我最近上班又遇到一个小难题了，就是如题所述：ViewPager预加载的问题。相信用过ViewPager的人大抵都有遇到过这种情况，网上的解决办法也就那么几个，终于在我自己不断试验之下，完美解决了（禁止了）ViewPager的预加载。
好了，首先来说明一下，什么是ViewPager的预加载：ViewPager有一个 “预加载”的机制，默认会把ViewPager当前位置的左右相邻页面预先初始化（俗称的预加载），它的默认值是 1，这样做的好处就是ViewPager左右滑动会更加流畅。
可是我的情况很特殊，因为我 5 个Fragment里有一个Fragment是有SurfaceView的，这样造成的问题就是，我ViewPager滑动到其相邻页面时，含有SurfaceView的页面就会被预先初始化，然后SurfaceView就开始预览了，只是我们看不到而已。同样的，当我们从含有SurfaceView的页面滑倒其相邻的页面时，SurfaceView并不会回调其surfaceDestory方法。于是这给我造成了极大的困扰。
ok，下面言归正传，到底该怎么禁止ViewPager的这个预加载问题呢？
方案1：网上大多数说法是 懒加载，即让ViewPager预加载初始化UI，而具体一些数据，网络访问请求等延迟加载。这是靠Fragment里有一个setUserVisibleHint(boolean isVisibleToUser)的方法，我们可以在这个方法里做判断，当其True可见时（即切换到某一个具体Fragment）时才去加载数据，这样可以省流量。但这里并不满足我的需求，因为某一个Fragment并不会在ViewPager滑动到其相邻的Fragment时销毁。这个只可以解决部分人问题。
首先我们来深入了解下ViewPager的预加载机制：
上文提到过，ViewPager默认预加载的数量是1，这一点我们可以在ViewPager源码里看到。
DEFAULT_OFFSCREEN_PAGES 这里就定义了默认值是1， 所以网上 有种解决方案 说调用ViewPager的setOffscreenPageLimit(int limit)，来设置ViewPager预加载的数量，但是这里很明确的告诉你，这种方案是不可行的，如下图ViewPager源码：
我们可以看到，如果你调用该方法传进来的值小于1是无效的，会被强行的拽回1。而且DEFAULT_OFFSCREEN_PAGES 这个值是private的，子类继承ViewPager也是不可见的。
方案2：然后网上有第二种说法，自定义一个ViewPager，把原生ViewPager全拷过来，修改这个DEFAULT_OFFSCREEN_PAGES 值为0。对，就是这种解决方案！！但是！！
但是！！！但是什么呢？但是我试过，没用。为什么呢？接下来就是本文的重点了。
因为现在Android都6.0了，版本都老高了，其实android虽然每个版本都有v4包，但是这些v4包是有差异的。这就造成高版本v4包里的ViewPager，即使你Copy它，将其DEFAULT_OFFSCREEN_PAGES的值改为0，还是不起作用的，其中的逻辑变了。具体哪里变了导致无效我也没有继续研究了，毕竟公司不会花时间让我研究这些啊。偷笑
完美解决方案：ok，所以关于禁止ViewPager预加载的完美解决方案就是，使用低版本v4包里的ViewPager，完全copy一份，将其中的DEFAULT_OFFSCREEN_PAGES值改为0即可。博主亲测 API 14 即 Android 4.0的v4包里ViewPager 有效。
当然，谷歌既然有这么一种ViewPager的机制肯定有它的道理，所以一般还是预加载的好。
最后，因为低版本的源码越来越少的人会去下载，这里直接把这个禁止了预加载的ViewPager贴上来，需要的人就拿去吧。copy就能用了。
package com.plumcot.usb.view;
import android.content.Context;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Parcel;
import android.os.Parcelable;
import android.os.SystemClock;
import android.support.v4.os.ParcelableCompat;
import android.support.v4.os.ParcelableCompatCreatorCallbacks;
import android.support.v4.view.KeyEventCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.support.v4.view.ViewConfigurationCompat;
import android.support.v4.widget.EdgeEffectCompat;
import android.util.AttributeSet;
import android.util.Log;
import android.view.FocusFinder;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SoundEffectConstants;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;
import android.view.animation.Interpolator;
import android.widget.Scroller;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class NoPreloadViewPager extends ViewGroup {
private static final String TAG = "NoPreLoadViewPager";
private static final boolean DEBUG = false;
private static final boolean USE_CACHE = false;
private static final int DEFAULT_OFFSCREEN_PAGES = 0;//默认是1
private static final int MAX_SETTLE_DURATION = 600; // ms
static class ItemInfo {
Object object;
int position;
boolean scrolling;
}
private static final Comparator COMPARATOR = new Comparator(){
@Override
public int compare(ItemInfo lhs, ItemInfo rhs) {
return lhs.position - rhs.position;
}};
private static final Interpolator sInterpolator = new Interpolator() {
public float getInterpolation(float t) {
// _o(t) = t * t * ((tension + 1) * t + tension)
// o(t) = _o(t - 1) + 1
t -= 1.0f;
return t * t * t + 1.0f;
}
};
private final ArrayList mItems = new ArrayList();
private PagerAdapter mAdapter;
private int mCurItem; // Index of currently displayed page.
private int mRestoredCurItem = -1;
private Parcelable mRestoredAdapterState = null;
private ClassLoader mRestoredClassLoader = null;
private Scroller mScroller;
private PagerObserver mObserver;
private int mPageMargin;
private Drawable mMarginDrawable;
private int mChildWidthMeasureSpec;
private int mChildHeightMeasureSpec;
private boolean mInLayout;
private boolean mScrollingCacheEnabled;
private boolean mPopulatePending;
private boolean mScrolling;
private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;
private boolean mIsBeingDragged;
private boolean mIsUnableToDrag;
private int mTouchSlop;
private float mInitialMotionX;
private float mLastMotionX;
private float mLastMotionY;
private int mActivePointerId = INVALID_POINTER;
private static final int INVALID_POINTER = -1;
private VelocityTracker mVelocityTracker;
private int mMinimumVelocity;
private int mMaximumVelocity;
private float mbaseLineFlingVelocity;
private float mFlingVelocityInfluence;
private boolean mFakeDragging;
private long mFakeDragBeginTime;
private EdgeEffectCompat mLeftEdge;
private EdgeEffectCompat mRightEdge;
private boolean mFirstLayout = true;
private onPageChangeListener mOnPageChangeListener;
public static final int SCROLL_STATE_IDLE = 0;
public static final int SCROLL_STATE_DRAGGING = 1;
public static final int SCROLL_STATE_SETTLING = 2;
private int mScrollState = SCROLL_STATE_IDLE;
public interface onPageChangeListener {
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);
public void onPageSelected(int position);
public void onPageScrollStateChanged(int state);
}
public static class SimpleonPageChangeListener implements onPageChangeListener {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
// This space for rent
}
@Override
public void onPageSelected(int position) {
// This space for rent
}
@Override
public void onPageScrollStateChanged(int state) {
// This space for rent
}
}
public NoPreloadViewPager(Context context) {
super(context);
initViewPager();
}
public NoPreloadViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
initViewPager();
}
void initViewPager() {
setWillNotDraw(false);
setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
setFocusable(true);
final Context context = getContext();
mScroller = new Scroller(context, sInterpolator);
final ViewConfiguration configuration = ViewConfiguration.get(context);
mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration);
mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
mLeftEdge = new EdgeEffectCompat(context);
mRightEdge = new EdgeEffectCompat(context);
float density = context.getResources().getDisplayMetrics().density;
mbaseLineFlingVelocity = 2500.0f * density;
mFlingVelocityInfluence = 0.4f;
}
private void setScrollState(int newState) {
if (mScrollState == newState) {
return;
}
mScrollState = newState;
if (monPageChangeListener != null) {
mOnPageChangeListener.onPageScrollStateChanged(newState);
}
}
public void setAdapter(PagerAdapter adapter) {
if (mAdapter != null) {
// mAdapter.unregisterDataSetObserver(mObserver);
mAdapter.startUpdate(this);
for (int i = 0; i < mItems.size(); i++) {
final ItemInfo ii = mItems.get(i);
mAdapter.destroyItem(this, ii.position, ii.object);
}
mAdapter.finishUpdate(this);
mItems.clear();
removeAllViews();
mCurItem = 0;
scrollTo(0, 0);
}
mAdapter = adapter;
if (mAdapter != null) {
if (mObserver == null) {
mObserver = new PagerObserver();
}
// mAdapter.registerDataSetObserver(mObserver);
mPopulatePending = false;
if (mRestoredCurItem >= 0) {
mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
setCurrentItemInternal(mRestoredCurItem, false, true);
mRestoredCurItem = -1;
mRestoredAdapterState = null;
mRestoredClassLoader = null;
} else {
populate();
}
}
}
public PagerAdapter getAdapter() {
return mAdapter;
}
public void setCurrentItem(int item) {
mPopulatePending = false;
setCurrentItemInternal(item, !mFirstLayout, false);
}
public void setCurrentItem(int item, boolean smoothScroll) {
mPopulatePending = false;
setCurrentItemInternal(item, smoothScroll, false);
}
public int getCurrentItem() {
return mCurItem;
}
void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
setCurrentItemInternal(item, smoothScroll, always, 0);
}
void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
if (mAdapter == null || mAdapter.getCount() <= 0) {
setScrollingCacheEnabled(false);
return;
}
if (!always && mCurItem == item && mItems.size() != 0) {
setScrollingCacheEnabled(false);
return;
}
if (item < 0) {
item = 0;
} else if (item >= mAdapter.getCount()) {
item = mAdapter.getCount() - 1;
}
final int pageLimit = mOffscreenPageLimit;
if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
// We are doing a jump by more than one page. To avoid
// glitches, we want to keep all current pages in the view
// until the scroll ends.
for (int i=0; i 0) {
duration += (duration / (velocity / mbaseLineFlingVelocity)) * mFlingVelocityInfluence;
} else {
duration += 100;
}
duration = Math.min(duration, MAX_SETTLE_DURATION);
mScroller.startScroll(sx, sy, dx, dy, duration);
invalidate();
}
void addNewItem(int position, int index) {
ItemInfo ii = new ItemInfo();
ii.position = position;
ii.object = mAdapter.instantiateItem(this, position);
if (index < 0) {
mItems.add(ii);
} else {
mItems.add(index, ii);
}
}
void dataSetChanged() {
// This method only gets called if our observer is attached, so mAdapter is non-null.
boolean needPopulate = mItems.size() < 3 && mItems.size() < mAdapter.getCount();
int newCurrItem = -1;
for (int i = 0; i < mItems.size(); i++) {
final ItemInfo ii = mItems.get(i);
final int newPos = mAdapter.getItemPosition(ii.object);
if (newPos == PagerAdapter.POSITION_UNCHANGED) {
continue;
}
if (newPos == PagerAdapter.POSITION_NONE) {
mItems.remove(i);
i--;
mAdapter.destroyItem(this, ii.position, ii.object);
needPopulate = true;
if (mCurItem == ii.position) {
// Keep the current item in the valid range
newCurrItem = Math.max(0, Math.min(mCurItem, mAdapter.getCount() - 1));
}
continue;
}
if (ii.position != newPos) {
if (ii.position == mCurItem) {
// Our current item changed position. Follow it.
newCurrItem = newPos;
}
ii.position = newPos;
needPopulate = true;
}
}
Collections.sort(mItems, COMPARATOR);
if (newCurrItem >= 0) {
// TODO This currently causes a jump.
setCurrentItemInternal(newCurrItem, false, true);
needPopulate = true;
}
if (needPopulate) {
populate();
requestLayout();
}
}
void populate() {
if (mAdapter == null) {
return;
}
// Bail now if we are waiting to populate. This is to hold off
// on creating views from the time the user releases their finger to
// fling to a new position until we have finished the scroll to
// that position, avoiding glitches from happening at that point.
if (mPopulatePending) {
if (DEBUG) Log.i(TAG, "populate is pending, skipping for now...");
return;
}
// Also, don't populate until we are attached to a window. This is to
// avoid trying to populate before we have restored our view hierarchy
// state and conflicting with what is restored.
if (getWindowToken() == null) {
return;
}
mAdapter.startUpdate(this);
final int pageLimit = mOffscreenPageLimit;
final int startPos = Math.max(0, mCurItem - pageLimit);
final int N = mAdapter.getCount();
final int endPos = Math.min(N-1, mCurItem + pageLimit);
if (DEBUG) Log.v(TAG, "populating: startPos=" + startPos + " endPos=" + endPos);
// Add and remove pages in the existing list.
int lastPos = -1;
for (int i=0; i endPos) && !ii.scrolling) {
if (DEBUG) Log.i(TAG, "removing: " + ii.position + " @ " + i);
mItems.remove(i);
i--;
mAdapter.destroyItem(this, ii.position, ii.object);
} else if (lastPos < endPos && ii.position > startPos) {
// The next item is outside of our range, but we have a gap
// between it and the last item where we want to have a page
// shown. Fill in the gap.
lastPos++;
if (lastPos < startPos) {
lastPos = startPos;
}
while (lastPos <= endPos && lastPos < ii.position) {
if (DEBUG) Log.i(TAG, "inserting: " + lastPos + " @ " + i);
addNewItem(lastPos, i);
lastPos++;
i++;
}
}
lastPos = ii.position;
}
// Add any new pages we need at the end.
lastPos = mItems.size() > 0 ? mItems.get(mItems.size()-1).position : -1;
if (lastPos < endPos) {
lastPos++;
lastPos = lastPos > startPos ? lastPos : startPos;
while (lastPos <= endPos) {
if (DEBUG) Log.i(TAG, "appending: " + lastPos);
addNewItem(lastPos, -1);
lastPos++;
}
}
if (DEBUG) {
Log.i(TAG, "Current page list:");
for (int i=0; i CREATOR
= ParcelableCompat.newCreator(new ParcelableCompatCreatorCallbacks() {
@Override
public SavedState createFromParcel(Parcel in, ClassLoader loader) {
return new SavedState(in, loader);
}
@Override
public SavedState[] newArray(int size) {
return new SavedState[size];
}
});
SavedState(Parcel in, ClassLoader loader) {
super(in);
if (loader == null) {
loader = getClass().getClassLoader();
}
position = in.readInt();
adapterState = in.readParcelable(loader);
this.loader = loader;
}
}
@Override
public Parcelable onSaveInstanceState() {
Parcelable superState = super.onSaveInstanceState();
SavedState ss = new SavedState(superState);
ss.position = mCurItem;
if (mAdapter != null) {
ss.adapterState = mAdapter.saveState();
}
return ss;
}
@Override
public void onRestoreInstanceState(Parcelable state) {
if (!(state instanceof SavedState)) {
super.onRestoreInstanceState(state);
return;
}
SavedState ss = (SavedState)state;
super.onRestoreInstanceState(ss.getSuperState());
if (mAdapter != null) {
mAdapter.restoreState(ss.adapterState, ss.loader);
setCurrentItemInternal(ss.position, false, true);
} else {
mRestoredCurItem = ss.position;
mRestoredAdapterState = ss.adapterState;
mRestoredClassLoader = ss.loader;
}
}
@Override
public void addView(View child, int index, LayoutParams params) {
if (mInLayout) {
addViewInLayout(child, index, params);
child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec);
} else {
super.addView(child, index, params);
}
if (USE_CACHE) {
if (child.getVisibility() != GONE) {
child.setDrawingCacheEnabled(mScrollingCacheEnabled);
} else {
child.setDrawingCacheEnabled(false);
}
}
}
ItemInfo infoForChild(View child) {
for (int i=0; i 0) {
final int oldScrollPos = getScrollX();
final int oldwwm = oldWidth + oldMargin;
final int oldScrollItem = oldScrollPos / oldwwm;
final float scrollOffset = (float) (oldScrollPos % oldwwm) / oldwwm;
final int scrollPos = (int) ((oldScrollItem + scrollOffset) * widthWithMargin);
scrollTo(scrollPos, getScrollY());
if (!mScroller.isFinished()) {
// We now return to your regularly scheduled scroll, already in progress.
final int newDuration = mScroller.getDuration() - mScroller.timePassed();
mScroller.startScroll(scrollPos, 0, mCurItem * widthWithMargin, 0, newDuration);
}
} else {
int scrollPos = mCurItem * widthWithMargin;
if (scrollPos != getScrollX()) {
completeScroll();
scrollTo(scrollPos, getScrollY());
}
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
mInLayout = true;
populate();
mInLayout = false;
final int count = getChildCount();
final int width = r-l;
for (int i = 0; i < count; i++) {
View child = getChildAt(i);
ItemInfo ii;
if (child.getVisibility() != GONE && (ii=infoForChild(child)) != null) {
int loff = (width + mPageMargin) * ii.position;
int childLeft = getPaddingLeft() + loff;
int childTop = getPaddingTop();
if (DEBUG) Log.v(TAG, "Positioning #" + i + " " + child + " f=" + ii.object
+ ":" + childLeft + "," + childTop + " " + child.getMeasuredWidth()
+ "x" + child.getMeasuredHeight());
child.layout(childLeft, childTop,
childLeft + child.getMeasuredWidth(),
childTop + child.getMeasuredHeight());
}
}
mFirstLayout = false;
}
@Override
public void computeScroll() {
if (DEBUG) Log.i(TAG, "computeScroll: finished=" + mScroller.isFinished());
if (!mScroller.isFinished()) {
if (mScroller.computeScrollOffset()) {
if (DEBUG) Log.i(TAG, "computeScroll: still scrolling");
int oldX = getScrollX();
int oldY = getScrollY();
int x = mScroller.getCurrX();
int y = mScroller.getCurrY();
if (oldX != x || oldY != y) {
scrollTo(x, y);
}
if (monPageChangeListener != null) {
final int widthWithMargin = getWidth() + mPageMargin;
final int position = x / widthWithMargin;
final int offsetPixels = x % widthWithMargin;
final float offset = (float) offsetPixels / widthWithMargin;
mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels);
}
// Keep on drawing until the animation has finished.
invalidate();
return;
}
}
// Done with scroll, clean up state.
completeScroll();
}
private void completeScroll() {
boolean needPopulate = mScrolling;
if (needPopulate) {
// Done with scroll, no longer want to cache view drawing.
setScrollingCacheEnabled(false);
mScroller.abortAnimation();
int oldX = getScrollX();
int oldY = getScrollY();
int x = mScroller.getCurrX();
int y = mScroller.getCurrY();
if (oldX != x || oldY != y) {
scrollTo(x, y);
}
setScrollState(SCROLL_STATE_IDLE);
}
mPopulatePending = false;
mScrolling = false;
for (int i=0; i 0 && scrollX == 0) || (dx < 0 && mAdapter != null &&
scrollX >= (mAdapter.getCount() - 1) * getWidth() - 1);
if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);
if (canScroll(this, false, (int) dx, (int) x, (int) y)) {
// Nested view has scrollable area under this point. Let it be handled there.
mInitialMotionX = mLastMotionX = x;
mLastMotionY = y;
return false;
}
if (xDiff > mTouchSlop && xDiff > yDiff) {
if (DEBUG) Log.v(TAG, "Starting drag!");
mIsBeingDragged = true;
setScrollState(SCROLL_STATE_DRAGGING);
mLastMotionX = x;
setScrollingCacheEnabled(true);
} else {
if (yDiff > mTouchSlop) {
// The finger has moved enough in the vertical
// direction to be counted as a drag... abort
// any attempt to drag horizontally, to work correctly
// with children that have scrolling containers.
if (DEBUG) Log.v(TAG, "Starting unable to drag!");
mIsUnableToDrag = true;
}
}
break;
}
case MotionEvent.ACTION_DOWN: {
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = ev.getY();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
if (mScrollState == SCROLL_STATE_SETTLING) {
// Let the user 'catch' the pager as it animates.
mIsBeingDragged = true;
mIsUnableToDrag = false;
setScrollState(SCROLL_STATE_DRAGGING);
} else {
completeScroll();
mIsBeingDragged = false;
mIsUnableToDrag = false;
}
if (DEBUG) Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY
+ " mIsBeingDragged=" + mIsBeingDragged
+ "mIsUnableToDrag=" + mIsUnableToDrag);
break;
}
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}
return mIsBeingDragged;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (mFakeDragging) {
// A fake drag is in progress already, ignore this real one
// but still eat the touch events.
// (It is likely that the user is multi-touching the screen.)
return true;
}
if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
// Don't handle edge touches immediately -- they may actually belong to one of our
// descendants.
return false;
}
if (mAdapter == null || mAdapter.getCount() == 0) {
// Nothing to present or scroll; nothing to touch.
return false;
}
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
final int action = ev.getAction();
boolean needsInvalidate = false;
switch (action & MotionEventCompat.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
completeScroll();
// Remember where the motion event started
mLastMotionX = mInitialMotionX = ev.getX();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
break;
}
case MotionEvent.ACTION_MOVE:
if (!mIsBeingDragged) {
final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
final float x = MotionEventCompat.getX(ev, pointerIndex);
final float xDiff = Math.abs(x - mLastMotionX);
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = Math.abs(y - mLastMotionY);
if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);
if (xDiff > mTouchSlop && xDiff > yDiff) {
if (DEBUG) Log.v(TAG, "Starting drag!");
mIsBeingDragged = true;
mLastMotionX = x;
setScrollState(SCROLL_STATE_DRAGGING);
setScrollingCacheEnabled(true);
}
}
if (mIsBeingDragged) {
// Scroll to follow the motion event
final int activePointerIndex = MotionEventCompat.findPointerIndex(
ev, mActivePointerId);
final float x = MotionEventCompat.getX(ev, activePointerIndex);
final float deltaX = mLastMotionX - x;
mLastMotionX = x;
float oldScrollX = getScrollX();
float scrollX = oldScrollX + deltaX;
final int width = getWidth();
final int widthWithMargin = width + mPageMargin;
final int lastItemIndex = mAdapter.getCount() - 1;
final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin);
final float rightBound =
Math.min(mCurItem + 1, lastItemIndex) * widthWithMargin;
if (scrollX < leftBound) {
if (leftBound == 0) {
float over = -scrollX;
needsInvalidate = mLeftEdge.onPull(over / width);
}
scrollX = leftBound;
} else if (scrollX > rightBound) {
if (rightBound == lastItemIndex * widthWithMargin) {
float over = scrollX - rightBound;
needsInvalidate = mRightEdge.onPull(over / width);
}
scrollX = rightBound;
}
// Don't lose the rounded component
mLastMotionX += scrollX - (int) scrollX;
scrollTo((int) scrollX, getScrollY());
if (monPageChangeListener != null) {
final int position = (int) scrollX / widthWithMargin;
final int positionOffsetPixels = (int) scrollX % widthWithMargin;
final float positionOffset = (float) positionOffsetPixels / widthWithMargin;
mOnPageChangeListener.onPageScrolled(position, positionOffset,
positionOffsetPixels);
}
}
break;
case MotionEvent.ACTION_UP:
if (mIsBeingDragged) {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) VelocityTrackerCompat.getXVelocity(
velocityTracker, mActivePointerId);
mPopulatePending = true;
final int widthWithMargin = getWidth() + mPageMargin;
final int scrollX = getScrollX();
final int currentPage = scrollX / widthWithMargin;
int nextPage = initialVelocity > 0 ? currentPage : currentPage + 1;
setCurrentItemInternal(nextPage, true, true, initialVelocity);
mActivePointerId = INVALID_POINTER;
endDrag();
needsInvalidate = mLeftEdge.onRelease() | mRightEdge.onRelease();
}
break;
case MotionEvent.ACTION_CANCEL:
if (mIsBeingDragged) {
setCurrentItemInternal(mCurItem, true, true);
mActivePointerId = INVALID_POINTER;
endDrag();
needsInvalidate = mLeftEdge.onRelease() | mRightEdge.onRelease();
}
break;
case MotionEventCompat.ACTION_POINTER_DOWN: {
final int index = MotionEventCompat.getActionIndex(ev);
final float x = MotionEventCompat.getX(ev, index);
mLastMotionX = x;
mActivePointerId = MotionEventCompat.getPointerId(ev, index);
break;
}
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
mLastMotionX = MotionEventCompat.getX(ev,
MotionEventCompat.findPointerIndex(ev, mActivePointerId));
break;
}
if (needsInvalidate) {
invalidate();
}
return true;
}
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
boolean needsInvalidate = false;
final int overScrollMode = ViewCompat.getOverScrollMode(this);
if (overScrollMode == ViewCompat.OVER_SCROLL_ALWAYS ||
(overScrollMode == ViewCompat.OVER_SCROLL_IF_CONTENT_SCROLLS &&
mAdapter != null && mAdapter.getCount() > 1)) {
if (!mLeftEdge.isFinished()) {
final int restoreCount = canvas.save();
final int height = getHeight() - getPaddingTop() - getPaddingBottom();
canvas.rotate(270);
canvas.translate(-height + getPaddingTop(), 0);
mLeftEdge.setSize(height, getWidth());
needsInvalidate |= mLeftEdge.draw(canvas);
canvas.restoreToCount(restoreCount);
}
if (!mRightEdge.isFinished()) {
final int restoreCount = canvas.save();
final int width = getWidth();
final int height = getHeight() - getPaddingTop() - getPaddingBottom();
final int itemCount = mAdapter != null ? mAdapter.getCount() : 1;
canvas.rotate(90);
canvas.translate(-getPaddingTop(),
-itemCount * (width + mPageMargin) + mPageMargin);
mRightEdge.setSize(height, width);
needsInvalidate |= mRightEdge.draw(canvas);
canvas.restoreToCount(restoreCount);
}
} else {
mLeftEdge.finish();
mRightEdge.finish();
}
if (needsInvalidate) {
// Keep animating
invalidate();
}
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// Draw the margin drawable if needed.
if (mPageMargin > 0 && mMarginDrawable != null) {
final int scrollX = getScrollX();
final int width = getWidth();
final int offset = scrollX % (width + mPageMargin);
if (offset != 0) {
// Pages fit completely when settled; we only need to draw when in between
final int left = scrollX - offset + width;
mMarginDrawable.setBounds(left, 0, left + mPageMargin, getHeight());
mMarginDrawable.draw(canvas);
}
}
}
public boolean beginFakeDrag() {
if (mIsBeingDragged) {
return false;
}
mFakeDragging = true;
setScrollState(SCROLL_STATE_DRAGGING);
mInitialMotionX = mLastMotionX = 0;
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
} else {
mVelocityTracker.clear();
}
final long time = SystemClock.uptimeMillis();
final MotionEvent ev = MotionEvent.obtain(time, time, MotionEvent.ACTION_DOWN, 0, 0, 0);
mVelocityTracker.addMovement(ev);
ev.recycle();
mFakeDragBeginTime = time;
return true;
}
public void endFakeDrag() {
if (!mFakeDragging) {
throw new IllegalStateException("No fake drag in progress. Call beginFakeDrag first.");
}
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int)VelocityTrackerCompat.getYVelocity(
velocityTracker, mActivePointerId);
mPopulatePending = true;
if ((Math.abs(initialVelocity) > mMinimumVelocity)
|| Math.abs(mInitialMotionX-mLastMotionX) >= (getWidth()/3)) {
if (mLastMotionX > mInitialMotionX) {
setCurrentItemInternal(mCurItem-1, true, true);
} else {
setCurrentItemInternal(mCurItem+1, true, true);
}
} else {
setCurrentItemInternal(mCurItem, true, true);
}
endDrag();
mFakeDragging = false;
}
public void fakeDragBy(float xOffset) {
if (!mFakeDragging) {
throw new IllegalStateException("No fake drag in progress. Call beginFakeDrag first.");
}
mLastMotionX += xOffset;
float scrollX = getScrollX() - xOffset;
final int width = getWidth();
final int widthWithMargin = width + mPageMargin;
final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin);
final float rightBound =
Math.min(mCurItem + 1, mAdapter.getCount() - 1) * widthWithMargin;
if (scrollX < leftBound) {
scrollX = leftBound;
} else if (scrollX > rightBound) {
scrollX = rightBound;
}
// Don't lose the rounded component
mLastMotionX += scrollX - (int) scrollX;
scrollTo((int) scrollX, getScrollY());
if (monPageChangeListener != null) {
final int position = (int) scrollX / widthWithMargin;
final int positionOffsetPixels = (int) scrollX % widthWithMargin;
final float positionOffset = (float) positionOffsetPixels / widthWithMargin;
mOnPageChangeListener.onPageScrolled(position, positionOffset,
positionOffsetPixels);
}
// Synthesize an event for the VelocityTracker.
final long time = SystemClock.uptimeMillis();
final MotionEvent ev = MotionEvent.obtain(mFakeDragBeginTime, time, MotionEvent.ACTION_MOVE,
mLastMotionX, 0, 0);
mVelocityTracker.addMovement(ev);
ev.recycle();
}
public boolean isFakeDragging() {
return mFakeDragging;
}
private void onSecondaryPointerUp(MotionEvent ev) {
final int pointerIndex = MotionEventCompat.getActionIndex(ev);
final int pointerId = MotionEventCompat.getPointerId(ev, 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;
mLastMotionX = MotionEventCompat.getX(ev, newPointerIndex);
mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
if (mVelocityTracker != null) {
mVelocityTracker.clear();
}
}
}
private void endDrag() {
mIsBeingDragged = false;
mIsUnableToDrag = false;
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
}
private void setScrollingCacheEnabled(boolean enabled) {
if (mScrollingCacheEnabled != enabled) {
mScrollingCacheEnabled = enabled;
if (USE_CACHE) {
final int size = getChildCount();
for (int i = 0; i < size; ++i) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
child.setDrawingCacheEnabled(enabled);
}
}
}
}
}
protected boolean canScroll(View v, boolean checkV, int dx, int x, int y) {
if (v instanceof ViewGroup) {
final ViewGroup group = (ViewGroup) v;
final int scrollX = v.getScrollX();
final int scrollY = v.getScrollY();
final int count = group.getChildCount();
// Count backwards - let topmost views consume scroll distance first.
for (int i = count - 1; i >= 0; i--) {
// TODO: Add versioned support here for transformed views.
// This will not work for transformed views in Honeycomb+
final View child = group.getChildAt(i);
if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight() &&
y + scrollY >= child.getTop() && y + scrollY < child.getBottom() &&
canScroll(child, true, dx, x + scrollX - child.getLeft(),
y + scrollY - child.getTop())) {
return true;
}
}
}
return checkV && ViewCompat.canScrollHorizontally(v, -dx);
}
@Override
public boolean dispatchKeyEvent(KeyEvent event) {
// Let the focused view and/or our descendants get the key first
return super.dispatchKeyEvent(event) || executeKeyEvent(event);
}
public boolean executeKeyEvent(KeyEvent event) {
boolean handled = false;
if (event.getAction() == KeyEvent.ACTION_DOWN) {
switch (event.getKeyCode()) {
case KeyEvent.KEYCODE_DPAD_LEFT:
handled = arrowScroll(FOCUS_LEFT);
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
handled = arrowScroll(FOCUS_RIGHT);
break;
case KeyEvent.KEYCODE_TAB:
if (KeyEventCompat.hasNoModifiers(event)) {
handled = arrowScroll(FOCUS_FORWARD);
} else if (KeyEventCompat.hasModifiers(event, KeyEvent.meta_SHIFT_ON)) {
handled = arrowScroll(FOCUS_BACKWARD);
}
break;
}
}
return handled;
}
public boolean arrowScroll(int direction) {
View currentFocused = findFocus();
if (currentFocused == this) currentFocused = null;
boolean handled = false;
View nextFocused = FocusFinder.getInstance().findNextFocus(this, currentFocused,
direction);
if (nextFocused != null && nextFocused != currentFocused) {
if (direction == View.FOCUS_LEFT) {
// If there is nothing to the left, or this is causing us to
// jump to the right, then what we really want to do is page left.
if (currentFocused != null && nextFocused.getLeft() >= currentFocused.getLeft()) {
handled = pageLeft();
} else {
handled = nextFocused.requestFocus();
}
} else if (direction == View.FOCUS_RIGHT) {
// If there is nothing to the right, or this is causing us to
// jump to the left, then what we really want to do is page right.
if (currentFocused != null && nextFocused.getLeft() <= currentFocused.getLeft()) {
handled = pageRight();
} else {
handled = nextFocused.requestFocus();
}
}
} else if (direction == FOCUS_LEFT || direction == FOCUS_BACKWARD) {
// Trying to move left and nothing there; try to page.
handled = pageLeft();
} else if (direction == FOCUS_RIGHT || direction == FOCUS_FORWARD) {
// Trying to move right and nothing there; try to page.
handled = pageRight();
}
if (handled) {
playSoundEffect(SoundEffectConstants.getContantForFocusDirection(direction));
}
return handled;
}
boolean pageLeft() {
if (mCurItem > 0) {
setCurrentItem(mCurItem-1, true);
return true;
}
return false;
}
boolean pageRight() {
if (mAdapter != null && mCurItem < (mAdapter.getCount()-1)) {
setCurrentItem(mCurItem+1, true);
return true;
}
return false;
}
@Override
public void addFocusables(ArrayList views, int direction, int focusableMode) {
final int focusableCount = views.size();
final int descendantFocusability = getDescendantFocusability();
if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
for (int i = 0; i < getChildCount(); i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
child.addFocusables(views, direction, focusableMode);
}
}
}
}
// we add ourselves (if focusable) in all cases except for when we are
// FOCUS_AFTER_DESCENDANTS and there are some descendants focusable. this is
// to avoid the focus search finding layouts when a more precise search
// among the focusable children would be more interesting.
if (
descendantFocusability != FOCUS_AFTER_DESCENDANTS ||
// No focusable descendants
(focusableCount == views.size())) {
// Note that we can't call the superclass here, because it will
// add all views in. So we need to do the same thing View does.
if (!isFocusable()) {
return;
}
if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE &&
isInTouchMode() && !isFocusableInTouchMode()) {
return;
}
if (views != null) {
views.add(this);
}
}
}
@Override
public void addTouchables(ArrayList views) {
// Note that we don't call super.addTouchables(), which means that
// we don't call View.addTouchables(). This is okay because a ViewPager
// is itself not touchable.
for (int i = 0; i < getChildCount(); i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
child.addTouchables(views);
}
}
}
}
@Override
protected boolean onRequestFocusInDescendants(int direction,
Rect previouslyFocusedRect) {
int index;
int increment;
int end;
int count = getChildCount();
if ((direction & FOCUS_FORWARD) != 0) {
index = 0;
increment = 1;
end = count;
} else {
index = count - 1;
increment = -1;
end = -1;
}
for (int i = index; i != end; i += increment) {
View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
if (child.requestFocus(direction, previouslyFocusedRect)) {
return true;
}
}
}
}
return false;
}
@Override
public boolean dispatchPopulateAccessibilityEvent(AccessibilityEvent event) {
// ViewPagers should only report accessibility info for the current page,
// otherwise things get very confusing.
// TODO: Should this note something about the paging container?
final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
final ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem &&
child.dispatchPopulateAccessibilityEvent(event)) {
return true;
}
}
}
return false;
}
private class PagerObserver extends DataSetObserver {
@Override
public void onChanged() {
dataSetChanged();
}
@Override
public void onInvalidated() {
dataSetChanged();
}
}
}
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持考高分网。
