollX/Y is different from what the app
// thinks it is.
//
int oldX = getScrollX();
int oldY = getScrollY();
int x = mScroller.getCurrX();
int y = mScroller.getCurrY();
if (oldX != x || oldY != y) {
final int range = getScrollRange();
final int overscrollMode = getOverScrollMode();
final boolean canOverscroll = overscrollMode == OVER_SCROLL_ALWAYS ||
(overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0);
overScrollBy(x - oldX, y - oldY, oldX, oldY, 0, range,
0, mOverflingDistance, false);
if (canOverscroll) {
if (y < 0 && oldY >= 0) {
mEdgeGlowTop.onAbsorb((int) mScroller.getCurrVelocity());
} else if (y > range && oldY <= range) {
mEdgeGlowBottom.onAbsorb((int) mScroller.getCurrVelocity());
}
}
}
if (!awakenScrollBars()) {
// Keep on drawing until the animation has finished.
postInvalidate();
}
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
scrollTo(getScrollX(), getScrollY());
}
/**
* * Fling the scroll view
* *
* * @param velocityY The initial velocity in the Y direction. Positive
* * numbers mean that the finger/cursor is moving down the screen,
* * which means we want to scroll towards the top.
*
*/
public void fling(int velocityY) {
if (getChildCount() > 0) {
int height = getHeight() - getPaddingBottom() - getPaddingTop();
int bottom = getChildAt(0).getHeight();
mScroller.fling(getScrollX(), getScrollY(), 0, velocityY, 0, 0, 0,
Math.max(0, bottom - height), 0, height / 2);
postInvalidate();
}
}
private void endDrag() {
mIsBeingDragged = false;
recycleVelocityTracker();
if (mEdgeGlowTop != null) {
mEdgeGlowTop.onRelease();
mEdgeGlowBottom.onRelease();
}
}
@Override
public void scrollTo(int x, int y) {
if (getChildCount() > 0) {
View child = getChildAt(0);
x = clamp(x, getWidth() - getPaddingRight() - getPaddingLeft(), child.getWidth());
y = clamp(y, getHeight() - getPaddingBottom() - getPaddingTop(), child.getHeight());
if (x != getScrollX() || y != getScrollY()) {
super.scrollTo(x, y);
}
}
}
@Override
public void setOverScrollMode(int mode) {
if (mode != OVER_SCROLL_NEVER) {
if (mEdgeGlowTop == null) {
Context context = getContext();
mEdgeGlowTop = new EdgeEffect(context);
mEdgeGlowBottom = new EdgeEffect(context);
}
} else {
mEdgeGlowTop = null;
mEdgeGlowBottom = null;
}
super.setOverScrollMode(mode);
}
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mEdgeGlowTop != null) {
final int scrollY = getScrollY();
if (!mEdgeGlowTop.isFinished()) {
final int restoreCount = canvas.save();
final int width = getWidth() - getPaddingLeft() - getPaddingRight();
canvas.translate(getPaddingLeft(), Math.min(0, scrollY));
mEdgeGlowTop.setSize(width, getHeight());
if (mEdgeGlowTop.draw(canvas)) {
postInvalidate();
}
canvas.restoreToCount(restoreCount);
}
if (!mEdgeGlowBottom.isFinished()) {
final int restoreCount = canvas.save();
final int width = getWidth() - getPaddingLeft() - getPaddingRight();
final int height = getHeight();
canvas.translate(-width + getPaddingLeft(),
Math.max(getScrollRange(), scrollY) + height);
canvas.rotate(180, width, 0);
mEdgeGlowBottom.setSize(width, height);
if (mEdgeGlowBottom.draw(canvas)) {
postInvalidate();
}
canvas.restoreToCount(restoreCount);
}
}
}
private static int clamp(int n, int my, int child) {
if (my >= child || n < 0) {
return 0;
}
if ((my + n) > child) {
return child - my;
}
return n;
}
}
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