View繪制流程
一圖勝千言
DecorView添加至窗口的流程
這里說明下Acitivity的onResume是在handleResumeActivity之前執行的，所以在onResume中獲取View的寬高為0。
實際上通用的繪制流程應該是從WindowManager#addView開始。
View measure()分析
首先View的Measure方法聲明為final，子類無法繼承，故關于View多態的實現就只能在onMeasure方法中實現
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//1.判斷View本身LayoutMode是否是視覺邊界布局，僅用于檢測ViewGroup
boolean optical = isLayoutModeOptical(this);
//2.判斷父容器是否是視覺布局邊界，是則重新調整測量規格（mParent可能是ViewRootImpl）
if (optical != isLayoutModeOptical(mParent)) {
//View的background需要設置.9背景圖才會生效，否則insets的left、right全為0
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);
}
// Suppress sign extension for the low bytes,禁止低位進行符號擴展
//3.根據當前測量規格生成一個與之對應的key（相同的測量規格產生的測量值肯定一樣的），供后續索引緩存測量值
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
//4.初始化測量緩存稀疏數組，該數組可自行擴容，只是初始化為2
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
//5.如果強制layout（eg.view.forceLayout（））或者本次測量規格與上次測量規格不同，進入該if語句
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
//6.如果需要強制layout則進行重新測量，反之則從緩存中查詢是否有與目前測量規格對應的key，
//如果有則取用緩存中的測量值，反之則執行onMeasure方法重新測量,未索引到返回一個負數
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
//7.targetSDK小于Kitkat版本（API20），sIgnoreMeasureCache則為true
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()");
}
//8.標志位賦值，表明需要在layout方法中執行onlayout方法
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
//9.緩存本次測量規格，及測量寬高
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
/**
* Return true if o is a ViewGroup that is laying out using optical bounds.
* @hide
*/
public static boolean isLayoutModeOptical(Object o) {
return o instanceof ViewGroup && ((ViewGroup) o).isLayoutModeOptical();
}
關于LayoutMode請參考Android LayoutMode需要翻墻
關于MeasureSpec請參考Android MeasureSpec
小結:
View的measure方法只是一個測量優化者，主要做了2級測量優化：
1.如果flag不為強制Layout或者與上次測量規格相比未改變，那么將不會進行重新測量（執行onMeasure方法），直接使用上次的測量值；
2.如果滿足非強制測量條件，即前后測量規格發生變化，則會先根據目前測量規格生成的key索引緩存數據，索引到就無需進行重新測量;如果targetSDK小于API 20則二級測量優化無效，依舊會重新測量，不會采用緩存測量值。
3.View#requestLayout 只會讓該View及其父容器重新走一遍，如果該View是ViewGroup，其里面的子View測量優化還是依舊有效的
View onMeasure()分析
View的onMeasure方法比較簡單，目的是將測量值賦給mMeasuredWidth和mMeasuredHeight
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) {
//未指定則使用最小值
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//解析測量規格獲得寬、高，這就是為何View無論你填match_parent還是wrap_content，它始終是填滿父容器的原因
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
//表明測量尺寸已經設置，與measure方法中的first clears the measured dimension flag相呼應
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
小結:
View的onMeasure方法才是真正的測量者，它根據測量規格以及其他條件來決定自己最終的測量大小。
需要注意，自定義View重寫該方法時，務必保證調用setMeasuredDimension()將測量寬、高存起來，measure方法分析中有提到，如果不調用該方法將會拋出非法狀態異常。
ViewGroup onMeasure分析
ViewGroup繼承至View實現了ViewParent接口，是一個抽象類。前面也提到View的measure方法不能被繼承，所以ViewGroup沒有measure方法。查看源碼發現它并沒有重寫onMeasure方法，那就去看看其實現類是否有重寫，就看最簡單的實現類FrameLayout。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
//判斷是否要再次測量layout_width/height屬性為match_parent的child
//即FrameLayout的layout_width/height屬性為wrap_content，則會再次測量屬性為match_parent的child
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
//首次遍歷測量子控件
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//GONE類型child不測量，這就是為何GONE不會占用位置，因為沒有測量
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
..................
}
}
..................
//保存自身的測量寬高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
//再次測量layout_width/height屬性為match_parent的child,部分View三次執行onMeasure的原因
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
..........
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
進入MeasureChildWithMargins函數
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//根據ViewGroup自身的測量規格生成child的測量規格
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);
//調用child的measure方法，并將child的測量規格傳遞給child
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
進入getChildMeasureSpec方法，看看到底是如何測量child的規格的
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//獲取父容器的測量模式、測量大小
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//計算出父容器允許child的最大尺寸
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// 父容器測量模式為精確模式
case MeasureSpec.EXACTLY:
//如果child的layout_width/height為具體的數值eg.20dp，那么child的測量規格就為大小20dp，模式為精確模式
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
//如果child的layout_width/height為MATCH_PARENT，那么child的測量規格就為大小父容器允許的最大值，模式為精確模式
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
}
//如果child的layout_width/height為WRAP_CONTENT，那么child的測量規格就為大小父容器允許的最大值，模式為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;
小結：
ViewGroup的測量主要是根據其自身測量規格，結合child的LayoutParams進行判斷分析，生成一個child的測量規格信息，傳遞給child的measure方法。所謂的測量規格即是一個建議，建議view的寬、高應該為多少，至于采取與否完全取決于view自己（嗯應該是取決于程序員O(∩_∩)O！）。
另ViewGroup提供了三個測量方法供我們使用，在實際運用中可以偷偷懶，不用自己去實現測量邏輯：
- measureChildWithMargins 測量單個child，margin參數有效
- measureChild 測量單個child，margin參數無效
- measureChildren 測量所有child內部調用measureChild
延伸閱讀：
【View為什么會至少執行2次onMeasure、onLayout】暫未發布
