从源码的角度分析View的绘制流程(2)
上篇文章已经分析到View绘制的三个阶段，想要了解View绘制的产生过程可以先去看看https://www.jianshu.com/p/a8d9a3a9848b这篇文章;本文将深入分析这三个过程.
1.Measure过程
Measure顾名思义就是测量，从上篇文章可以了解到View系统的绘制流程会从ViewRoot的performTraversals()方法中开始，执行performMeasure(),在其内部调用View的measure()方法.我们来看一看其内部的实现
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
measure()方法接收两个参数，widthMeasureSpec和heightMeasureSpec，这两个值分别用于确定视图的宽度和高度的规格和大小。这里有个概念就是MeasureSpec测量规格，其实这个东西就是父容器根据子View的layoutParams来确定的一个布局要求。也就是说子View的MeasureSpec是由父View的MeasureSpec和子View的LayoutParams通过简单的计算得出来的；
MeasureSpec是一个大小跟模式的组合值,MeasureSpec中的值是一个整型（32位）将size和mode打包成一个Int型，其中高两位是mode，后面30位存的是size，是为了减少对象的分配开支。
specMode一共有三种类型：
EXACTLY：一般是设置了明确的值或者是MATCH_PARENT
AT_MOST：表示子布局限制在一个最大值内，一般为WARP_CONTENT
UNSPECIFIED：表示子布局想要多大就多大，很少使用
我们来看看MeasureSpec是怎么的出来的？
那么你可能会有疑问了，widthMeasureSpec和heightMeasureSpec这两个值又是从哪里得到的呢？通常情况下，这两个值都是由父视图经过计算后传递给子视图的，说明父视图会在一定程度上决定子视图的大小。但是最外层的根视图，它的widthMeasureSpec和heightMeasureSpec又是从哪里得到的呢？这就需要去分析ViewRootImpl中的源码了，观察performTraversals()方法可以发现如下代码：
private void performTraversals() {
......
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed! mWidth="
+ mWidth + " measuredWidth=" + host.getMeasuredWidth()
+ " mHeight=" + mHeight
+ " measuredHeight=" + host.getMeasuredHeight()
+ " coveredInsetsChanged=" + contentInsetsChanged);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
......
}
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
performTraversals 中会执行performMeasure我们看到的mView其实就是DecorView,View的绘制从DecorView开始， 在mView.measure()的时候调用getRootMeasureSpec获得两个MeasureSpec做为参数，在getRootMeasureSpec的方法中可以看到，这里使用了MeasureSpec.makeMeasureSpec()方法来组装一个MeasureSpec，当rootDimension参数等于MATCH_PARENT的时候，MeasureSpec的specMode就等于EXACTLY，当rootDimension等于WRAP_CONTENT的时候，MeasureSpec的specMode就等于AT_MOST。并且MATCH_PARENT和WRAP_CONTENT时的specSize都是等于windowSize的，也就意味着根视图总是会充满全屏的。
我们发现DecorView和它的父类Framelayout都没有measure方法，ViewGroup也没有因此他应该会调用View里面的measure方法接着会走DecorView的onMeasure().查看源码可以发现他会走super.onMeasure(widthMeasureSpec, heightMeasureSpec);也就是会走FrameLayout的onMeasure()，我们看一下它的源码
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
可以看到它里面就是遍历子View再执行子View的measure方法，我们可以看到getChildMeasureSpec获取子类的测量规格MeasureSpec
// spec参数 表示父View的MeasureSpec
// padding参数 父View的Padding+子View的Margin，父View的大小减去这些边距，才能精确算出
// 子View的MeasureSpec的size
// childDimension参数 表示该子View内部LayoutParams属性的值（lp.width或者lp.height）
// 可以是wrap_content、match_parent、一个精确指(an exactly size),
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec); //获得父View的mode
int specSize = MeasureSpec.getSize(spec); //获得父View的大小
//父View的大小-自己的Padding+子View的Margin，得到值才是子View的大小。
int size = Math.max(0, specSize - padding);
int resultSize = 0; //初始化值，最后通过这个两个值生成子View的MeasureSpec
int resultMode = 0; //初始化值，最后通过这个两个值生成子View的MeasureSpec
switch (specMode) {
// Parent has imposed an exact size on us
//1、父View是EXACTLY的 ！
case MeasureSpec.EXACTLY:
//1.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//1.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//1.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST 。
}
break;
// Parent has imposed a maximum size on us
//2、父View是AT_MOST的 ！
case MeasureSpec.AT_MOST:
//2.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//2.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST
}
//2.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST
}
break;
// Parent asked to see how big we want to be
//3、父View是UNSPECIFIED的 ！
case MeasureSpec.UNSPECIFIED:
//3.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY
}
//3.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0; //size为0！ ,其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode为 UNSPECIFIED
}
//3.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0; //size为0! ，其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode为 UNSPECIFIED
}
break;
}
//根据上面逻辑条件获取的mode和size构建MeasureSpec对象。
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
从以上可以得出我们之前提过的结论，子View的MeasureSpec是由父View的MeasureSpec和子View的LayoutParams计算出来，getChildMeasureSpec就是这个算法过程
layout过程
在performTraversals中我们知道measure过程结束后，视图的大小就已经测量好了，接下来就是layout的过程，查看performLayout源码我们重点关注host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());host就是上面说的mView也就是DecorView因此我们去查看ViewGroup的layout方法
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
代码可以看个大概，LayoutTransition是用于处理ViewGroup增加和删除子视图的动画效果，也就是说如果当前ViewGroup未添加LayoutTransition动画，或者LayoutTransition动画此刻并未运行，那么调用super.layout(l, t, r, b)，继而调用到ViewGroup中的onLayout，否则将mLayoutSuppressed设置为true，等待动画完成时再调用requestLayout()。
这个函数是final 不能重写，所以ViewGroup的子类都会调用这个函数，layout 的具体实现是在super.layout(l, t, r, b)里面做的，那么我接下来看一下View类的layout函数
public void layout(int l, int t, int r, int b) {
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
}
}
重点关注onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
我们发现onLayout是个空实现，因为onLayout()过程是为了确定视图在布局中所在的位置，而这个操作应该是由布局来完成的，即父视图决定子视图的显示位置。既然如此，我们来看下ViewGroup中的onLayout()方法是怎么写的吧，代码如下：
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);
ViewGroup中多了关键字abstract的修饰，要求其子类必须重载onLayout函数。那我们就看看FrameLayout的onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
layoutChildren里面代码
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
迭代每一个子类，再调用子类的layout。接着执行onLayout，其他的ViewGroup也是类似的
draw过程
在测量和确定位置之后就会执行draw，ViewRootImpl中的代码会继续执行并创建出一个Canvas对象，然后调用View的draw()方法来执行具体的绘制工作。draw()方法内部的绘制过程总共可以分为六步，其中第二步和第五步在一般情况下很少用到，因此这里我们只分析简化后的绘制过程。代码如下所示：
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
注释写得比较清楚，一共分成6步，看到注释没有（ // skip step 2 & 5 if possible (common case)）除了2 和 5之外 我们一步一步来看：
1、第一步：背景绘制
看注释即可，不是重点
private void drawBackground(Canvas canvas) {
Drawable final Drawable background = mBackground;
......
//mRight - mLeft, mBottom - mTop layout确定的四个点来设置背景的绘制区域
if (mBackgroundSizeChanged) {
background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false; rebuildOutline();
}
......
//调用Drawable的draw() 把背景图片画到画布上
background.draw(canvas);
......
}
2、第三步，对View的内容进行绘制。
onDraw(canvas) 方法是view用来draw 自己的，具体如何绘制，颜色线条什么样式就需要子View自己去实现，View.java 的onDraw(canvas) 是空实现，ViewGroup 也没有实现，每个View的内容是各不相同的，所以需要由子类去实现具体逻辑。
3、第4步 对当前View的所有子View进行绘制
dispatchDraw(canvas) 方法是用来绘制子View的，View.java 的dispatchDraw()方法是一个空方法,因为View没有子View,不需要实现dispatchDraw ()方法，ViewGroup就不一样了，它实现了dispatchDraw ()方法：
@Override
protected void dispatchDraw(Canvas canvas) {
...
if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
for (int i = 0; i < count; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
} else {
for (int i = 0; i < count; i++) {
final View child = children[getChildDrawingOrder(count, i)];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
}
......
}
代码一眼看出，就是遍历子View然后drawChild(),drawChild()方法实际调用的是子View.draw()方法,ViewGroup类已经为我们实现绘制子View的默认过程，这个实现基本能满足大部分需求，所以ViewGroup类的子类（LinearLayout,FrameLayout）也基本没有去重写dispatchDraw方法，我们在实现自定义控件，除非比较特别，不然一般也不需要去重写它， drawChild()的核心过程就是为子视图分配合适的cavas剪切区，剪切区的大小正是由layout过程决定的，而剪切区的位置取决于滚动值以及子视图当前的动画。设置完剪切区后就会调用子视图的draw()函数进行具体的绘制了。
4、第6步 对View的滚动条进行绘制
不是重点，知道有这东西就行，onDrawScrollBars 的一句注释 ：Request the drawing of the horizontal and the vertical scrollbar. The scrollbars are painted only if they have been awakened first.
一张图看下整个draw的递归流程。
image.png
到此整个绘制过程基本讲述完毕了。
本文基于安卓6.0源码分析，中间有很多代码已经省略，要查看全部源码，请自行查阅
参考资料：
https://www.jianshu.com/p/5a71014e7b1b
https://blog.csdn.net/guolin_blog/article/details/16330267
