UI繪制的詳細步驟
1. 測量performMeasure
->view.measure
->view.onMeasure
->view.setMeasuredDimension
->setMeasuredDimensionRaw
2. 布局performLayout
->view.layout
->view.onLayout
3. 繪制performDraw
->ViewRootImpl.draw(fullRedrawNeeded)
->ViewRootImpl.drawSoftware
->view.draw(Canvas)
1. 測量performMeasure：
先看ViewRootImpl.class源碼中的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);
}
}
調(diào)用了View.class的measure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
//讀取緩存
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
...
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
//調(diào)用onMeasure
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;
}
...
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
//設置緩存
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
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;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
代碼邏輯還算是比較清晰的方椎，調(diào)用步驟一如開頭總結(jié)出來的，為了更好的理解View繪制的過程钧嘶，我們再來理解一個類 MeasureSpec.class
了解MeasureSpec
View的布局參數(shù) 包含 模式 + 尺寸 的信息
而MeasureSpec 作為一個32位的int值棠众，則記錄了 模式 + 尺寸 這些信息
00 000000000000000000000000000000
【前2位代表模式SpecMode，后30位代表尺寸SpecSize】
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
3左移30位有决，轉(zhuǎn)為二進制：
11 000000000000000000000000000000
~MODE_MASK 則為：
00 111111111111111111111111111111
- public static final int UNSPECIFIED = 0 << MODE_SHIFT;
0左移30位闸拿，轉(zhuǎn)為二進制：
00 000000000000000000000000000000
父容器不對View做任何限制，一般系統(tǒng)內(nèi)部使用
public static final int EXACTLY = 1 << MODE_SHIFT;
1左移30位书幕，轉(zhuǎn)為二進制：
01 000000000000000000000000000000
父容器檢測出View的大小新荤，View的大小就是SpecSize LayoutParams match_parent 以及 固定大小public static final int AT_MOST = 2 << MODE_SHIFT;
2左移30位，轉(zhuǎn)為二進制：
10 000000000000000000000000000000
父容器制定一個可用大小按咒，View的大小不能超過這個值迟隅，LayoutParams wrap_content
如何生成一個MeasureSpec呢但骨？Android已經(jīng)給你提供方法了，傳入一個mode智袭，一個size奔缠，makeMeasureSpec方法就會返回一個MeasureSpec給你。而通過getMode和getSize就能從傳入的MeasureSpec中取出mode和size吼野。
public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
@MeasureSpecMode int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
View的測量-確定DecorView的MeasureSpec
DecorView的MeasureSpec由窗口大小和自身LayoutParams共同決定校哎，遵循如下規(guī)則：
- LayoutParams.MATCH_PARENT ：精確模式，窗口大小
- LayoutParams.WRAP_CONTENT ：最大模式瞳步，最大為窗口大小
- 固定大忻贫摺：精確模式，大小為LayoutParams的大小
回想剛才的分析流程单起，performMeasure方法里執(zhí)行了mView.measure方法抱怔，measure方法中又執(zhí)行了onMeasure方法，而對于我們要探究的DecorView來說嘀倒，我們就要看一下繼承自FrameLayout的DecorView.class是否重寫了onMeasure方法屈留，而不是只看View.class中的實現(xiàn)了，不同View子類對于onMeasure测蘑、onLayout灌危、onDraw的重寫，是我們的學習重點碳胳。
DecorView的onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
...
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
可以看到里面還調(diào)用了super.onMeasure勇蝙，那我們再來看看DecorView父類FrameLayout的onMeasure實現(xiàn)：
@Override
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();
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) {
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());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
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);
}
}
}
其中，在for循環(huán)中調(diào)用了ViewGroup.class的 measureChildWithMargins方法
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);
}
在ViewGroup.class的 measureChildWithMargins方法之中挨约，以及在FrameLayout.class的setMeasuredDimension方法之后味混，也都調(diào)用了ViewGroup.class的getChildMeasureSpec方法：
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;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
for循環(huán)之后，調(diào)用了View.class的setMeasuredDimension方法烫罩，以及setMeasuredDimensionRaw方法：
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;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
- onMeasure方法傳入的參數(shù)widthMeasureSpec惜傲，heightMeasureSpec就是當前容器的測量規(guī)格。
- for循環(huán)調(diào)用measureChildWithMargins贝攒，而這個方法就是測量FrameLayout中子控件的寬高，其中的getChildMeasureSpec方法就是獲取子控件的測量規(guī)格时甚。
getChildMeasureSpec(int spec, int padding, int childDimension)這個方法有三個參數(shù)：
spec 表示父容器的測量規(guī)格隘弊，
padding 就不說了，
childDimension 子控件的布局參數(shù)（MarginLayoutParams）對應的尺寸荒适。
方法中根據(jù)父容器的不同mode模式梨熙，給子控件的resultSize resultMode賦值。
最后通過MeasureSpec.makeMeasureSpec(resultSize, resultMode)方法將其打包刀诬。
- for循環(huán)不是把子View都測量了個遍么咽扇，然后調(diào)用了一個方法setMeasuredDimension，這個方法是設定父控件自身的寬高的，因為父控件的寬高可能是和子控件的寬高也有關系的质欲。
階段總結(jié)：
View的MeasureSpec由父容器的MeasureSpec和自身的LayoutParams決定
- 子View長寬為固定树埠，則無視父類模式，肯定為EXACTLY模式嘶伟，且長寬都為childSize
- 子View長寬為match_parent怎憋，則跟隨父類的模式，且長寬都為parentSize九昧，UNSPECIFIED模式為0
- 子View長寬為wrap_content绊袋，則模式都為AT_MOST，且長寬都為暫定為parentSize
【注：parentSize都是父控件的剩余大小铸鹰，減去padding之類的長度】
ViewGroup
->measure
->onMeasure【通過measureChildWithMargins方法測量子控件的寬高癌别，調(diào)用子控件的measure】
->setMeasureDimension
->setMeasureDimensionRaw【保存自己的寬高】
View
->measure
->onMeasure
->setMeasureDimension() 【getDefaultSize() 】
->setMeasureDimensionRaw
注意View.class的setMeasureDimension
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
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;
//子控件為wrap_content時
case MeasureSpec.AT_MOST:
//子控件為match_parent時
case MeasureSpec.EXACTLY:
//都賦值為父控件的specSize
result = specSize;
break;
}
return result;
}
實戰(zhàn)技巧
自定義View時，如果不重寫onMeasure蹋笼，則match_parent和wrap_content的效果一樣规个。 此時需要視情況決定是否重寫onMeasure方法，另需注意姓建，當自定義類型為容器ViewGroup時诞仓，重寫onMeasure方法要測量子View寬高后再設置自身寬高，非ViewGroup的View則不需測量子View速兔。
2. 布局performLayout
先看ViewRootImpl.class源碼中的performLayout方法：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
//這個mView就是頂層View
final View host = mView;
if (host == null) {
return;
}
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
mInLayout = false;
}
看見 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());墅拭，追蹤一下View.class的layout方法：
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);
...
}
...
}
可以看到有一處調(diào)用setFrame方法，就是對mLeft涣狗，mTop谍婉，mBottom，mRight進行賦值镀钓，確定了這四個值穗熬，View的位置可以說就固定下來了。然后調(diào)用了onLayout(changed, l, t, r, b)丁溅，但是點進去發(fā)現(xiàn)View.class的onLayout方法是空方法唤蔗，也就是說是留給子類重寫的。如果我們自定義的是容器類型的，則需在onLayout里擺放子View的位置，如果我們自定義的是View趁窃，則無需重寫脚粟。
距離FrameLayout：
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
//遍歷子控件對子控件進行擺放，并調(diào)用子View的layout方法，形成遞歸操作
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);
}
}
}
階段總結(jié)
ViewGroup
->layout【確定自己的位置，4個點的位置】
->onLayout【負責子View的布局，自定義時需要重寫】View
->layout【確定自己的位置作煌，4個點的位置】
3. 繪制performDraw
先看ViewRootImpl.class源碼
private void performDraw() {
...
try {
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
} 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);
...
}
這時候掘殴，代碼追蹤draw方法來到了View.class，通過觀察注釋即可得知draw的繪制步驟
* 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)
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;
}
...
}
因為涉及到子View的繪制粟誓，我們看一下ViewGroup.class中對dispatchDraw的實現(xiàn)
@Override
protected void dispatchDraw(Canvas canvas) {
...
//遍歷子控件奏寨，通過drawChild繪制子控件
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
階段總結(jié)
ViewGroup
->繪制背景drawBackground(Canvas)
-> 繪制自己onDraw(Canvas)【自定義時需重寫】
-> 繪制子View dispatchDraw(Canvas)
-> 繪制前景，滾動條等裝飾 onDrawForeground(Canvas)View
->繪制背景drawBackground(Canvas)
-> 繪制自己onDraw(Canvas)【自定義時需重寫】
-> 繪制前景努酸，滾動條等裝飾 onDrawForeground(Canvas)
總結(jié)
自定義View時
ViewGroup
-> onMeasure
-> onLayout
-> onDraw【可選服爷，例如容器中裝載的都是系統(tǒng)控件，則不用重寫】View
-> onMeasure
-> onDraw【可選】
