一、高度设置wrap_content或者具体的值无效
ViewPager高度设置为wrap_content或者具体的高度值无效，是因为ViewPager的onMeasure方法在度量宽高的时候，在方法体的最开始就直接调用了setMeasuredDimension()方法将自身的宽高度量，但是并没有在其onMeasure()计算完其具体的子View的宽高之后，重新度量一次自身的宽高
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// For simple implementation, our internal size is always 0.
// We depend on the container to specify the layout size of
// our view. We can't really know what it is since we will be
// adding and removing different arbitrary views and do not
// want the layout to change as this happens.
setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
getDefaultSize(0, heightMeasureSpec));
...
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
从这里我们可以看到，ViewPager的宽高会受其父容器的宽高的限制，但是并不会因为自身子View的宽高而影响ViewPager的宽高。
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
// ViewPager的度量宽高
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
看setMeasuredDimension的源码调用可以看出，当父容器的高度确定时，ViewPager的宽高其实就是父容器的宽高，ViewPager就是在onMeasure方法一进来的时候就直接填充满整个父容器的剩余空间。在计算孩子节点之前，就已经计算好了ViewPager的宽高，在计算完孩子节点之后，并不会再去重新计算ViewPager的宽高。
二、解决ViewPager的wrap_content或者100dp这样具体高度无效的问题
自定义一个ViewPager，根据子View的宽高重新度量ViewPager的宽高。其实做法就是在自定义onMeasure的super.onMeasure(widthMeasureSpec, heightMeasureSpec);之前重新计算heightMeasureSpec，将原本ViewPager接收的父容器的限定的heightMeasureSpec替换成我们自定义的heightMeasureSpec。
public class NoScrollWrapContentViewPager extends ViewPager {
private boolean noScroll = false;
private int current;
private int height = 0;
/**
* 保存position与对于的View
*/
private HashMap<Integer, View> mChildrenViews = new LinkedHashMap<Integer, View>();
public NoScrollWrapContentViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
}
public NoScrollWrapContentViewPager(Context context) {
super(context);
}
public void setNoScroll(boolean noScroll) {
this.noScroll = noScroll;
}
@Override
public void scrollTo(int x, int y) {
super.scrollTo(x, y);
}
@Override
public boolean onTouchEvent(MotionEvent arg0) {
if (noScroll)
return false;
else
return super.onTouchEvent(arg0);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent arg0) {
if (noScroll)
return false;
else
return super.onInterceptTouchEvent(arg0);
}
@Override
public void setCurrentItem(int item, boolean smoothScroll) {
super.setCurrentItem(item, smoothScroll);
}
@Override
public void setCurrentItem(int item) {
super.setCurrentItem(item);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// int height = 0;
// for (int i = 0; i < getChildCount(); i++) {
// View child = getChildAt(i);
// child.measure(widthMeasureSpec,
// MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
// int h = child.getMeasuredHeight();
// if (h > height)
// height = h;
// }
// heightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
// MeasureSpec.EXACTLY);
// super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (mChildrenViews.size() > current) {
View child = mChildrenViews.get(current);
child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
height = child.getMeasuredHeight();
}
heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
public void resetHeight(int current) {
this.current = current;
if (mChildrenViews.size() > current) {
LinearLayout.LayoutParams layoutParams = (LinearLayout.LayoutParams) getLayoutParams();
if (layoutParams == null) {
layoutParams = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, height);
} else {
layoutParams.height = height;
}
setLayoutParams(layoutParams);
}
}
/**
* 保存position与对于的View
*/
public void setObjectForPosition(View view, int position) {
mChildrenViews.put(position, view);
}
}
但是这样的做法，会有种问题，即在ViewPager的子View是采用LinearLayout作为根布局的时候，并且给LinearLayout设置了固定的高度值，那么会出现ViewPager动态高度无效的问题
其实具体的做法，就是仿造measureChild的做法，自定义子View的heightMeasureSpec然后度量整个子View，其实子View的宽度也可以这样做。
public class MyViewPager extends ViewPager {
private static final String TAG = "MyViewPager";
public MyViewPager(@NonNull Context context) {
super(context);
}
public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int height = 0;
int count = getChildCount();
for (int i=0;i<count;i++) {
View child = getChildAt(i);
// 自定义子View的MeasureSpec，可以参考measureChild的做法，需要通过子View的LayoutParams
ViewGroup.LayoutParams lp = child.getLayoutParams();
child.measure(widthMeasureSpec, getChildMeasureSpec(heightMeasureSpec, 0, lp.height));
int h = child.getMeasuredHeight();
Log.e(TAG, "测量的子View的宽高：" + h);
if (h > height) {
height = h;
}
}
heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
三、ViewPager缓存机制解析
1.setOffscreenPageLimit为什么缓存的页面数不能低于1？
这里其实是源码层做了限制，在setOffscreenPageLimit中设置了一个默认值，而这个默认值的大小为1
private static final int DEFAULT_OFFSCREEN_PAGES = 1;
public void setOffscreenPageLimit(int limit) {
if (limit < DEFAULT_OFFSCREEN_PAGES) {
Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " +
DEFAULT_OFFSCREEN_PAGES);
limit = DEFAULT_OFFSCREEN_PAGES;
}
if (limit != mOffscreenPageLimit) {
mOffscreenPageLimit = limit;
populate();
}
}
所以从这里可以看出，ViewPager的最小缓存的limit是1，而不能小于1，当小于1的时候就会被强制的设置为1。
而populate()函数就是用来处理ViewPager的缓存的。
populate()的生命周期是与Adapter的生命周期绑定的。
其实在setOffscreenPageLimit()的时候，调用的populate()，而populate()内部调用的
public void populate() {
populate(mCurItem);
}
而pupulate(int newCurrentItem)方法在另一处调用的地方就是在setCurrentItem。
其实ViewPager缓存都是基于ItemInfo这个类来进行的，
void populate(int newCurrentItem) {
ItemInfo oldCurInfo = null;
int focusDirection = View.FOCUS_FORWARD;
// 重置mCurItem，当ViewPager从当前页面滑动到下一个页面时
// 这个时候mCurItem就会发生改变，populate的参数是新页面的position
// 所以需要修改mCurItem为新的position
if (mCurItem != newCurrentItem) {
focusDirection = mCurItem < newCurrentItem ? View.FOCUS_RIGHT : View.FOCUS_LEFT;
// 取出上一个页面的当前信息
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.
// 如果正在填充，则排序子View的绘制顺序，保证绘制填充过程
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;
}
// 调用mAdapter开始更新，将新的页面的数据更新
mAdapter.startUpdate(this);
// 计算缓存的偏移量，发生偏移后的区间的最大值和最小值position
final int pageLimit = mOffscreenPageLimit;
// 当前item的位置的最左边开始缓存的位置
final int startPos = Math.max(0, mCurItem - pageLimit);
final int N = mAdapter.getCount();// ViewPager的adapter的页面数量
// 当前item的位置最右边的缓存的位置
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;
// 遍历所有的缓存中的mItems，判断是否有新的mCurItem的数据已经在缓存中
// 如果有则取出
for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
final ItemInfo ii = mItems.get(curIndex);
if (ii.position >= mCurItem) {
if (ii.position == mCurItem) curItem = ii;
break;
}
}
// 如果缓存中没有心的mCurItem对应的数据，则添加新的Item数据
// 创建一个Adapter的新的item
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 = getPaddedWidth();
final float leftWidthNeeded = clientWidth <= 0 ? 0 :
2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
// 这里的mCurItem是已经切换到了新的Fragment的index
// 这部分内容是对预加载的Fragment进行处理的。如果当前的pos值与开始
// 需要缓存的值相比，小于，则判断mItems中是否有存在不需要缓存的Fragment，
// 如果有，则删除，并且将mAdapter中的销毁。下面的endPos的也是同理。
// 如果是需要新加入缓存的，则通过调用addNewItem，将需要加入缓存的
// Fragment添加到ItemInfo对象中，然后添加到mItems中
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);
// 销毁item
// 一般调用container.removeView((View) object);
// 即从container中删除对应的object
// 这个container其实就是ViewPager
mAdapter.destroyItem(this, pos, ii.object);
if (DEBUG) {
Log.i(TAG, "populate() - destroyItem() with pos: " + pos +
" 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;
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);
// 如果缓存中存在超出缓存界限的ItemInfo，则remove
// 并且调用Adapter.destroyItem销毁
mAdapter.destroyItem(this, pos, ii.object);
if (DEBUG) {
Log.i(TAG, "populate() - destroyItem() with pos: " + pos +
" 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);
}
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);
}
}
// setPrimaryItem方法其实就是具体去调用对应的Fragment的显示隐藏的回调的
// 方法，在这里会先判断被选择的item是否是与当前的item一致，
// 如果被选择的fragment与当前的一致，则什么都不做，如果不一致，
// 则将当前的Fragment的显示隐藏的回调置为false，将新的被选择的Fragment
// 置为true，并且更新当前Fragment对象缓存
// 在创建item之后，设置给adapter
mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null);
// 如果adapter需要更新新的item，那么在调用setPrimaryItem之后
// 就会调用finishUpdate结束更新
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) {
final Rect focusRect;
if (currentFocused == null) {
focusRect = null;
} else {
focusRect = mTempRect;
currentFocused.getFocusedRect(mTempRect);
offsetDescendantRectToMyCoords(currentFocused, mTempRect);
offsetRectIntoDescendantCoords(child, mTempRect);
}
if (child.requestFocus(focusDirection, focusRect)) {
break;
}
}
}
}
}
}
看下ViewPager.addNewItem的源码
其实ViewPager.addNewItem就是通过调用Adapter.instantiateItem来创建对应的View，并且将View保存到ItemInfo中的object属性，并且判断ViewPager缓存中是否已经有ItemInfo，如果没有，则添加，如果有则做修改替换
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;
}
四、FragmentStatePagerAdapter源码解析
@SuppressWarnings("deprecation")
public abstract class FragmentStatePagerAdapter extends PagerAdapter {
private static final String TAG = "FragmentStatePagerAdapt";
private static final boolean DEBUG = false;
@Retention(RetentionPolicy.SOURCE)
@IntDef({BEHAVIOR_SET_USER_VISIBLE_HINT, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT})
private @interface Behavior { }
/**
* Indicates that {@link Fragment#setUserVisibleHint(boolean)} will be called when the current
* fragment changes.
*
* @deprecated This behavior relies on the deprecated
* {@link Fragment#setUserVisibleHint(boolean)} API. Use
* {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT} to switch to its replacement,
* {@link FragmentTransaction#setMaxLifecycle}.
* @see #FragmentStatePagerAdapter(FragmentManager, int)
*/
@Deprecated
public static final int BEHAVIOR_SET_USER_VISIBLE_HINT = 0;
/**
* Indicates that only the current fragment will be in the {@link Lifecycle.State#RESUMED}
* state. All other Fragments are capped at {@link Lifecycle.State#STARTED}.
*
* @see #FragmentStatePagerAdapter(FragmentManager, int)
*/
public static final int BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT = 1;
private final FragmentManager mFragmentManager;
private final int mBehavior;
private FragmentTransaction mCurTransaction = null;
private ArrayList<Fragment.SavedState> mSavedState = new ArrayList<>();
private ArrayList<Fragment> mFragments = new ArrayList<>();
private Fragment mCurrentPrimaryItem = null;
private boolean mExecutingFinishUpdate;
/**
* Constructor for {@link FragmentStatePagerAdapter} that sets the fragment manager for the
* adapter. This is the equivalent of calling
* {@link #FragmentStatePagerAdapter(FragmentManager, int)} and passing in
* {@link #BEHAVIOR_SET_USER_VISIBLE_HINT}.
*
* <p>Fragments will have {@link Fragment#setUserVisibleHint(boolean)} called whenever the
* current Fragment changes.</p>
*
* @param fm fragment manager that will interact with this adapter
* @deprecated use {@link #FragmentStatePagerAdapter(FragmentManager, int)} with
* {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT}
*/
@Deprecated
public FragmentStatePagerAdapter(@NonNull FragmentManager fm) {
this(fm, BEHAVIOR_SET_USER_VISIBLE_HINT);
}
/**
* Constructor for {@link FragmentStatePagerAdapter}.
*
* If {@link #BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT} is passed in, then only the current
* Fragment is in the {@link Lifecycle.State#RESUMED} state, while all other fragments are
* capped at {@link Lifecycle.State#STARTED}. If {@link #BEHAVIOR_SET_USER_VISIBLE_HINT} is
* passed, all fragments are in the {@link Lifecycle.State#RESUMED} state and there will be
* callbacks to {@link Fragment#setUserVisibleHint(boolean)}.
*
* @param fm fragment manager that will interact with this adapter
* @param behavior determines if only current fragments are in a resumed state
*/
public FragmentStatePagerAdapter(@NonNull FragmentManager fm,
@Behavior int behavior) {
mFragmentManager = fm;
mBehavior = behavior;
}
/**
* Return the Fragment associated with a specified position.
*/
@NonNull
public abstract Fragment getItem(int position);
@Override
public void startUpdate(@NonNull ViewGroup container) {
if (container.getId() == View.NO_ID) {
throw new IllegalStateException("ViewPager with adapter " + this
+ " requires a view id");
}
}
@SuppressWarnings("deprecation")
@NonNull
@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {
// If we already have this item instantiated, there is nothing
// to do. This can happen when we are restoring the entire pager
// from its saved state, where the fragment manager has already
// taken care of restoring the fragments we previously had instantiated.
if (mFragments.size() > position) {
Fragment f = mFragments.get(position);
if (f != null) {
return f;
}
}
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
Fragment fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
if (mSavedState.size() > position) {
Fragment.SavedState fss = mSavedState.get(position);
if (fss != null) {
fragment.setInitialSavedState(fss);
}
}
while (mFragments.size() <= position) {
mFragments.add(null);
}
// 调用fragment的对应的方法
// 比如setUserVisibleHint
// 此时将fragment添加到事务中，但是还没提交事务
// 优先调用了setUserVisibleHint，而提交事务是在finishUpdate中
// 只有提交事务了才会执行生命周期
fragment.setMenuVisibility(false);
if (mBehavior == BEHAVIOR_SET_USER_VISIBLE_HINT) {
fragment.setUserVisibleHint(false);
}
mFragments.set(position, fragment);
mCurTransaction.add(container.getId(), fragment);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
}
return fragment;
}
// TODO(b/141958824): Suppressed during upgrade to AGP 3.6.
@SuppressWarnings("ReferenceEquality")
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment) object;
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object
+ " v=" + ((Fragment)object).getView());
while (mSavedState.size() <= position) {
mSavedState.add(null);
}
mSavedState.set(position, fragment.isAdded()
? mFragmentManager.saveFragmentInstanceState(fragment) : null);
mFragments.set(position, null);
mCurTransaction.remove(fragment);
if (fragment.equals(mCurrentPrimaryItem)) {
mCurrentPrimaryItem = null;
}
}
@Override
@SuppressWarnings({"ReferenceEquality", "deprecation"})
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
// 在切换ViewPager的时候，会调用adapter的setPrimaryItem方法
// 切换的时候，会将当前Fragment的setUserVisibleHint设置为false
// 会将目标fragment的setUserVisibleHint设置为true
// 即当前fragment需要隐藏，目标fragment需要显示。
if (mCurrentPrimaryItem != null) {
mCurrentPrimaryItem.setMenuVisibility(false);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
mCurTransaction.setMaxLifecycle(mCurrentPrimaryItem, Lifecycle.State.STARTED);
} else {
mCurrentPrimaryItem.setUserVisibleHint(false);
}
}
fragment.setMenuVisibility(true);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.RESUMED);
} else {
fragment.setUserVisibleHint(true);
}
mCurrentPrimaryItem = fragment;
}
}
@Override
public void finishUpdate(@NonNull ViewGroup container) {
if (mCurTransaction != null) {
// We drop any transactions that attempt to be committed
// from a re-entrant call to finishUpdate(). We need to
// do this as a workaround for Robolectric running measure/layout
// calls inline rather than allowing them to be posted
// as they would on a real device.
if (!mExecutingFinishUpdate) {
// 提交事务
// 在ViewPager.populate方法中调用
// 当执行完Adapter的startUpdate、addNewItem
// instantiateItem、destroyItem、setPrimaryItem
// 之后，才会调用finishUpdate
try {
mExecutingFinishUpdate = true;
mCurTransaction.commitNowAllowingStateLoss();
} finally {
mExecutingFinishUpdate = false;
}
}
mCurTransaction = null;
}
}
@Override
public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
return ((Fragment)object).getView() == view;
}
@Override
@Nullable
public Parcelable saveState() {
Bundle state = null;
if (mSavedState.size() > 0) {
state = new Bundle();
Fragment.SavedState[] fss = new Fragment.SavedState[mSavedState.size()];
mSavedState.toArray(fss);
state.putParcelableArray("states", fss);
}
for (int i=0; i<mFragments.size(); i++) {
Fragment f = mFragments.get(i);
if (f != null && f.isAdded()) {
if (state == null) {
state = new Bundle();
}
String key = "f" + i;
mFragmentManager.putFragment(state, key, f);
}
}
return state;
}
@Override
public void restoreState(@Nullable Parcelable state, @Nullable ClassLoader loader) {
if (state != null) {
Bundle bundle = (Bundle)state;
bundle.setClassLoader(loader);
Parcelable[] fss = bundle.getParcelableArray("states");
mSavedState.clear();
mFragments.clear();
if (fss != null) {
for (int i=0; i<fss.length; i++) {
mSavedState.add((Fragment.SavedState)fss[i]);
}
}
Iterable<String> keys = bundle.keySet();
for (String key: keys) {
if (key.startsWith("f")) {
int index = Integer.parseInt(key.substring(1));
Fragment f = mFragmentManager.getFragment(bundle, key);
if (f != null) {
while (mFragments.size() <= index) {
mFragments.add(null);
}
f.setMenuVisibility(false);
mFragments.set(index, f);
} else {
Log.w(TAG, "Bad fragment at key " + key);
}
}
}
}
}
}
从分析FragmentStatePagerAdapter来看，setUserVisibleHint方法会优先于Fragment的生命周期函数 执行。因为在FragmentStatePagerAdapter中提交事务，是在调用finishUpdate方法中进行的，只有提交事务的时候，才会去执行Fragment的生命周期。
FragmentStatePagerAdapter中的instantiateItem和destroyItem都实现了对fragment的事务的添加和删除，而finishUpdate实现了事务的提交，所以在实现FragmentStatePagerAdapter的时候，并不需要重写instantiateItem和destroyItem
