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ViewPager内有适配器，适配的是一个个的View，Fragment的本质是一个View
为什么说Fragment本质是个View
Fragment对View的封装，是使用组合
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ViewPager.populate()
void populate() { populate(mCurItem);} void populate(int newCurrentItem) { ItemInfo oldCurInfo = null; if (mCurItem != newCurrentItem) { oldCurInfo = infoForPosition(mCurItem); mCurItem = newCurrentItem; } if (mAdapter == null) { sortChildDrawingOrder(); 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..."); sortChildDrawingOrder(); 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 (N != mExpectedAdapterCount) { String resName; try { resName = getResources().getResourceName(getId()); } catch (Resources.NotFoundException e) { resName = Integer.toHexString(getId()); } throw new IllegalStateException("The application's PagerAdapter changed the adapter's" + " contents without calling PagerAdapter#notifyDataSetChanged!" + " Expected adapter item count: " + mExpectedAdapterCount + ", found: " + N + " Pager id: " + resName + " Pager class: " + getClass() + " Problematic adapter: " + mAdapter.getClass()); } // Locate the currently focused item or add it if needed. int curIndex = -1; 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; } } // 第一次进来，什么数据都没有 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; // 当前页左边的缓存和去缓存处理 for (int pos = mCurItem - 1; pos >= 0; pos--) { if (extraWidthLeft >= leftWidthNeeded && pos < startPos) { if (ii == null) { break; } if (pos == ii.position && !ii.scrolling) { // 把左边要销毁的View移除缓存 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; } } float extraWidthRight = curItem.widthFactor; itemIndex = curIndex + 1; if (extraWidthRight < 2.f) { ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null; final float rightWidthNeeded = clientWidth <= 0 ? 0 : (float) getPaddingRight() / (float) clientWidth + 2.f; // 当前页右边的缓存和去缓存处理 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; } } } calculatePageOffsets(curItem, curIndex, oldCurInfo); mAdapter.setPrimaryItem(this, mCurItem, curItem.object); } if (DEBUG) { Log.i(TAG, "Current page list:"); for (int i = 0; i < mItems.size(); i++) { Log.i(TAG, "#" + i + ": page " + mItems.get(i).position); } } mAdapter.finishUpdate(this); // Check width measurement of current pages and drawing sort order. // Update LayoutParams as needed. final int childCount = getChildCount(); for (int i = 0; i < childCount; i++) { final View child = getChildAt(i); final LayoutParams lp = (LayoutParams) child.getLayoutParams(); lp.childIndex = i; if (!lp.isDecor && lp.widthFactor == 0.f) { // 0 means requery the adapter for this, it doesn't have a valid width. final ItemInfo ii = infoForChild(child); if (ii != null) { lp.widthFactor = ii.widthFactor; lp.position = ii.position; } } } sortChildDrawingOrder(); if (hasFocus()) { View currentFocused = findFocus(); ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null; if (ii == null || ii.position != mCurItem) { for (int i = 0; i < getChildCount(); i++) { View child = getChildAt(i); ii = infoForChild(child); if (ii != null && ii.position == mCurItem) { if (child.requestFocus(View.FOCUS_FORWARD)) { break; } } } } } }
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addNewItem()
ItemInfo addNewItem(int position, int index) { ItemInfo ii = new ItemInfo(); ii.position = position; // 根据不同类型的adapter返回不同的类型 // 比如FragmentPagerAdapter返回的是fragment 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; }
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FragmentViewPager.instantiateItem()
@NonNull public Object instantiateItem(@NonNull ViewGroup container, int position) { if (this.mCurTransaction == null) { this.mCurTransaction = this.mFragmentManager.beginTransaction(); } long itemId = this.getItemId(position); String name = makeFragmentName(container.getId(), itemId); Fragment fragment = this.mFragmentManager.findFragmentByTag(name); if (fragment != null) { // 如果fragment没有被销毁，直接attach this.mCurTransaction.attach(fragment); } else { fragment = this.getItem(position); this.mCurTransaction.add(container.getId(), fragment, makeFragmentName(container.getId(), itemId)); } if (fragment != this.mCurrentPrimaryItem) { fragment.setMenuVisibility(false); if (this.mBehavior == 1) { this.mCurTransaction.setMaxLifecycle(fragment, State.STARTED); } else { fragment.setUserVisibleHint(false); } } return fragment; }
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FragmentPagerAdapter.setPrimaryItem()
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) { Fragment fragment = (Fragment)object; if (fragment != this.mCurrentPrimaryItem) { if (this.mCurrentPrimaryItem != null) { // 旧的当前页设置为不展示 this.mCurrentPrimaryItem.setMenuVisibility(false); if (this.mBehavior == 1) { if (this.mCurTransaction == null) { this.mCurTransaction = this.mFragmentManager.beginTransaction(); } this.mCurTransaction.setMaxLifecycle(this.mCurrentPrimaryItem, State.STARTED); } else { this.mCurrentPrimaryItem.setUserVisibleHint(false); } } // 当前页设置为展示 fragment.setMenuVisibility(true); if (this.mBehavior == 1) { if (this.mCurTransaction == null) { this.mCurTransaction = this.mFragmentManager.beginTransaction(); } this.mCurTransaction.setMaxLifecycle(fragment, State.RESUMED); } else { fragment.setUserVisibleHint(true); } this.mCurrentPrimaryItem = fragment; } }
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FragmentPagerAdapter.finishUpdate()
public void finishUpdate(@NonNull ViewGroup container) { if (this.mCurTransaction != null) { if (!this.mExecutingFinishUpdate) { try { this.mExecutingFinishUpdate = true; this.mCurTransaction.commitNowAllowingStateLoss(); } finally { this.mExecutingFinishUpdate = false; } } this.mCurTransaction = null; } }
finishUpdate是抽象方法，不同适配器有不同的finish方式
ViewPager三步：缓存页面、设置当前页面、销毁页面
预加载在缓存页面设置