Android知识总结
一、测量
1.1、onMeasure
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//设置父容器测量高度，没有根据子视图决定，
//导致子view无法决定父容器的大小，这时需要我们重写 onMeasure 方法。
setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
getDefaultSize(0, heightMeasureSpec));
final int measuredWidth = getMeasuredWidth();
final int maxGutterSize = measuredWidth / 10;
//设置mGutterSize的值，后面再讲mGutterSize
mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);
// ViewPager的显示区域只能显示对于一个View
//childWidthSize和childHeightSize为一个View的可用宽高大小
//即去除了ViewPager内边距后的宽高
int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();
//1.先对Decor View进行测量
//下面这个循环是只针对Decor View的，即用于装饰ViewPager的View
int size = getChildCount();
for (int i = 0; i < size; ++i) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//1.1 如果该View是Decor View,即用于装饰ViewPager的View
if (lp != null && lp.isDecor) {
//1.2 获取Decor View的在水平方向和竖直方向上的Gravity
final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
//1.3 默认Dedor View模式对应的宽高是wrap_content
int widthMode = MeasureSpec.AT_MOST;
int heightMode = MeasureSpec.AT_MOST;
//1.4 记录Decor View是在垂直方向上还是在水平方向上占用空间
boolean consumeVertical = vgrav == Gravity.TOP || vgrav == Gravity.BOTTOM;
boolean consumeHorizontal = hgrav == Gravity.LEFT || hgrav == Gravity.RIGHT;
//1.5 consumeHorizontal：如果是在垂直方向上占用空间，
// 那么水平方向就是match_parent，即EXACTLY
//而垂直方向上具体占用多少空间，还得由Decor View决定
//consumeHorizontal也是同理
if (consumeVertical) {
widthMode = MeasureSpec.EXACTLY;
} else if (consumeHorizontal) {
heightMode = MeasureSpec.EXACTLY;
}
//1.6 宽高大小，初始化为ViewPager可视区域中子View可用空间
int widthSize = childWidthSize;
int heightSize = childHeightSize;
//1.7 如果宽度不是wrap_content，那么width的测量模式就是EXACTLY
//如果宽度既不是wrap_content又不是match_parent，那么说明是用户
//在布局文件写的具体的尺寸，直接将widthSize设置为这个具体尺寸
if (lp.width != LayoutParams.WRAP_CONTENT) {
widthMode = MeasureSpec.EXACTLY;
if (lp.width != LayoutParams.FILL_PARENT) {
widthSize = lp.width;
}
}
//1.8 同1.7
if (lp.height != LayoutParams.WRAP_CONTENT) {
heightMode = MeasureSpec.EXACTLY;
if (lp.height != LayoutParams.FILL_PARENT) {
heightSize = lp.height;
}
}
//1.9 合成Decor View的宽高specification（包含尺寸和模式的整数）
final int widthSpec = MeasureSpec.makeMeasureSpec(widthSize, widthMode);
final int heightSpec = MeasureSpec.makeMeasureSpec(heightSize, heightMode);
//1.10 对DecorView进行测量
child.measure(widthSpec, heightSpec);
//1.11 如果Decor View占用了ViewPager的垂直方向的空间
//需要将子View的竖直方向可用的空间减去DecorView的高度，
//同理，水平方向上也做同样的处理
if (consumeVertical) {
childHeightSize -= child.getMeasuredHeight();
} else if (consumeHorizontal) {
childWidthSize -= child.getMeasuredWidth();
}
}
}
}
//2.子View默认宽高的specification（包含尺寸和模式的整数）
mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);
//3.确保我们需要显示的fragment已经被我们创建好了
//populate()比较复杂，后面再详细介绍
mInLayout = true;
populate();
mInLayout = false;
// 4.再对子View进行测量
size = getChildCount();
for (int i = 0; i < size; ++i) {
final View child = getChildAt(i);
//4.1 visibility为GONE的无需测量
if (child.getVisibility() != GONE) {
if (DEBUG) Log.v(TAG, "Measuring #" + i + " " + child
+ ": " + mChildWidthMeasureSpec);
//4.2 获取子View的LayoutParams
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//4.3 只针对子View而不对Decor View测量
if (lp == null || !lp.isDecor) {
//4.4 LayoutParams的widthFactor是取值为[0,1]的浮点数，
// 用于表示子View占ViewPager显示区域中子View可用宽度的比例，
// 即(childWidthSize * lp.widthFactor)表示当前子View的实际宽度
final int widthSpec = MeasureSpec.makeMeasureSpec(
(int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
//4.5 对当前子View进行测量
child.measure(widthSpec, mChildHeightMeasureSpec);
}
}
}
}
1.2、onLayout
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
//1.以下局部变量很简单，不再解释
final int count = getChildCount();
int width = r - l;
int height = b - t;
int paddingLeft = getPaddingLeft();
int paddingTop = getPaddingTop();
int paddingRight = getPaddingRight();
int paddingBottom = getPaddingBottom();
final int scrollX = getScrollX();
//2.Decor View 数量
int decorCount = 0;
//3.首先对Decor View进行layout，再对普通子View进行layout，
// 之所以先对Decor View布局，是为了让普通子View能有合适的偏移
//下面循环主要是针对Decor View
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//3.1 visibility不为GONE才layout
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//3.2 左边和顶部的边距初始化为0
int childLeft = 0;
int childTop = 0;
if (lp.isDecor) {//3.3 只针对Decor View
//3.4 获取水平或垂直方向上的Gravity
final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
//3.5 根据水平方向上的Gravity，确定childLeft以及paddingRight
switch (hgrav) {
default://没有设置水平方向Gravity时（左中右），childLeft就取paddingLeft
childLeft = paddingLeft;
break;
case Gravity.LEFT://水平方向Gravity为left，Decor View往最左边靠
childLeft = paddingLeft;
paddingLeft += child.getMeasuredWidth();
break;
case Gravity.CENTER_HORIZONTAL://将Decor View居中摆放
childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
paddingLeft);
break;
case Gravity.RIGHT://将Decor View往最右边靠
childLeft = width - paddingRight - child.getMeasuredWidth();
paddingRight += child.getMeasuredWidth();
break;
}
//3.6 与3.5同理
switch (vgrav) {
default:
childTop = paddingTop;
break;
case Gravity.TOP:
childTop = paddingTop;
paddingTop += child.getMeasuredHeight();
break;
case Gravity.CENTER_VERTICAL:
childTop = Math.max((height - child.getMeasuredHeight()) / 2,
paddingTop);
break;
case Gravity.BOTTOM:
childTop = height - paddingBottom - child.getMeasuredHeight();
paddingBottom += child.getMeasuredHeight();
break;
}
//3.7 上面计算的childLeft是相对ViewPager的左边计算的，
//还需要加上x方向已经滑动的距离scrollX
childLeft += scrollX;
//3.8 对Decor View布局
child.layout(childLeft, childTop,
childLeft + child.getMeasuredWidth(),
childTop + child.getMeasuredHeight());
//3.9 将Decor View数量+1
decorCount++;
}
}
}
//4.普通子View的宽度
final int childWidth = width - paddingLeft - paddingRight;
// Page views. Do this once we have the right padding offsets from above.
//5.下面针对普通子View布局，在此之前我们已经得到正确的偏移量了
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//5.1 ItemInfo 是ViewPager静态内部类，
// 它保存了普通子View的position、offset等信息，是对普通子View的一个抽象描述
ItemInfo ii;
//5.2 infoForChild通过传入View查询对应的ItemInfo对象
if (!lp.isDecor && (ii = infoForChild(child)) != null) {
//计算当前子View的左边偏移量
int loff = (int) (childWidth * ii.offset);
//将左边距+左边偏移量得到最终子View左边位置
int childLeft = paddingLeft + loff;
int childTop = paddingTop;
//5.3 如果当前子View需要进行测量（measure），当这个子View是在Layout期间新添加新的，
// 那么这个子View需要进行测量，即needsMeasure为true
if (lp.needsMeasure) {
//5.4 标记已经测量过了
lp.needsMeasure = false;
//5.5 下面过程跟onMeasure类似
final int widthSpec = MeasureSpec.makeMeasureSpec(
(int) (childWidth * lp.widthFactor),
MeasureSpec.EXACTLY);
final int heightSpec = MeasureSpec.makeMeasureSpec(
(int) (height - paddingTop - paddingBottom),
MeasureSpec.EXACTLY);
child.measure(widthSpec, heightSpec);
}
if (DEBUG) Log.v(TAG, "Positioning #" + i + " " + child + " f=" + ii.object
+ ":" + childLeft + "," + childTop + " " + child.getMeasuredWidth()
+ "x" + child.getMeasuredHeight());
//5.6 对普通子View进行layout
child.layout(childLeft, childTop,
childLeft + child.getMeasuredWidth(),
childTop + child.getMeasuredHeight());
}
}
}
//6. 将部分局部变量保存到实例变量中
mTopPageBounds = paddingTop;
mBottomPageBounds = height - paddingBottom;
mDecorChildCount = decorCount;
//7. 如果是第一次layout，则将ViewPager滑动到第一个子View的位置
if (mFirstLayout) {
scrollToItem(mCurItem, false, 0, false);
}
//8. 标记已经布局过了，即不再是第一次布局了
mFirstLayout = false;
}
二、处理缓存的执行流程
public boolean onTouchEvent(MotionEvent ev) {
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
mScroller.abortAnimation();
mPopulatePending = false;
populate();
// Remember where the motion event started
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = mInitialMotionY = ev.getY();
mActivePointerId = ev.getPointerId(0);
break;
}
}
}
void populate() {
populate(mCurItem);
}
void populate(int newCurrentItem) {
ItemInfo oldCurInfo = null;
if (mCurItem != newCurrentItem) {
oldCurInfo = infoForPosition(mCurItem);
mCurItem = newCurrentItem;
}
if (mAdapter == null) {
sortChildDrawingOrder();
return;
}
if (mPopulatePending) {
if (DEBUG) Log.i(TAG, "populate is pending, skipping for now...");
sortChildDrawingOrder();
return;
}
if (getWindowToken() == null) {
return;
}
// 开始适配
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;
}
}
if (curItem == null && N > 0) {
// 创建适配的Item数据
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) {
//移出当前位置 item
mItems.remove(itemIndex);
// 销毁适配的Item数据
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;
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);
// 设置当前显示的Item
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);
}
}
// 通过事务的方式，执行fragment生命周期函数
mAdapter.finishUpdate(this);
//获取 item 的个数
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;
}
}
}
}
}
}
1.1、开始适配
调用PagerAdapter中的startUpdate方法
public void startUpdate(@NonNull ViewGroup container) {
startUpdate((View) container);
}
- 1)、执行
FragmentPagerAdapter
public void startUpdate(@NonNull ViewGroup container) {
if (container.getId() == View.NO_ID) {
throw new IllegalStateException("ViewPager with adapter " + this
+ " requires a view id");
}
}
- 2)、执行
FragmentStatePagerAdapter
public void startUpdate(@NonNull ViewGroup container) {
if (container.getId() == View.NO_ID) {
throw new IllegalStateException("ViewPager with adapter " + this
+ " requires a view id");
}
}
1.2、创建 item
调用ViewPager中的addNewItem方法
//缓存了当前的 Fragment
private final ArrayList mItems = new ArrayList();
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;
}
static class ItemInfo {
Object object; //fragment
int position; //当前的位置
boolean scrolling;
float widthFactor; //宽度
float offset;
}
- 1)、执行
FragmentPagerAdapter的instantiateItem
public Object instantiateItem(@NonNull ViewGroup container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
//创建的 fragment 不是当前的fragment
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
} else {
//创建的fragment 不显示
fragment.setUserVisibleHint(false);
}
}
return fragment;
}
- 2)、执行
FragmentStatePagerAdapter的instantiateItem
public Object instantiateItem(@NonNull ViewGroup container, int position) {
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);
}
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;
}
1.3、销毁适配的Item
调用PagerAdapter中的destroyItem方法
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
destroyItem((View) container, position, object);
}
- 1)、执行
FragmentPagerAdapter
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, "Detaching item #" + getItemId(position) + ": f=" + object
+ " v=" + (fragment.getView()));
//解绑视图，fragment实例还保留在FragmentManager 中
mCurTransaction.detach(fragment);
if (fragment == mCurrentPrimaryItem) {
mCurrentPrimaryItem = null;
}
}
- 2)、执行
FragmentStatePagerAdapter
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);
}
//保存State 信息，可在 onSaveInstanceState(Bundle) 方法中保存fragment的 Bundle 信息
mSavedState.set(position, fragment.isAdded()
? mFragmentManager.saveFragmentInstanceState(fragment) : null);
mFragments.set(position, null);
//移出视图， FragmentManager 中的fragment会被彻底移除
mCurTransaction.remove(fragment);
if (fragment == mCurrentPrimaryItem) {
mCurrentPrimaryItem = null;
}
}
1.4、设置当前显示的Item
调用PagerAdapter中的addNewItem方法
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
setPrimaryItem((View) container, position, object);
}
- 1)、执行
FragmentPagerAdapter
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
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 {
//当前fragment不显示
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 {
//创建的 fragment 显示
fragment.setUserVisibleHint(true);
}
//更新 mCurrentPrimaryItem
mCurrentPrimaryItem = fragment;
}
}
- 2)、执行
FragmentStatePagerAdapter
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
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 {
//setUserVisibleHint 不是生命周期方法，在 fragment 提交事务之前执行
//导致在生命周期之前执行
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;
}
}
1.5、结束适配
调用PagerAdapter中的finishUpdate方法
public void finishUpdate(@NonNull ViewGroup container) {
finishUpdate((View) container);
}
- 1)、执行
FragmentPagerAdapter
public void finishUpdate(@NonNull ViewGroup container) {
if (mCurTransaction != null) {
//提交事务执行生命周期函数，由此决定setUserVisibleHint在生命周期之前执行
mCurTransaction.commitNowAllowingStateLoss();
mCurTransaction = null;
}
}
- 2)、执行
FragmentStatePagerAdapter
public void finishUpdate(@NonNull ViewGroup container) {
if (mCurTransaction != null) {
mCurTransaction.commitNowAllowingStateLoss();
mCurTransaction = null;
}
}
三、Adapter
2.1、FragmentStatePagerAdapter 不可以缓存
FragmentStatePagerAdapter 会销毁不需要的fragment，事务提交后，FragmentManager 中的fragment会被彻底移除。
public class MyFragmentPagerAdapter extends FragmentStatePagerAdapter {
private static final String TAG = MyFragmentPagerAdapter.class.getSimpleName();
private List fragmentList;
public MyFragmentPagerAdapter(FragmentManager fm, List flist) {
super(fm,FragmentStatePagerAdapter.BEHAVIOR_SET_USER_VISIBLE_HINT);
fragmentList = flist;
}
@Override
public Fragment getItem(int i) {
Log.v(TAG, "getItem: " + i);
return fragmentList.get(i);
}
@Override
public int getCount() {
return fragmentList.size();
}
// 早期 Android 监听 ViewPager 的 Item 什么时候移除 记录下标
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
super.destroyItem(container, position, object);
}
}
2.2、FragmentPagerAdapter 可以缓存
对于不再需要的fragment， FragmentPagerAdapter 会选择调用事务的 detach(Fragment) 方法来处理它，而非 remove(Fragment) 方法。
FragmentPagerAdapter 只是销毁了fragment的视图，fragment实例还保留在FragmentManager 中。
public class MyFragmentPagerAdapter extends FragmentPagerAdapter {
private static final String TAG = "MyFragmentPagerAdapter";
private List fragmentList;
public MyFragmentPagerAdapter(FragmentManager fm, List flist) {
super(fm);
fragmentList = flist;
}
@Override
public Fragment getItem(int i) {
return fragmentList.get(i);
}
@Override
public int getCount() {
return fragmentList.size();
}
}
四、重写 onMeasure 方法
public class MyViewPager extends ViewPager {
public MyViewPager(@NonNull Context context) {
super(context);
}
public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
//主要解决父View大小没有有子View决定的问题
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int hSpec, wSpec;
int finalH = 0, finalW = 0;
final int count = getChildCount();
View child;
//找出子View中最大的宽和高
for (int i = 0; i < count; i++) {
child = getChildAt(i);
measureChild(child, widthMeasureSpec, heightMeasureSpec);
int childHeight = child.getMeasuredHeight();
int childWidth = child.getMeasuredWidth();
finalH = Math.max(finalH, childHeight);
finalW = Math.max(finalW, childWidth);
}
//设置模式为 MeasureSpec.EXACTLY
wSpec = MeasureSpec.makeMeasureSpec(finalW, MeasureSpec.EXACTLY);
hSpec = MeasureSpec.makeMeasureSpec(finalH, MeasureSpec.EXACTLY);
setMeasuredDimension(wSpec, hSpec);
}
}