ViewPager源码分析（1）：onMeasure、onLayout

我的简书同步发布：ViewPager源码分析（1）：onMeasure、onLayout

转载请注明出处：【huachao1001的专栏：http://blog.csdn.net/huachao1001】

在网上看了一些面经，感觉ViewPager被问到的概率还是蛮大的，于是决定去好好研究ViewPager源码，一步一步去琢磨ViewPager的实现，并写到博文里来~。

我们知道，ViewPager是一个ViewGroup，而我们平时自己自定义ViewGroup时，除了至少写两个构造函数以外，onMeasure和onLayout这两个函数基本上是必须要去写的。今天先把onMeasure和onLayout两个函数仔细研究研究~

onMeasure

ViewPager将子View分为两种，一种是Decor View 用于装饰ViewPager，它可能需要占用一些空间；另一种是普通的子View，也就是我们横滑时显示的各个View。onMeasure首先是对Decor View进行测量，然后再对普通的子View进行测量。详细的注释如下：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//根据布局文件，设置尺寸信息，默认大小为0
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）
//（PS：mChildWidthMeasureSpec并没有再次用到，个人感觉有点多余）
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);
}
}
}
}

onLayout

我们知道ViewPager的子View是水平摆放的，所以在onLayout中，大部分工作的就是计算childLeft，即子View的左边位置，而顶部位置基本上是一样的，从以下代码中可以体现的出来：

@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;
}

好了，最基本的onMeasure和onLayout已经分析完了，现在还不能滑动，接下来的文章将去分析一下ViewPager的滑动~