一。属性
1.setPageMargin
2.setOffscreenPageLimit
二。
1.什么时候调用PagerAdapter.instantiateItem
//在populate中，遍历mCurItem之前的， addNewItem满足这个条件的pos == ii.position && !ii.scrolling
遍历mCurItem之后的， addNewItem满足这个条件的pos == ii.position && !ii.scrolling
ItemInfo addNewItem(int position, int index) {
ItemInfo ii = new ItemInfo();
ii.position = position;
ii.object = mAdapter.instantiateItem(this, position);
ii.widthFactor = mAdapter.getPageWidth(position);
if (index < 0 || index >= mItems.size()) {
mItems.add(ii);
} else {
mItems.add(index, ii);
}
return ii;
} void populate(int newCurrentItem) {
... ...
// Locate the currently focused item or add it if needed.
ItemInfo curItem = null;
for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
final ItemInfo ii = mItems.get(curIndex);
if (ii.position >= mCurItem) {
if (ii.position == mCurItem) curItem = ii;
break;
}
}
if (curItem == null && N > 0) {
curItem = addNewItem(mCurItem, curIndex);
}
... ...
// Fill 3x the available width or up to the number of offscreen
// pages requested to either side, whichever is larger.
// If we have no current item we have no work to do.
if (curItem != null) {
float extraWidthLeft = 0.f;
int itemIndex = curIndex - 1;
ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
final int clientWidth = getClientWidth();
final float leftWidthNeeded = clientWidth <= 0 ? 0 :
2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
for (int pos = mCurItem - 1; pos >= 0; pos--) {
if (extraWidthLeft >= leftWidthNeeded && pos < startPos) {
if (ii == null) {
break;
}
if (pos == ii.position && !ii.scrolling) {
mItems.remove(itemIndex);
mAdapter.destroyItem(this, pos, ii.object);
if (DEBUG) {
Log.i(TAG, "populate() - destroyItem() with pos: " + pos
+ " view: " + ((View) ii.object));
}
itemIndex--;
curIndex--;
ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
}
} else if (ii != null && pos == ii.position) {
extraWidthLeft += ii.widthFactor;
itemIndex--;
ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
} else {
ii = addNewItem(pos, itemIndex + 1);
extraWidthLeft += ii.widthFactor;
curIndex++;
ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
}
}
... ...
if (extraWidthRight < 2.f) {
ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
final float rightWidthNeeded = clientWidth <= 0 ? 0 :
(float) getPaddingRight() / (float) clientWidth + 2.f;
for (int pos = mCurItem + 1; pos < N; pos++) {
if (extraWidthRight >= rightWidthNeeded && pos > endPos) {
if (ii == null) {
break;
}
if (pos == ii.position && !ii.scrolling) {
mItems.remove(itemIndex);
mAdapter.destroyItem(this, pos, ii.object);
if (DEBUG) {
Log.i(TAG, "populate() - destroyItem() with pos: " + pos
+ " view: " + ((View) ii.object));
}
ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
}
} else if (ii != null && pos == ii.position) {
extraWidthRight += ii.widthFactor;
itemIndex++;
ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
} else {
ii = addNewItem(pos, itemIndex);
itemIndex++;
extraWidthRight += ii.widthFactor;
ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
}
}
}
} 2.什么时候调用PagerAdapter.destroyItem
//在dataSetChanged中遍历mItems，利用PagerAdapter.getItemPosition() destroyItem pos
//在populate中，遍历mCurItem之前的， destroyItem满足这个条件的pos == ii.position && !ii.scrolling
遍历mCurItem之后的， destroyItem满足这个条件的pos == ii.position && !ii.scrolling
void dataSetChanged() {
... ...
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--;
if (!isUpdating) {
mAdapter.startUpdate(this);
isUpdating = true;
}
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, adapterCount - 1));
needPopulate = true;
}
continue;
}
... ...
}原理：
3.什么时候调用populate(item)
item是curItem
void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
... ...
if (mFirstLayout) {
// We don't have any idea how big we are yet and shouldn't have any pages either.
// Just set things up and let the pending layout handle things.
mCurItem = item;
if (dispatchSelected) {
dispatchOnPageSelected(item);
}
requestLayout();
} else {
populate(item);
scrollToItem(item, smoothScroll, velocity, dispatchSelected);
}
}
在onRestoreInstanceState，
onTouchEvent(MotionEvent ev) MotionEvent.ACTION_UP mIsBeingDragged==true时，
程序员调用setCurrentItem，setAdapter, endFakeDrag时
dataSetChanged，
private class PagerObserver extends DataSetObserver {
PagerObserver() {
}
@Override
public void onChanged() {
dataSetChanged();
}
@Override
public void onInvalidated() {
dataSetChanged();
}
}
PagerAdapter.java:
public void notifyDataSetChanged() {
synchronized (this) {
if (mViewPagerObserver != null) {
mViewPagerObserver.onChanged();
}
}
mObservable.notifyChanged();
}也就说除非用户自己调用setCurrentItem，setAdapter, endFakeDrag，PagerAdapter.notifyDataSetChanged ，只有在ACTION_UP时，才会调用populate(item)
