写在前面的话
按照之前写的节奏来的话，这篇改对View的整个测量、布局和绘制过程进行分析了。在之前的Activity显示到Window的过程中了解到performTraversals()这个方法会执行performMeasure()去测量View的大小，performLayout()去将子View放到合适的位置上，performDraw()将View真正绘制出来。
1. measure的过程
1.1 在测量前，先看下MeasureSpec
MeasureSpec理解为测量规格，从源码中可以知道测量规格包括了测量模式（SpecMode）和大小（SpecSize），这个规格通过一个int型来表示。其中int的高2位代表了测量模式，低30位代表了大小。我们都知道两位可以有四种组合情况，而Android中View有三种测量模式，分别是：
- UNSPECIFIED（0 << 30）：子View可以想要任意大小
- EXACTLY（1 << 30）：父容器已经检测出子View所需要的精确大小，View的大小即为SpecSize的大小，他对应于布局参数中的MATCH_PARENT，或者精确值
- AT_MOST（2 << 30）：父容器指定了一个大小，即SpecSize，子View的大小不能超过这个SpecSize的大小
通过测量规格获取测量模式和大小：
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
// 获得SpecMode
@MeasureSpecMode
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
// 获得SpecSize
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
从上面可以看到，获得SpecMode时，需要和MODE_MASK（0x30000000）进行与运算，因为低30位全为0，高2位都为1，所以最终的结果就是高2位<<30的值，也就是我们三个测量模式中的一个。
同理，在获得SpecSize时，我们需要将SpecMode去除，获得低30位的值，所以这里进行的是与上非MODE_MASK运算，即获取低30位的值（SpecSize）。
1.2 getRootMeasureSpec方法
在执行performMeasure()方法前，会执行ViewRootImpl中的getRootMeasureSpec方法，通过这个方法来获得跟布局的测量规格。
// mWidth和mHeight的值是通过
//if (mWidth != frame.width() || mHeight != frame.height()) {
//mWidth = frame.width();
//mHeight = frame.height();
//}
//赋值，这里等于Window窗口的宽高
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
//rootDimension是decorView的params的参数，这里为MATCH_PARENT，所以测量模式是EXACTLY
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;
}
1.3 View的measure方法
在获得了宽高的测量规格后，将会执行performMeasure()方法，performMeasure()方法会调用DecorView的measure()方法，DecorView和其父类并没有重写这个measure方法，最终会调用View的measure方法。
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//判断当前View的layoutMode是不是LAYOUT_MODE_OPTICAL_BOUNDS，这种情况很少
boolean optical = isLayoutModeOptical(this);
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);
}
// 作为缓存的key
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
// 需要布局
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
// 如果是强制布局的话，则需要重新去调用onMeasure方法，否则去缓存中获取
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
// 放到缓存中
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
// onMeasure中需要去设置测量的结果，View的默认实现是设置默认的大小，这个大小根据测量模式来确定
// 如果是UNSPECIFIED：未指定的话则大小为建议的最小值
// 如果是AT_MOST||EXACTLY，那么返回值为SpecSize
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
// 真正为mMeasuredWidth和mMeasuredHeight赋值
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
// 为mMeasuredWidth和mMeasuredHeight赋值
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
从代码中可以看到，如果我们需要进行布局的话，首先判断是否为强制布局，如果不是的话获得mMeasureCache中当前测量规格的位置。如果没有这个缓存，则说明需要去进行onMeasure方法去测量真正的宽高，最后将当前宽高的测量规格保存到缓存中。
在调用View的onMeasure方法时，我们需要调用setMeasuredDimension方法来设置具体的宽高，当调用了这个方法后，会通过setMeasuredDimensionRaw方法来给mMeasuredWidth和mMeasuredHeight赋值，这样我们通过getMeasuredWidthAndState()获取mMeasuredWidth值或者通过getMeasuredWidth()来获取mMeasuredWidth & MEASURED_SIZE_MASK(0x00ffffff)值。
上面写到的都是View里面关于测量的方法，从这里我们就看出来了，View的测量确定了View的四个点的位置以及测量的宽高。ViewGroup作为View的子类，其并没有重写onMeasure方法，作为ViewGroup的子类基本上都会重写onMeasure方法，通过onMeasure方法来测量子View的大小，通过子View的大小最终来确定自己的大小。
下面是FrameLayout的测量过程：
FrameLayout.java
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
// 如果当前的测量模式不是EXACTLY，则需要统计拥有MATCH_PARENT属性的子View
// 在设置完成当前layout的宽高后，需要重新测量拥有MATCH_PARENT属性的子View
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
// 如果子View不是隐藏状态，则需要测量
if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 测量子View
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
// 设置最大宽度，每个子View和当前的最大宽度进行比较
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);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
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());
}
// 为当前layout设置宽高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
// 如果有MATCH_PARENT属性的子View大于1的话，则需要重新去测量这些子View
count = mMatchParentChildren.size();
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);
}
}
}
ViewGroup.java
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
// 两个测量规格都有了，接着通过child.measure去测量自身大小
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
ViewGroup.java
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// EXACTLY如果是精确大小的话，则根据child的大小来计算具体大小
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
// 如果设置有具体值，则结果设置具体值，模式为EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
// MATCH_PARENT则设置父View的大小，模式为EXACTLY
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
// 结果设置成父View大小，并且测量模式设置成AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// AT_MOST模式
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
// 设置具体大小，并且测量模式是EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} 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.
// 设置成父View大小，并且模式是AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
// 设置成父View大小，并且模式是AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// UNSPECIFIED模式
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
// 设置具体大小，并且测量模式是EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
// sUseZeroUnspecifiedMeasureSpec = targetSdkVersion < M
// 通过targetSdkVersion来判断size设置为0还是父View的size，并且测量模式设置UNSPECIFIED
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
// 通过targetSdkVersion来判断size设置为0还是父View的size，并且测量模式设置UNSPECIFIED
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
View.java
// 这个我的理解是解析size并使这个size拥有一个状态
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
final int specMode = MeasureSpec.getMode(measureSpec);
final int specSize = MeasureSpec.getSize(measureSpec);
final int result;
switch (specMode) {
// 如果是AT_MOST模式，则需要判断需要的size和测量规格的SpecSize的大小
// 如果需要的size>SpecSize，那么使用SpecSize，并且设置标志位为MEASURED_STATE_TOO_SMALL = 0x01000000
// 这个标志为代表了当前测量规格的大小小于所需大小
case MeasureSpec.AT_MOST:
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
// 如果测量模式是EXACTLY，那么这个结果就是specSize
case MeasureSpec.EXACTLY:
result = specSize;
break;
// 其他情况都是所需的size
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
//MEASURED_STATE_MASK = 0xff000000，这里的结果都带有一个状态，这个状态的用处不详。。。
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
从上面FrameLayout的测量过程我们可以看到，整个流程如下：
流程图
测量过程：
- FrameLayout调用onMeasure方法，开始测量
- FrameLayout的onMeasure方法中调用了其父类的measureChildWithMargins方法去测量子View的大小
- measureChildWithMargins通过getChildMeasureSpec方法获得子View的测量规格后调用子View的measure方法
- 子View通过调用onMeasure方法最终通过setMeasuredDimension设置具体的测量后的宽高
- FrameLayout在获得所有子View的结果后，获取其中的最大值，并且如果有背景图的话，获取子View和背景图的最大值，同样通过setMeasuredDimension设置FrameLayout的大小
2. layout过程
measure之后就会进行layout过程。layout其实就是对View的left、top、right、bottom这四个点位置的确定的过程。从源码可以看到，View中实现了layout方法，ViewGroup对其进行了Override，但是ViewGroup会调用super.layout(l, t, r, b)，所以最终还是进入View的layout方法。
在ViewGroup中onLayout是一个抽象方法，这就意味着所有的子类需要实现这个抽象方法。一般来说，每个不同的layout都有不同的实现，这样就构成了我们Android各种布局。当然了，自定义控件中关于onLayout的实现也是很重要的。下面还是关于FrameLayout的layout的实现：
View.java
public void layout(int l, int t, int r, int b) {
// mPrivateFlags3的赋值是在measure方法中，多次测量是在不是强制layout并且有缓存的情况下进行赋值的
// 这种情况需要重新调用onMeasure方法，对View重新设置大小
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;
// 是否使用视觉边界布局效果，这个并不影响这个流程，因为setOpticalFrame也会调用setFrame方法
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 如果有改变，则需要重新布局
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 调用onLayout方法进行布局
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
// 调用OnLayoutChangeListener的onLayoutChange方法
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)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);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
View.java
// 返回值是boolean，代表是否位置改变了，如果和以前的位置不同，则说明改变了，需要重新布局
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
// 如果和以前的位置不同，则说明改变了，需要重新布局
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
// 原宽高和新宽高如果不一致，则说明尺寸改变了
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// 使我们旧的位置无效
// Invalidate our old position
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
// 如果尺寸改变了，调用sizeChange方法，这里面会调用onSizeChanged方法
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
FrameLayout.java
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
FrameLayout.java
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
// 获得当前View的四个可布局的点
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;
}
// 获得布局的方向，如果没有设置flag PFLAG2_LAYOUT_DIRECTION_RESOLVED_RTL，则为从左到右布局
// 右到左布局在有些国家会出现这种情况
final int layoutDirection = getLayoutDirection();
// 获得当前布局的绝对显示位置，这里会根据布局方向来设置具体是从左边开始还是右边开始
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
// 垂直方法的显示位置
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
// 水平方向显示位置的设置
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
// 水平居中
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
// 从右边开始
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
// 左边和默认的情况都是左边开始布局
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
// 垂直方向的布局
switch (verticalGravity) {
// 顶部开始
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
// 垂直居中
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
// 底部开始
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
// 默认顶部开始
default:
childTop = parentTop + lp.topMargin;
}
// 子View布局
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
layout流程图
layout的过程就是确定当前View的left、top、right、bottom这四个点位置，通过这四个点可以确定这个View的位置，从而在绘制的时候正确绘制。layout的过程和measure的过程不同，layout的过程是先确定自己的位置在确定其子View的位置。
3. draw
draw过程在之前的Activity显示到Window的过程中有写到过，在整个过程中，会调用View的draw方法：
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) {
// 绘制背景
drawBackground(canvas);
}
// 通常来算是跳过2和5部分。这里只看下跳过2和5部分时，整个流程
// 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
// 调用onDraw方法，去绘制内容
if (!dirtyOpaque) onDraw(canvas);
// 分发绘制事件
// Step 4, draw the children
dispatchDraw(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// 绘制装饰内容
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// we're done...
return;
}
......
}
从代码中我们可以了解到，整个绘制过程一共有6步，但是通常来说第2步和第5步不会调用：
- 绘制背景 drawBackground
- 保存画布图层
- 绘制内容 onDraw
- 分发绘制事件（绘制子View） dispatchDraw
- 绘制并恢复图层
- 绘制装饰 onDrawForeground
3.1 drawBackground
private void drawBackground(Canvas canvas) {
final Drawable background = mBackground;
// 背景为空，直接返回
if (background == null) {
return;
}
// 设置背景的边界值
setBackgroundBounds();
// Attempt to use a display list if requested.
if (canvas.isHardwareAccelerated() && mAttachInfo != null
&& mAttachInfo.mHardwareRenderer != null) {
mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode);
final RenderNode renderNode = mBackgroundRenderNode;
if (renderNode != null && renderNode.isValid()) {
setBackgroundRenderNodeProperties(renderNode);
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
return;
}
}
// 滚动的x和y值
final int scrollX = mScrollX;
final int scrollY = mScrollY;
// 没有滚动，直接绘制
if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
// 将canvas移动后再绘制
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
void setBackgroundBounds() {
// 如果背景尺寸改变并且背景不为空，这设置其边界为0，0，width，height
if (mBackgroundSizeChanged && mBackground != null) {
mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
rebuildOutline();
}
}
drawBackground方法比较简单，总体来说就是绘制View的背景，当然根据背景是否存在，是否页面滚动了来绘制背景。
3.2 onDraw
onDraw是没有具体实现的内容，一般来说在自定义View的时候，很多时候会重写onDraw方法来绘制真正要实现的内容。
3.3 dispatchDraw
从注释来看，dispatchDraw作为绘制子View的开始，其在View中是空实现。在ViewGroup中有dispatchDraw方法的具体实现：
ViewGroup.java
// 我们只关注如何绘制childView，内容省略了一大部分
// 这里面可以看到会调用drawChild去绘制其子View
@Override
protected void dispatchDraw(Canvas canvas) {
......
// 我们只关注如何绘制childView
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
......
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
// 绘制其子View
more |= drawChild(canvas, child, drawingTime);
}
}
......
}
ViewGroup.java
// 子View会调用其返回值为boolean的draw方法去绘制
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
View.java
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
// 关于硬件加速模式
......
// 动画相关
......
// 硬件加速相关，通过updateDisplayListIfDirty获取显示列表的renderNode最后下面绘制
if (drawingWithRenderNode) {
// Delay getting the display list until animation-driven alpha values are
// set up and possibly passed on to the view
renderNode = updateDisplayListIfDirty();
if (!renderNode.isValid()) {
// Uncommon, but possible. If a view is removed from the hierarchy during the call
// to getDisplayList(), the display list will be marked invalid and we should not
// try to use it again.
renderNode = null;
drawingWithRenderNode = false;
}
}
......
// 如果使用缓存去绘制，则通过cache绘制，否则还是会调用View的draw(canvas)方法绘制
if (!drawingWithDrawingCache) {
// 硬件加速的话通过drawRenderNode去绘制，在之前讲过了最后会调用View的draw(canvas)方法绘制
if (drawingWithRenderNode) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
} else {
// Fast path for layouts with no backgrounds
// 无背景的快速绘制
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
//
dispatchDraw(canvas);
} else {
// 调用View的draw(canvas)方法绘制
draw(canvas);
}
}
} else if (cache != null) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
// no layer paint, use temporary paint to draw bitmap
Paint cachePaint = parent.mCachePaint;
if (cachePaint == null) {
cachePaint = new Paint();
cachePaint.setDither(false);
parent.mCachePaint = cachePaint;
}
cachePaint.setAlpha((int) (alpha * 255));
canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
} else {
// use layer paint to draw the bitmap, merging the two alphas, but also restore
int layerPaintAlpha = mLayerPaint.getAlpha();
if (alpha < 1) {
mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
}
canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
if (alpha < 1) {
mLayerPaint.setAlpha(layerPaintAlpha);
}
}
}
......
return more;
}
绘制子View的过程要相对繁琐一些，通过View的另一个draw方法来绘制子View，并且这个方法包括了动画相关，硬件加速相关。上面的代码有一部分省略了，其中比较重要的是两个地方：
- 硬件加速相关过程：在之前的Activity显示到Window的过程中有写到过，主要是通过
updateDisplayListIfDirty方法获取显示列表的renderNode最后通过硬件加速绘制。 - 软件绘制过程：需要判断是否使用缓存，如果使用缓存的话，直接绘制缓存，否则的话还需要按照上面的绘制流程一步步进行。
3.4 onDrawForeground
看这个名称可以认为是绘制前景，其中包括了滚动条、滚动指示器等。当然了，可以通过重写这个方法去绘制任何想要的前景。
public void onDrawForeground(Canvas canvas) {
// 滚动指示器绘制
onDrawScrollIndicators(canvas);
// 滚动条绘制
onDrawScrollBars(canvas);
// 前景
final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
if (foreground != null) {
if (mForegroundInfo.mBoundsChanged) {
mForegroundInfo.mBoundsChanged = false;
final Rect selfBounds = mForegroundInfo.mSelfBounds;
final Rect overlayBounds = mForegroundInfo.mOverlayBounds;
if (mForegroundInfo.mInsidePadding) {
selfBounds.set(0, 0, getWidth(), getHeight());
} else {
selfBounds.set(getPaddingLeft(), getPaddingTop(),
getWidth() - getPaddingRight(), getHeight() - getPaddingBottom());
}
final int ld = getLayoutDirection();
Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(),
foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld);
foreground.setBounds(overlayBounds);
}
// 前景的绘制
foreground.draw(canvas);
}
}
3.5 绘制顺序
这里还是使用扔物线大神的一张图来表示绘制顺序吧。
绘制顺序.jpg
3.6 draw的总结
整个绘制过程是一个自上向下的过程，在这个过程中先绘制自身的背景（drawBackground）、内容（onDraw），接着绘制子View（dispatchDraw）。子View的绘制过程又和上面的过程一样，当所有的子View绘制完成后，会执行装饰的绘制（onDrawForeground）
写在后面的话
还是按照计划来的，整个流程已经写到了测量、布局和绘制的过程。总体来说感觉网上有些资料还是不够靠谱，如果自己不去看一遍的话，可能会有许多坑等着你来填。
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