ViewPager

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FragmentPagerAdapterinstantiateItem
    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FragmentStatePagerAdapterinstantiateItem
    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);
    }
}

三、总结

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