Android View 原始碼解析(四) - LinearLayout原始碼分析
摘要：
LinearLayout 原始碼分析
measure過程
主要過程
根據佈局方向選擇measure過程分支
初始化相關變數
對View進行第一次測量
mTotalLength的再次測量
二次測量部分View和對為測量的子View進行測量
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LinearLayout 原始碼分析
measure過程
主要過程
- 根據佈局方向選擇measure過程分支
- 初始化相關變數
- 對View進行第一次測量
- mTotalLength的再次測量
- 二次測量部分View和對為測量的子View進行測量
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//判斷佈局方向
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
measureVertical和measureHorizontal只是佈局方向上的區別 以下主要分析measureVertical方法
初始化相關變數
//mTotalLength是記錄內部使用的高度也就是子View的高度和 而不是LinearLayout的高度 mTotalLength = 0; //子檢視的最大寬度(不包括layout_weight>0的子View) int maxWidth = 0; int childState = 0; int alternativeMaxWidth = 0; //子檢視的最大寬度(僅包含layout_weight>0的子View) int weightedMaxWidth = 0; //子檢視是否均為fillParent 用於判斷是否需要重新計算 boolean allFillParent = true; //權重值的總和 float totalWeight = 0; //子View的數量(統一級別下) final int count = getVirtualChildCount(); //高度寬度模式 final int widthMode = MeasureSpec.getMode(widthMeasureSpec); final int heightMode = MeasureSpec.getMode(heightMeasureSpec); //子View的寬度是否需要由父View決定 boolean matchWidth = false; boolean skippedMeasure = false; //第幾個子View的baseLine作為LinearLayout的基準線 final int baselineChildIndex = mBaselineAlignedChildIndex; //mUseLargestChild為是否使用最大子元素的尺寸作為標準再次測量 final boolean useLargestChild = mUseLargestChild; //子View中最高高度 int largestChildHeight = Integer.MIN_VALUE;
第一次測量
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
// 測量為null的子檢視的高度
// measureNullChild() 暫時返回 0 便於擴充套件
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
//Visibility為Gone的時候跳過該View
// getChildrenSkipCount()方法同樣返回0 便於擴充套件
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
//根據showDivider的值(通過hasDividerBeforeChildAt()) 來決定當前子View是否需要新增分割線的高度
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
//會將子view的LayoutParams強轉為父View的LayoutParams型別
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)child.getLayoutParams();
totalWeight += lp.weight;
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
// 滿足該條件的話 不需要現在計算該子檢視的高度 測量工作會在之後進行
// 若子View的height=0 且weight> 0 則說明該View希望使用的是LinearLayout的剩餘空間
// LinearLayout是EXACTLY模式的說明LinearLayout高度已經確定 不需要依賴子View的測量結果來計算自己 就無需測量該子View
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
skippedMeasure = true;
} else {
//測量子View
int oldHeight = Integer.MIN_VALUE;
//當前View的height=0 且weight> 0 則說明該LinearLayout的高度需要靠子View測量(不需要的在上面分支處理了)
//將子View的高度設為-1 防止子View高度為0
if (lp.height == 0 && lp.weight > 0) {
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
//呼叫子View的measureChildWithMargins() 對子View進行測量
//第四個引數表示當前已使用的寬度因為是豎直模式 所以為0
//最後一個引數表示已使用的高度 如果之前的子View或者當前的View有weight屬性 則當前子檢視使用 LinearLayout 的所有高度 已使用的高度為0
measureChildBeforeLayout(child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
//測量完成後 重置子View高度
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
// 比較child測量前後總高度 取較大值
///getNextLocationOffset() 返回0 便於擴充套件
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
// 設定最高子檢視大小
if (useLargestChild) {
largestChildHeight = Math.max(childHeight, largestChildHeight);
}
}
// mBaselineChildTop 表示指定的 baseline 的子檢視的頂部高度
if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
mBaselineChildTop = mTotalLength;
}
// 設定為 baseline 的子檢視的前面不允許設定 weiget 屬性
if (i < baselineChildIndex && lp.weight > 0) {
throw new RuntimeException("A child of LinearLayout with index "
+ "less than mBaselineAlignedChildIndex has weight > 0, which "
+ "won't work.Either remove the weight, or don't set "
+ "mBaselineAlignedChildIndex.");
}
// 寬度測量相關
boolean matchWidthLocally = false;
//當LinearLayout非EXACTLY模式 並且自View為MATCH_PARENT時
//設定matchWidth和matchWidthLocally為true
//該子View佔據LinearLayout水平方向上所有空間
if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
matchWidth = true;
matchWidthLocally = true;
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
//對一堆變數賦值
maxWidth = Math.max(maxWidth, measuredWidth);
childState = combineMeasuredStates(childState, child.getMeasuredState());
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
if (lp.weight > 0) {
weightedMaxWidth = Math.max(weightedMaxWidth,
matchWidthLocally ? margin : measuredWidth);
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
}
i += getChildrenSkipCount(child, i);
}
二次測量mTotalLength
//根據hasDividerBeforeChildAt得到showDivider的值是否為end 來判斷是否需要加上divider的高度
if (mTotalLength > 0 && hasDividerBeforeChildAt(count))
mTotalLength += mDividerHeight;
}
//如果高度測量模式為AT_MOST或者UNSPECIFIED 則進行二次測量 且設定了measureWithLargestChild
if (useLargestChild && (heightMode == MeasureSpec.AT_MOST ||
heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
child.getLayoutParams();
// 計算所有子View的高度之和
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
}
就是需要useLargestChild
而 mUseLargestChild = a.getBoolean(R.styleable.LinearLayout_measureWithLargestChild, false);
就是說僅在LinearLayout的measureWithLargestChild屬性設定為True時(預設為false)才可能出現某個child被二次測量
例項如下
mTotalLength的二次測量
二次測量部分View和對為測量的子View進行測量
//加上padding的值
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
//minHeight和當前使用的高度比較取較大值
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
//根據heightMeasureSpec協助計算heightSizeAndState的大小
//resolveSizeAndState方法之後會分析
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
// Either expand children with weight to take up available space or
// shrink them if they extend beyond our current bounds. If we skipped
// measurement on any children, we need to measure them now.
//delta為額外的空間 及LinearLayout中未被分配的空間(可以為負)
int delta = heightSize - mTotalLength;
if (skippedMeasure || delta != 0 && totalWeight > 0.0f) {
//skippedMeasure為第一次測量下對跳過測量的子View設定的
//weightSum為權重和 如果設定了總權重則使用我們所設定的如果沒有則使用子View的weight和
float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
//測量什麼的
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child.getVisibility() == View.GONE) {
continue;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
// Child said it could absorb extra space -- give him his share
//計算weight屬性分配的大小
int share = (int) (childExtra * delta / weightSum);
//權重和減去已經分配權重
weightSum -= childExtra;
//剩餘高度減去分配的高度
delta -= share;
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight +
lp.leftMargin + lp.rightMargin, lp.width);
// TODO: Use a field like lp.isMeasured to figure out if this
// child has been previously measured
if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
//子檢視已經被測量過
//非EXACTLY view需要加上share
int childHeight = child.getMeasuredHeight() + share;
if (childHeight < 0) {
childHeight = 0;
}
//重新測量View
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
} else {
//如果當前是EXACTLY模式 說明沒有被測量 需要進行測量
//子檢視首次被測量
//EXACTLY模式下 將weight佔比的高度分配給子View
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
MeasureSpec.EXACTLY));
}
// Child may now not fit in vertical dimension.
childState = combineMeasuredStates(childState, child.getMeasuredState()
& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
}
//處理子檢視寬度
final int margin =lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
lp.width == LayoutParams.MATCH_PARENT;
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
// TODO: Should we recompute the heightSpec based on the new total length?
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
weightedMaxWidth);
// We have no limit, so make all weighted views as tall as the largest child.
// Children will have already been measured once.
if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
//使用最大子檢視高度測量
child.measure(
MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(largestChildHeight,
MeasureSpec.EXACTLY));
}
}
}
}
if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
maxWidth = alternativeMaxWidth;
}
maxWidth += mPaddingLeft + mPaddingRight;
// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
if (matchWidth) {
forceUniformWidth(count, heightMeasureSpec);
}
resolveSizeAndState方法 定義在View中
/**
* Utility to reconcile a desired size and state, with constraints imposed
* by a MeasureSpec. Will take the desired size, unless a different size
* is imposed by the constraints. The returned value is a compound integer,
* with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
* optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the
* resulting size is smaller than the size the view wants to be.
*
* @param size How big the view wants to be.
* @param measureSpec Constraints imposed by the parent.
* @param childMeasuredState Size information bit mask for the view's
*children.
* @return Size information bit mask as defined by
*{@link #MEASURED_SIZE_MASK} and
*{@link #MEASURED_STATE_TOO_SMALL}.
*/
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) {
case MeasureSpec.AT_MOST:
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
delta為負的相關解析
相關程式碼及效果如下
負delta相關解析
根據之前的measure流程分析一下
- 相關變數初始化
- 第一次測量 兩個子TextView都會被測量 TextView1.height = TextView1.height = 500dp 則mToatalLength為1000dp
- mToatalLength再次測量跳過
- 計算delta delta = heightSize - mTotalLength 根據resolveSizeAndState方法 父LinearLayout是EXACTLY模式 所以最終heightSize為500dp delta = -500dp
- 根據weight分配剩餘空間 TextView1.height = 500 + 1 / 5 * (- 500) = 400 dp
TextView2.height = 500 + 4 / 5 * (- 500) = 100 dp
layout過程
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
我們可以看出 同樣是分成水平和豎直兩個方向的 同樣分析豎直 方向下的layout過程
/**
* Position the children during a layout pass if the orientation of this
* LinearLayout is set to {@link #VERTICAL}.
*
* @see #getOrientation()
* @see #setOrientation(int)
* @see #onLayout(boolean, int, int, int, int)
* @param left
* @param top
* @param right
* @param bottom
*/
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
//父View預設子View的寬度
final int width = right - left;
//子View的右側預設位置
int childRight = width - mPaddingRight;
// 子View的可用空間大小
int childSpace = width - paddingLeft - mPaddingRight;
//子View的個數
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
//根據LinearLayout設定的對其方式 設定第一個子View的Top值
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
//遍歷各個子View
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//LinearLayout中子View的寬和高有measure過程決定
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//獲取子View的LayoutParams
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
//根據子View的對其方式設定Left值
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
//如果有分割線 新增分割線的高度
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
//子View的top修改
childTop += lp.topMargin;
//用setChildFrame()方法設定子控制元件控制元件的在父控制元件上的座標軸
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
draw 原始碼分析
protected void onDraw(Canvas canvas) {
if (mDivider == null) {
return;
}
if (mOrientation == VERTICAL) {
drawDividersVertical(canvas);
} else {
drawDividersHorizontal(canvas);
}
}
同樣主要分析垂直方向的處理
void drawDividersVertical(Canvas canvas) {
final int count = getVirtualChildCount();
//根據計算好的座標繪製對應的子View
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child != null && child.getVisibility() != GONE) {
if (hasDividerBeforeChildAt(i)) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int top = child.getTop() - lp.topMargin - mDividerHeight;
drawHorizontalDivider(canvas, top);
}
}
}
//繪製分割線
if (hasDividerBeforeChildAt(count)) {
final View child = getLastNonGoneChild();
int bottom = 0;
if (child == null) {
bottom = getHeight() - getPaddingBottom() - mDividerHeight;
} else {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
bottom = child.getBottom() + lp.bottomMargin;
}
drawHorizontalDivider(canvas, bottom);
}
}
void drawHorizontalDivider(Canvas canvas, int top) {
mDivider.setBounds(getPaddingLeft() + mDividerPadding, top,
getWidth() - getPaddingRight() - mDividerPadding, top + mDividerHeight);
mDivider.draw(canvas);
}
以上
【附錄】
資料圖