重温View绘制原理（二）
好记性不如烂笔头。生活中多做笔记，不仅可以方便自己，还可以方便他人。
(下面的源码大部分是来自API 28)
紧接着上一篇文章重温View绘制原理（一），继续看view绘制原理。
1. View绘制流程
view的绘制是从根视图 ViewRoot 的 performTraversals() 方法开始，从上到下遍历整个视图树，每个 View 控制负责绘制自己，而 ViewGroup 还需要负责通知自己的子 View 进行绘制操作。视图操作的过程可以分为三个步骤，分别是测量(Measure)、布局(Layout)和绘制(Draw)。performTraversals 方法在viewRoot的实现类 ViewRootImpl 里面：
- measure方法用于测量View的宽高
- layout方法用于确定View在父容器中的位置
- draw方法负责将View绘制在屏幕上
view绘制流程图：
看看performTraversals方法源码：
private void performTraversals() {
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
...
// 测量
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
// 布局
performLayout(lp, mWidth, mHeight);
...
// 绘制
performDraw();
...
}
依次调用performMeasure、performLayout、performDraw方法，分别完成顶级View的measure、layout、draw流程。performMeasure会调用measure方法，而measure又会调用onMeasure方法，在onMeasure方法中又会对子元素进行measure，这样重复下去就完成了整个View树的遍历。
performLayout、performDraw传递过程也非常类似，不过performDraw是在draw方法中通过dispatchDraw方法实现的。
measure过程决定了View的宽高，而Layout方法则确定了四个顶点的坐标和实际的宽高(往往等于measure中计算的宽高)，draw方法则决定了View的显示。只有完成了draw方法才能正确显示在屏幕上。
2. MeasureSpec
MeasureSpec是measure的重要参数。
MeasureSpec 表示的是一个 32 位的整数值，它的高 2 位表示测量模式 SpecMode，低 30 位表示某种测量模式下的规格大小 SpecSize（PS：这里用到了位运算进行状态压缩来节省内存）。MeasureSpec 是 View 类的一个静态内部类，用来说明应该如何测量这个View，有三种模式：
-
UNSPECIFIED：不指定测量模式，父视图没有限制子视图的大小，子视图可以是想要的任何尺寸，通常用于系统内部，应用开发中很少使用到。
-
EXACTLY：精确测量模式，当该视图的 layout_width 或者 layout_height 指定为具体数值或者 match_parent 时生效，表示父视图已经决定了子视图的精确大小，这种模式下 View 的测量值就是 SpecSize 的值。
-
AT_MOST：最大值模式，当前视图的 layout_width 或者 layout_height 指定为 wrap_content 时生效，此时子视图的尺寸可以是不超过父视图运行的最大尺寸的任何尺寸。
下表是普通View的MeasureSpec的创建规则对应表：
| childLayoutParams/parentSpecParams | EXACTLY | AT_MOST | UNSPECIFIED |
|---|---|---|---|
| dp/px | EXACTLY childSIze | EXACTLY childSIze | EXACTLY childSIze |
| match_parent | EXACTLY parentSize | AT_MOST parentSize | UNSPECIFIED 0 |
| wrap_content | AT_MOST parentSize | AT_MOST parentSize | UNSPECIFIED 0 |
3. Measure
View的绘制从测量开始，看看performMeasure（）方法：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
具体操作是分发给 ViewGroup 的，由 ViewGroup 在它的 measureChild 方法中传递给子 View。ViewGroup 通过遍历自身所有的子 View，并逐个调用子 View 的 measure 方法实现测量操作。
3.1 View的measure过程
View的measure过程由measure方法来完成，measure方法是一个final方法，不能重写，它会调用VIew的onMeasure方法。onMeasure方法中会调用getDefaultSize方法，而getDefault方法中又会调用getSuggestedWidth和getSuggestedHeight方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
getDefaultSize方法所返回的就是测量后的View的大小。
接着看getSuggestedWidth和getSuggestedHeight方法:
/**
* Returns the suggested minimum width that the view should use. This
* returns the maximum of the view's minimum width
* and the background's minimum width
* ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
*
* When being used in {@link #onMeasure(int, int)}, the caller should still
* ensure the returned width is within the requirements of the parent.
*
* @return The suggested minimum width of the view.
*/
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
它在没有指定background的情况下，返回的是minSize这一属性对应的值，而在指定了背景的情况下，返回的是背景drawable的getMinimumWidth / getMinimumHeight方法对应的值
这两个方法在Drawable有原始宽度的情况下返回原始宽度，否则返回0
从getDefaultSize方法可以看出，View的宽高由specSize决定。
3.2 ViewGroup的measure过程
ViewGroup除了完成自己的measure过程，还会遍历调用子元素的measure方法，然后子元素再次递归执行，ViewGroup是一个抽象类，因此没有重写View的onMeasure方法。但它提供了一个measureChildren的方法，如下:
/**
* Ask all of the children of this view to measure themselves, taking into
* account both the MeasureSpec requirements for this view and its padding.
* We skip children that are in the GONE state The heavy lifting is done in
* getChildMeasureSpec.
*
* @param widthMeasureSpec The width requirements for this view
* @param heightMeasureSpec The height requirements for this view
*/
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
可以看到，ViewGroup执行measure时，会遍历子元素，调用measureChild方法对子元素进行measure。
在measureChild方法中，取出子元素的LayoutParams，通过getChildMeasureSpec方法创建子元素MeasureSpec，然后传递给View的measure方法进行测量。
ViewGroup没有定义测量具体过程，因为它是个抽象类。具体的测量过程的onMeasure方法需要子类来实现，由于它的子类的特性可能会很大不同，所以没法做统一处理(如LinearLayout和RelativeLayout)。
4. Layout
Layout流程的作用是ViewGroup确定子元素的位置。当ViewGroup被确定后，在onLayout中会遍历所有子元素并调用layout方法，在layout方法中会调用onLayout方法。layout方法确定View的位置，而onLayout方法则确定所有子元素的位置。
ViewRootImpl 的 performLayout 如下：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (host == null) {
return;
}
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Log.v(mTag, "Laying out " + host + " to (" +
host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
mInLayout = false;
}
先看View的layout方法：
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
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;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
final boolean wasLayoutValid = isLayoutValid();
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if (!wasLayoutValid && isFocused()) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
if (canTakeFocus()) {
// We have a robust focus, so parents should no longer be wanting focus.
clearParentsWantFocus();
} else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) {
// This is a weird case. Most-likely the user, rather than ViewRootImpl, called
// layout. In this case, there's no guarantee that parent layouts will be evaluated
// and thus the safest action is to clear focus here.
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
clearParentsWantFocus();
} else if (!hasParentWantsFocus()) {
// original requestFocus was likely on this view directly, so just clear focus
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
// otherwise, we let parents handle re-assigning focus during their layout passes.
} else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
View focused = findFocus();
if (focused != null) {
// Try to restore focus as close as possible to our starting focus.
if (!restoreDefaultFocus() && !hasParentWantsFocus()) {
// Give up and clear focus once we've reached the top-most parent which wants
// focus.
focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
}
}
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}
首先，通过setFrame方法设定View四个顶点的位置（初始化mLeft，mRight，mTop，mBottom）。四个顶点一旦确定，则在父容器中的位置也确定了，接着便会调用onLayout方法，来让父容器确定子容器的位置。onLayout同样和具体布局有关，因此View和ViewGroup均没有实现onLayout方法。
5. Draw
draw流程是将View绘制到屏幕上。先看看performDraw 方法：
private void performDraw() {
....
try {
boolean canUseAsync = draw(fullRedrawNeeded);
....
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
...
}
private boolean draw(boolean fullRedrawNeeded) {
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
...
mView.draw(canvas);
...
}
最终调用到每个 View 的 draw 方法绘制每个具体的 View，绘制基本上可以分为六个步骤:
public void draw(Canvas canvas) {
...
// Step 1, draw the background, if needed
if (!dirtyOpaque) {
drawBackground(canvas);
}
...
// Step 2, save the canvas' layers
saveCount = canvas.getSaveCount();
...
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
canvas.drawRect(left, top, right, top + length, p);
...
canvas.restoreToCount(saveCount);
...
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}
View绘制过程的传递是通过dispatchDraw实现的。dispatchDraw会遍历调用所有子元素的draw方法。这样draw事件就一层层传递了下来。
它有个比较特殊的setWillNotDraw方法。如果一个View不需要绘制任何内容，在我们设定这个标记为true后，系统就会对其进行相应优化。一般View没有启用这个标记位。但ViewGroup是默认启用的。
它对实际开发的意义在于：我们的自定义控件继承于ViewGroup并且不具备绘制功能时，可以开启这个标记位方便系统进行后续优化。
6. 结束语
关于view绘制的原理在网上也特别多，时间久了也容易忘记，看一遍别人的，还不如顺便把他们的记录下来，方便自己以后温习。
重要参考：
-
https://blog.csdn.net/qq_21556263/article/details/82768420
-
https://www.jianshu.com/p/c151efe22d0d