作者学习View的三部曲:Measure、Layout、Draw的时候,就像 Android进阶系列0—View的工作流程:measure,layout,draw小结说的,借助《Android开发艺术探索》和郭霖大神的博客学习,但是作为一个level not very high的人,总觉得像在记忆,不符合自己的学习习惯。博主的几篇博文里面,Android进阶系列4和5是比较符合个人学习习惯的,跟着代码学习流程和细节。在读完 从ViewRootImpl类分析View绘制的流程一文之后对View的绘制流程有豁然开朗的感觉,按这个思路,更容易get到View的绘制。默默地说一句,作者这篇文章题目起的不好,或许换个题目可以被更多人读到。 言归正传,博主从DecorView开始理解View的Measure、Layout、Draw流程,上篇文章Android进阶系列6-从DecorView开始的View绘制流程已经说完了DecorView怎样从handleResumeActivty()一步步地准备进入到View的三部曲。这篇文章就在它的基础上,讲三部曲具体的过程。
1.measure
执行performMeasure测量之前通过getRootMeasureSpec方法获得DecorView的测量规格,查看getRootMeasureSpec源码
/**
* Figures out the measure spec for the root view in a window based on it's
* layout params.
*
* @param windowSize
* The available width or height of the window
*
* @param rootDimension
* The layout params for one dimension (width or height) of the
* window.
*
* @return The measure spec to use to measure the root view.
*/
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
//匹配父容器时,测量模式为MeasureSpec.EXACTLY,测量大小直接为屏幕的大小,也就是充满真个屏幕
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
//包裹内容时,测量模式为MeasureSpec.AT_MOST,测量大小直接为屏幕大小,也就是充满真个屏幕
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
//其他情况时,测量模式为MeasureSpec.EXACTLY,测量大小为DecorView顶层视图布局设置的大小。
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
DecorView的rootDimension为啥是Exactly,博主没想明白,知道的同学在评论中告诉博主。 得到measureSpec后,performMeasure()方法调用了 View中measure()方法进行测量。由于DecorView继承自FrameLayout,FrameLayout的父类是ViewGroup,ViewGroup是View的子类,因此可在这几个类中寻找方法的定义(layout,draw过程也是如此)。DecorView的代码很长这里就不贴出来了。查看DecorView的源码发现,DecorView没有重写measure方法,其他父类也是,只有View中定义了measure方法,所以查看View的measure方法。
View的measure
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
......
//如果上一次的测量规格和这次不一样,则条件满足,重新测量视图View的大小
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);//重要方法
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;
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
//保存当前绘制数据,和下次的绘制数据进行比较
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}
view的measure方法调用了onMeasure方法且measure是final方法,不能被重写。DecorView重写了onMeasure方法,又调用了FrameLayout的onMeasure方法,而ViewGroup没有重写View的onMeasure方法。所以我们直接分析FrameLayout和ViewGroup的onMeasure方法即可。为什么ViewGroup没有重写onMeasure方法呢?我们猜测是因为onMeasure才是真正测量本身的方法,不同的ViewGroup布局不同需要具体的onMeasure实现,ViewGroup就没有给出统一的测量方法。我们不妨先查看View的onMeasure方法,一会儿再去看DecorView和FrameLayout中的onMeasure。
View的onMeasure
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
调用了setMeasuredDimension方法,并利用getDefaultSize设置方法参数,getDefauldSize代码如下
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
//获得测量模式
int specMode = MeasureSpec.getMode(measureSpec);
//获得父亲容器留给子视图View的大小
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;
}
可见setMeasuredDimension是真正绘制View大小的方法,某种程度上可以验证我们对onMeasure的猜想。此外,View最终的大小是由布局大小和父容器的测量规格共同决定,自定义View如果没有重写onMeasure方法,那么MeasureSpec.AT_MOST和MeasureSpec.EXACTLY下的测量大小是一样的,都是Match_Parent的效果。对于getSuggestedMinumWidth就不做分析了,很少用到。
说完View的measure,再看DecorView和FrameLayout的onMeasure,DecorView的onMeasure做了一些参数设置,查看FrameLayout的onMeasure方法查看一番
FrameLayout onMeasure
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
..............
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//测量FrameLayout下每个子视图View的宽和高
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);
}
}
}
}
// 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());
}
//设置当前FrameLayout测量结果,此方法的调用表示当前View测量的结束。
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
}
onMeasure中调用measureChildWithMargins方法遍历FrameLayout的每个子View,该方法在FrameLayout的父类ViewGroup中实现。遍历完之后调用setMeasuredDimension确定FrameLayout大小。
ViewGroup measureChildWithMargins
measureChildWithMargins代码如下:
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);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
该方法中调用 getChildMeasureSpec方法由父布局的MeasureSpec和子View的LayoutParams共同决定子View的MeasureSpec,并调用子View的measure进行绘制。getChildMeasureSpec的具体操作方式如代码所示:
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) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
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.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
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.
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.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// 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
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
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
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
代码比较简单,相信都能看懂,不做过多解释。根布局DecorView的测量规格中的测量模式是MeasureSpec.EXACTLY,测量大小是整个窗口大小。因此上面代码分支走MeasureSpec.EXACTLY。子视图View的测量规格由其宽和高参数决定。
从DecorView开始的measure流程就说完了,流程如图:
注意:
- View的measure方法是final类型的,子类不可以重写。子类可以通过重写onMeasure方法来测量自己的大小或者ViewGroup类型子类继续调用子View的measure方法
- View(直接继承自View的相关View)的大小由父容器和自身LayoutParams决定。为什么一定有父容器呢?每个Activity相关的View都是从DecorView开始的加载过程,所以说一定有父容器了
2.Layout
performLayout代码如下:
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
......
//mView就是DecorView
final View host = mView;
......
//DecorView请求布局
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}
调用了DecorView的layout方法,查看DecorView代码后发现,layout在ViewGroup重写了layout方法,并调用父类View的layout方法。重写的layout方法加了关于滑动的判断,无关紧要,去View看layout方法实现。
View layout()
public void layout(int l, int t, int r, int b) {
//判断是否需要重新测量
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
//保存上一次View的四个位置
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//设置当前视图View的左,顶,右,底的位置,并且判断布局是否有改变
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
//如果布局有改变,条件成立,则视图View重新布局
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//调用onLayout,将具体布局逻辑留给子类实现
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
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;
}
里面有两个关键方法setFrame(l, t, r, b)和onLayout(changed, l, t, r, b)。按名字来看setFrame应该已经布局完毕,那onLayout又是干啥的,挨个来看。
view setFrame()
/*Assign a size and position to this view.*/
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
//当上,下,左,右四个位置有一个和上次的值不一样都会重新布局
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
//得到本次和上次的宽和高
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
//判断本次View的宽高和上次View的宽高是否相等
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
//清楚上次布局的位置
invalidate(sizeChanged);
//保存当前View的最新位置
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
//如果当前View的尺寸有所变化
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
......
return changed;
}
mLeft,mTop,mRight,mBottom保存当前View的最新位置,到此当前View的布局基本结束,也可以发现,此时调用getWidth方法才会得到有效的宽度。
说完了setFrame(),再看看onLayout干啥的
View onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
空方法,啥都没干。看下DecorView中onLayout方法
DecorView onLayout
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
getOutsets(mOutsets);
if (mOutsets.left > 0) {
offsetLeftAndRight(-mOutsets.left);
}
if (mOutsets.top > 0) {
offsetTopAndBottom(-mOutsets.top);
}
}
主要调用了父类的onLayout方法,查看FrameLayout的onLayout方法实现
FrameLayout onLayout
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
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);
}
}
}
onLayout中调用了layoutChildren,layoutChildren遍历所有child,并调用child的layout方法对自己布局。可以知道,FrameLayout自身的布局是由View的layout处理的,onLayout处理了所有FrameLayout子布局的布局操作。在ViewGroup中,onLayout是一个抽象方法,必须由子View处理。为什么呢?因为onLayout方法是处理子View布局的,而不同的布局结构,对子View的处理方式不一,ViewGroup不提供统一的实现,ViewGroup的子View按实际需要实现。
这样以来,layout的过程就讲完了。
View Draw
从performDraw方法中调用drawSoftware()方法
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
.......
try {
//调整画布的位置
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
//调用View类中的成员方法draw开始绘制View视图
mView.draw(canvas);
}
......
return true;
}
只保留了绘制相关的主要代码,可以看到调用了view.draw方法,DecorView的draw方法如下所示
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
查看DecorView父类的draw方法,在View中有定义,代码较长
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);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// 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
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
}
官方的注释给的很详细了,我们清楚大概流程就行,
完整的是左边显示的六步,不需要边距渐变效果时,可精简为四步。其他四步暂且不管,想了解的可以这里查看。我们看见绘制当前视图和绘制子视图如何实现的。
protected void onDraw(Canvas canvas) {
}
protected void dispatchDraw(Canvas canvas) {
}
View中的两个方法都是空方法,去查看下子类的实现情况。DecorView中的onDraw方法如下
@Override
public void onDraw(Canvas c) {
super.onDraw(c);
mBackgroundFallback.draw(mContentRoot, c, mContentParent);
}
再看父类FrameLayout的onDraw未重载,一直调用到View的onDraw空方法。由此可见,onDraw由各个View自行实现,负责自身的绘制。
再看dispatchDraw方法,在ViewGroup中重写了该方法,代码较长,只看关键部分。
@Override
protected void dispatchDraw(Canvas canvas) {
......
//遍历绘制当前视图的子视图View
for (int i = 0; i < childrenCount; i++) {
int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
......
}
遍历所有子View调用drawChild绘制。drawChild方法如下:
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
调用 child.draw方法,又开始了新的一轮onDraw,disPatchDraw等操作。Draw的大概流程就是:
总结
至此View的measure,layout,draw流程就讲完了,这次没有机械的按照View或者ViewGroup的代码直接去总结该怎么。而是从DecorView的绘制开始,由上而下的去看整个绘制的过程,能理解到为什么会是这样,希望能让大家接受起来更顺畅。一般我们遇到的View绘制,都是从DecorView发起的,后面博主把WindowManger,Window理解了,再来做相应的补充。
很惭愧,做了一点微小的贡献!