我的CSDN博客同步發(fā)布：ViewPager源碼分析（1）：onMeasure橙数、onLayout

轉(zhuǎn)載請注明出處：【huachao1001的簡書：http://www.reibang.com/users/0a7e42698e4b/latest_articles】

在網(wǎng)上看了一些面經(jīng)荸型，感覺ViewPager被問到的概率還是蠻大的投储，于是決定去好好研究ViewPager源碼衡招，一步一步去琢磨ViewPager的實現(xiàn)融师，并寫到博文里來~。

我們知道蚁吝，ViewPager是一個ViewGroup旱爆，而我們平時自己自定義ViewGroup時，除了至少寫兩個構(gòu)造函數(shù)以外窘茁，onMeasure和onLayout這兩個函數(shù)基本上是必須要去寫的怀伦。今天先把onMeasure和onLayout兩個函數(shù)仔細研究研究~

onMeasure

ViewPager將子View分為兩種，一種是Decor View 用于裝飾ViewPager山林，它可能需要占用一些空間房待；另一種是普通的子View，也就是我們橫滑時顯示的各個View驼抹。onMeasure首先是對Decor View進行測量桑孩，然后再對普通的子View進行測量。詳細的注釋如下：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //根據(jù)布局文件框冀，設(shè)置尺寸信息流椒，默認大小為0
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
            getDefaultSize(0, heightMeasureSpec));

    final int measuredWidth = getMeasuredWidth();
    final int maxGutterSize = measuredWidth / 10;
    //設(shè)置mGutterSize的值，后面再講mGutterSize
    mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);

    // ViewPager的顯示區(qū)域只能顯示對于一個View
    //childWidthSize和childHeightSize為一個View的可用寬高大小
    //即去除了ViewPager內(nèi)邊距后的寬高
    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(huán)是只針對Decor View的明也，即用于裝飾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,即用于裝飾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(yīng)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占用空間
                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占用空間宣虾，
                // 那么水平方向就是match_parent，即EXACTLY
                //而垂直方向上具體占用多少空間温数，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 寬高大小绣硝，初始化為ViewPager可視區(qū)域中子View可用空間
                int widthSize = childWidthSize;
                int heightSize = childHeightSize;
                //1.7 如果寬度不是wrap_content，那么width的測量模式就是EXACTLY
                //如果寬度既不是wrap_content又不是match_parent撑刺，那么說明是用戶
                //在布局文件寫的具體的尺寸鹉胖，直接將widthSize設(shè)置為這個具體尺寸
                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(shù)）
                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占用了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(shù)）
    //（PS：mChildWidthMeasureSpec并沒有再次用到甫菠，個人感覺有點多余）
    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(jīng)被我們創(chuàng)建好了
    //populate()比較復(fù)雜败许，后面再詳細介紹
    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為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是取值為[0,1]的浮點數(shù)，
                // 用于表示子View占ViewPager顯示區(qū)域中子View可用寬度的比例淑蔚，
                // 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);
            }
        }
    }
}

onLayout

我們知道ViewPager的子View是水平擺放的市殷，所以在onLayout中，大部分工作的就是計算childLeft刹衫，即子View的左邊位置醋寝，而頂部位置基本上是一樣的，從以下代碼中可以體現(xiàn)的出來：

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    //1.以下局部變量很簡單带迟，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(shù)量
    int decorCount = 0;

    //3.首先對Decor View進行l(wèi)ayout音羞，再對普通子View進行l(wèi)ayout，
    // 之所以先對Decor View布局仓犬，是為了讓普通子View能有合適的偏移
    //下面循環(huán)主要是針對Decor View
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        //3.1 visibility不為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為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(jù)水平方向上的Gravity嗅绰，確定childLeft以及paddingRight
                switch (hgrav) {
                    default://沒有設(shè)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為left搀继，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的左邊計算的窘面，
                //還需要加上x方向已經(jīng)滑動的距離scrollX
                childLeft += scrollX;
                //3.8 對Decor View布局
                child.layout(childLeft, childTop,
                        childLeft + child.getMeasuredWidth(),
                        childTop + child.getMeasuredHeight());
                //3.9 將Decor View數(shù)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布局，在此之前我們已經(jīng)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(tài)內(nèi)部類叽躯，
            // 它保存了普通子View的position财边、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(yīng)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）点骑，當這個子View是在Layout期間新添加新的酣难，
                // 那么這個子View需要進行測量，即needsMeasure為true
                if (lp.needsMeasure) {
                    //5.4 標記已經(jīng)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l(wèi)ayout
                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黑滴，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(jīng)布局過了憨募，即不再是第一次布局了
    mFirstLayout = false;
}

好了，最基本的onMeasure和onLayout已經(jīng)分析完了袁辈，現(xiàn)在還不能滑動菜谣，接下來的文章將去分析一下ViewPager的滑動~