ViewPager 一、測量 二、執行流程 三、Adapter 四、重寫 onMeasure 方法

Android知識總結

一、測量

1.1、onMeasure

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //設置父容器測量高度，沒有根據子視圖決定，
    //導致子view無法決定父容器的大小，這時需要我們重寫  onMeasure 方法。
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
            getDefaultSize(0, heightMeasureSpec));
 
    final int measuredWidth = getMeasuredWidth();
    final int maxGutterSize = measuredWidth / 10;
    //設置mGutterSize的值，後面再講mGutterSize
    mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);
 
    // ViewPager的顯示區域只能顯示對於一個View
    //childWidthSize和childHeightSize爲一個View的可用寬高大小
    //即去除了ViewPager內邊距後的寬高
    int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
    int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();
 
    //1.先對Decor View進行測量
    //下面這個循環是隻針對Decor View的，即用於裝飾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 如果該View是Decor View,即用於裝飾ViewPager的View
            if (lp != null && lp.isDecor) {
                //1.2 獲取Decor View的在水平方向和豎直方向上的Gravity
                final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                //1.3 默認Dedor View模式對應的寬高是wrap_content
                int widthMode = MeasureSpec.AT_MOST;
                int heightMode = MeasureSpec.AT_MOST;
 
                //1.4 記錄Decor View是在垂直方向上還是在水平方向上佔用空間
                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佔用空間，
                // 那麼水平方向就是match_parent，即EXACTLY
                //而垂直方向上具體佔用多少空間，還得由Decor View決定
                //consumeHorizontal也是同理
                if (consumeVertical) {
                    widthMode = MeasureSpec.EXACTLY;
                } else if (consumeHorizontal) {
                    heightMode = MeasureSpec.EXACTLY;
                }
                //1.6 寬高大小，初始化爲ViewPager可視區域中子View可用空間
                int widthSize = childWidthSize;
                int heightSize = childHeightSize;
                //1.7 如果寬度不是wrap_content，那麼width的測量模式就是EXACTLY
                //如果寬度既不是wrap_content又不是match_parent，那麼說明是用戶
                //在佈局文件寫的具體的尺寸，直接將widthSize設置爲這個具體尺寸
                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的寬高specification（包含尺寸和模式的整數）
                final int widthSpec = MeasureSpec.makeMeasureSpec(widthSize, widthMode);
                final int heightSpec = MeasureSpec.makeMeasureSpec(heightSize, heightMode);
                //1.10 對DecorView進行測量
                child.measure(widthSpec, heightSpec);
                //1.11 如果Decor View佔用了ViewPager的垂直方向的空間
                //需要將子View的豎直方向可用的空間減去DecorView的高度，
                //同理，水平方向上也做同樣的處理
                if (consumeVertical) {
                    childHeightSize -= child.getMeasuredHeight();
                } else if (consumeHorizontal) {
                    childWidthSize -= child.getMeasuredWidth();
                }
            }
        }
    }
 
    //2.子View默認寬高的specification（包含尺寸和模式的整數）
    mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
    mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);
 
    //3.確保我們需要顯示的fragment已經被我們創建好了
    //populate()比較複雜，後面再詳細介紹
    mInLayout = true;
    populate();
    mInLayout = false;
 
    // 4.再對子View進行測量
    size = getChildCount();
    for (int i = 0; i < size; ++i) {
        final View child = getChildAt(i);
        //4.1 visibility爲GONE的無需測量
        if (child.getVisibility() != GONE) {
            if (DEBUG) Log.v(TAG, "Measuring #" + i + " " + child
                    + ": " + mChildWidthMeasureSpec);
            //4.2 獲取子View的LayoutParams
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            //4.3 只針對子View而不對Decor View測量
            if (lp == null || !lp.isDecor) {
                //4.4 LayoutParams的widthFactor是取值爲[0,1]的浮點數，
                // 用於表示子View佔ViewPager顯示區域中子View可用寬度的比例，
                // 即(childWidthSize * lp.widthFactor)表示當前子View的實際寬度
                final int widthSpec = MeasureSpec.makeMeasureSpec(
                        (int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
                //4.5 對當前子View進行測量
                child.measure(widthSpec, mChildHeightMeasureSpec);
            }
        }
    }
}

1.2、onLayout

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    //1.以下局部變量很簡單，不再解釋
    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 數量
    int decorCount = 0;
 
    //3.首先對Decor View進行layout，再對普通子View進行layout，
    // 之所以先對Decor View佈局，是爲了讓普通子View能有合適的偏移
    //下面循環主要是針對Decor View
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        //3.1 visibility不爲GONE才layout
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            //3.2 左邊和頂部的邊距初始化爲0
            int childLeft = 0;
            int childTop = 0;
            if (lp.isDecor) {//3.3 只針對Decor View
                //3.4 獲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 根據水平方向上的Gravity，確定childLeft以及paddingRight
                switch (hgrav) {
                    default://沒有設置水平方向Gravity時（左中右），childLeft就取paddingLeft
                        childLeft = paddingLeft;
                        break;
                    case Gravity.LEFT://水平方向Gravity爲left，Decor View往最左邊靠
                        childLeft = paddingLeft;
                        paddingLeft += child.getMeasuredWidth();
                        break;
                    case Gravity.CENTER_HORIZONTAL://將Decor View居中擺放
                        childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                paddingLeft);
                        break;
                    case Gravity.RIGHT://將Decor View往最右邊靠
                        childLeft = width - paddingRight - child.getMeasuredWidth();
                        paddingRight += child.getMeasuredWidth();
                        break;
                }
                //3.6 與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 上面計算的childLeft是相對ViewPager的左邊計算的，
                //還需要加上x方向已經滑動的距離scrollX
                childLeft += scrollX;
                //3.8 對Decor View佈局
                child.layout(childLeft, childTop,
                        childLeft + child.getMeasuredWidth(),
                        childTop + child.getMeasuredHeight());
                //3.9 將Decor View數量+1
                decorCount++;
            }
        }
    }
    //4.普通子View的寬度
    final int childWidth = width - paddingLeft - paddingRight;
    // Page views. Do this once we have the right padding offsets from above.
    //5.下面針對普通子View佈局，在此之前我們已經得到正確的偏移量了
    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靜態內部類，
            // 它保存了普通子View的position、offset等信息，是對普通子View的一個抽象描述
            ItemInfo ii;
            //5.2 infoForChild通過傳入View查詢對應的ItemInfo對象
            if (!lp.isDecor && (ii = infoForChild(child)) != null) {
            //計算當前子View的左邊偏移量
                int loff = (int) (childWidth * ii.offset);
                //將左邊距+左邊偏移量得到最終子View左邊位置
                int childLeft = paddingLeft + loff;
                int childTop = paddingTop;
                //5.3 如果當前子View需要進行測量（measure），當這個子View是在Layout期間新添加新的，
                // 那麼這個子View需要進行測量，即needsMeasure爲true
                if (lp.needsMeasure) {
                    //5.4 標記已經測量過了
                    lp.needsMeasure = false;
                    //5.5 下面過程跟onMeasure類似
                    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 對普通子View進行layout
                child.layout(childLeft, childTop,
                        childLeft + child.getMeasuredWidth(),
                        childTop + child.getMeasuredHeight());
            }
        }
    }
    //6. 將部分局部變量保存到實例變量中
    mTopPageBounds = paddingTop;
    mBottomPageBounds = height - paddingBottom;
    mDecorChildCount = decorCount;
    //7. 如果是第一次layout，則將ViewPager滑動到第一個子View的位置
    if (mFirstLayout) {
        scrollToItem(mCurItem, false, 0, false);
    }
    //8. 標記已經佈局過了，即不再是第一次佈局了
    mFirstLayout = false;
}

二、執行流程

    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;
        }
        // 開始適配
        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) {
            // 創建適配的Item數據
            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) {
                        //移出當前位置 item
                        mItems.remove(itemIndex);
                        // 銷燬適配的Item數據
                        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);
            // 設置當前顯示的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);
            }
        }
        // 通過事務的方式，執行fragment生命週期函數
        mAdapter.finishUpdate(this);

        //獲取 item 的個數
        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、開始適配

調用PagerAdapter中的startUpdate方法

    public void startUpdate(@NonNull ViewGroup container) {
        startUpdate((View) container);
    }

• 1)、執行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)、執行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、創建 item

調用PagerAdapter中的addNewItem方法

    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;
    }

• 1)、執行FragmentPagerAdapter

    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));
        }
        //創建的 fragment 不是當前的fragment
        if (fragment != mCurrentPrimaryItem) {
            fragment.setMenuVisibility(false);
            if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
                mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
            } else {
                 //創建的fragment 不顯示
                fragment.setUserVisibleHint(false);
            }
        }

        return fragment;
    }

• 2)、執行FragmentStatePagerAdapter

    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、銷燬適配的Item

調用PagerAdapter中的destroyItem方法

    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        destroyItem((View) container, position, object);
    }

• 1)、執行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()));
        //解綁視圖，fragment實例還保留在FragmentManager 中
        mCurTransaction.detach(fragment);
        if (fragment == mCurrentPrimaryItem) {
            mCurrentPrimaryItem = null;
        }
    }

• 2)、執行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);
        //移出視圖， FragmentManager 中的fragment會被徹底移除
        mCurTransaction.remove(fragment);
        if (fragment == mCurrentPrimaryItem) {
            mCurrentPrimaryItem = null;
        }
    }

1.4、設置當前顯示的Item

調用PagerAdapter中的addNewItem方法

    public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        setPrimaryItem((View) container, position, object);
    }

• 1)、執行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 {
                    //當前fragment不顯示
                    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 {
                //創建的 fragment 顯示
                fragment.setUserVisibleHint(true);
            }
            //更新 mCurrentPrimaryItem 
            mCurrentPrimaryItem = fragment;
        }
    }

• 2)、執行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 不是生命週期方法，在 fragment 提交事務之前執行
                    //導致在生命週期之前執行
                    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、結束適配

調用PagerAdapter中的finishUpdate方法

    public void finishUpdate(@NonNull ViewGroup container) {
        finishUpdate((View) container);
    }

• 1)、執行FragmentPagerAdapter

    public void finishUpdate(@NonNull ViewGroup container) {
        if (mCurTransaction != null) {
            //提交事務執行生命週期函數，由此決定setUserVisibleHint在生命週期之前執行
            mCurTransaction.commitNowAllowingStateLoss();
            mCurTransaction = null;
        }
    }

• 2)、執行FragmentStatePagerAdapter

    public void finishUpdate(@NonNull ViewGroup container) {
        if (mCurTransaction != null) {
            mCurTransaction.commitNowAllowingStateLoss();
            mCurTransaction = null;
        }
    }

三、Adapter

2.1、FragmentStatePagerAdapter 不可以緩存

FragmentStatePagerAdapter 會銷燬不需要的fragment，事務提交後，FragmentManager 中的fragment會被徹底移除。

public class MyFragmentPagerAdapter extends FragmentStatePagerAdapter {

    private static final String TAG = MyFragmentPagerAdapter.class.getSimpleName();

    private List<Fragment> fragmentList;

    public MyFragmentPagerAdapter(FragmentManager fm, List<Fragment> 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   監聽 ViewPager 的 Item  什麼時候移除  記錄下標
    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        super.destroyItem(container, position, object);
    }
}

2.2、FragmentPagerAdapter 可以緩存

對於不再需要的fragment， FragmentPagerAdapter 會選擇調用事務的 detach(Fragment) 方法來處理它，而非 remove(Fragment) 方法。
FragmentPagerAdapter 只是銷燬了fragment的視圖，fragment實例還保留在FragmentManager 中。

 */
public class MyFragmentPagerAdapter extends FragmentPagerAdapter {
    private static final String TAG = "MyFragmentPagerAdapter";

    private List<Fragment> fragmentList;
    public MyFragmentPagerAdapter(FragmentManager fm, List<Fragment> flist) {
        super(fm);
        fragmentList = flist;
    }
    @Override
    public Fragment getItem(int i) {
        return fragmentList.get(i);
    }

    @Override
    public int getCount() {
        return fragmentList.size();
    }
}

四、重寫 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);
    }

    //主要解決父View大小沒有有子View決定的問題
    @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中最大的寬和高
        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);
        }
        //設置模式爲 MeasureSpec.EXACTLY
        wSpec = MeasureSpec.makeMeasureSpec(finalW, MeasureSpec.EXACTLY);
        hSpec = MeasureSpec.makeMeasureSpec(finalH, MeasureSpec.EXACTLY);
        setMeasuredDimension(wSpec, hSpec);
    }
}