View的工作原理之Measure過程原始碼學習(四)
上一篇文章,學習了ViewGroup和View的measure流程。文章最後講到,本文將會學習ViewGroup和普通View的onMeasure方法的工作。
因為ViewGroup是抽象類,它並沒有實現onMeasure方法,而是選擇讓它的子類實現。之前學習DecorView的measure過程時,說到DecorView的父類繼承自FrameLayout,FrameLayout又繼承自ViewGroup,並且重寫了onMeasure方法。那我們就來看一下FrameLayout的onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int count = getChildCount(); //... 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()); //... 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); } } }
從程式碼可以看到,child.measure方法的呼叫引數childWidthMeasureSpec和childHeightMeasureSpec的生成,是與父容器的MeasureSpec值和child的LayoutParams有關的。很明顯子元素的測量值,是去除了子元素本身的margin和padding值的。檢視LinearLayout和RelativeLayout的原始碼,可以發現它們的onMeasure方法中,呼叫child.measure方法的呼叫引數也是去除了子元素本身的margin和padding值的。由此,我們可以知道,在自定義ViewGroup的時候,我們需要在重寫的onMeasure方法中處理子元素的margin和padding值。注意到上述程式碼中子元素的childWidthMeasureSpec值可以是呼叫了getChildMeasureSpec獲得的(childHeightMeasureSpec同理)。下面我們看一下它的原始碼:
//VIewGroup.java 6602行
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);
}
從上面的getChildMeasureSpec方法原始碼(這裡我們依然不分析UNSPECIFIED模式),結合上面FrameLayout(或者LinearLayout/RelativeLayout)的onMeasure方法,可以知道,getChildMeasureSpec方法的三個引數是:父容器的測量規格值、子元素的LayoutParams中的margin和子元素的padding值的和、子元素的LayoutParams中的width(或者LayoutParams中的height)。
從原始碼可以總結出子元素MeasureSpec建立的規律,如下表。表格中,第一行表示父容器的測量模式,第一列表示子元素在父容器的約束下,子元素LayoutParams的值:
x:parentSpecMode y:childLayoutParams |
EXACTLY |
AT_MOST |
UNSPECIFIED |
dp/px |
EXACTLY childSize |
EXACTLY childSize |
EXACTLY childSize |
MATCH_PARENT |
EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
WRAP_CONTENT |
AT_MOST parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
其中:parentSize是值父容器的測量值的大小減去子元素margin和padding佔用的值。childSize就是子元素的LayoutParams中的width或者LayoutParams中的height的值。
下面我們看一下,ViewGroup給我們提供的一些關於Measure過程的方法。
//ViewGroup.java 6522行
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
//ViewGroup.java 6542行
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
ViewGroup能夠擁有子元素,所以它的measure過程除了測量自己的大小以外,還需要遍歷去呼叫所有子元素的measure方法。遍歷子元素經行測量這個過程可以通過呼叫measureChildren方法實現,也可以自己在自定義的ViewGroup的onMeasure方法中實現。
measureChildren方法很簡單,就是遍歷ViewGroup的子元素,再根據子元素的可見狀態呼叫measureChild方法。在measureChild方法中建立子元素的MeasureSpec,然後呼叫子元素的measure方法,如果子元素是ViewGroup那麼重複這個過程。如果子元素是View,那就回到了View的measure過程了。
//VIewGroup.java 6568行
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);
}
自定義ViewGroup的時候,可以自己實現對子元素的margin和padding的處理,也可以呼叫在這個方法。
普通View的onMeasure方法相對ViewGroup簡單些,根據父容器剩餘空間,處理普通View的margin和padding,如果有額外的需求,加上這些需求的處理計算出普通View的寬高,呼叫setMeasuredDimension設定普通View全域性寬高值即可。