上两篇文章主要讲述了view的measure过程,主要分析了xml文件中控件的height和width设置成不同值的时候,经过测量之后,如何计算出控件的真实高度。所以也就验证了我们经常所说的measure过程就是把match_parent等值转化成在具体设备上的具体的值。
本文主要分析一下layout的过程,同样我们以LinearLayout的layout过程为例。
在ViewRoot的performTraversals方法中首先是measure过程,然后接着是layout,layout开始也是从host.layout方法开始的。
host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);
host是一个viewGroup,ViewGroup继承自view,然而view中的layout是final的,所以这里调用的仍然是view的layout方法,然后调用了onLayout方法,所以我们从linearLayout的onLayout方法中开始分析
public final void layout(int l, int t, int r, int b) {
boolean changed = setFrame(l, t, r, b);
if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_LAYOUT);
}
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~LAYOUT_REQUIRED;
}
mPrivateFlags &= ~FORCE_LAYOUT;
}
linearLayout的layout过程如下:
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical();
} else {
layoutHorizontal();
}
}
void layoutVertical() {
final int paddingLeft = mPaddingLeft;
int childTop = mPaddingTop;
int childLeft;
final int width = mRight - mLeft;
int childRight = width - mPaddingRight;
// Space available for child 实际上可用的宽度空间
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.HORIZONTAL_GRAVITY_MASK;
//根据LinearLayotu的gravity的值计算childTop的位置
if (majorGravity != Gravity.TOP) {
switch (majorGravity) {
case Gravity.BOTTOM:
childTop = mBottom - mTop + mPaddingTop - mTotalLength;
break;
case Gravity.CENTER_VERTICAL:
childTop += ((mBottom - mTop) - mTotalLength) / 2;
break;
}
}
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//拿到子View的LayoutParams
final LinearLayout.LayoutParams lp =(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
//计算子View在水平方向的childLeft
switch (gravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.LEFT:
childLeft = paddingLeft + lp.leftMargin;
break;
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
default:
childLeft = paddingLeft;
break;
}
childTop += lp.topMargin;
// 调用child.layout方法设置child的布局位置
setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
首先根据mRight - mLeft获得width,根据width - paddingLeft - mPaddingRight获得childSpace,也就是实际上我们可以用的宽度。然后根据LinearLayotu的gravity的值计算childTop的位置:
- 为Gravity.BOTTOM:
childTop = mBottom - mTop + mPaddingTop - mTotalLength;
在这里当测量出来的mTotalLength总高度足够大的时候会出现childTop是负数的情况,也就是child的上面一部分会显示不全的情况,对应的图就像是下面的情况,图中上面的蓝色框内的一部分是不会显示的。
- 为Gravity.CENTER_VERTICAL:
childTop += ((mBottom - mTop) - mTotalLength) / 2;
对应的图片如下所示:
接着循环遍历子view,如果子view为可见的,则计算出子view在水平方向上的childLeft,这里我们讨论的是垂直方向的布局,这里为什么会出现还要计算view在水平方向上的left呢,因为就算是垂直布局,每个子view他们也有marginleft和paddingLeft值。最后调用setChildFrame设置子view的布局位置,我们可以进入此方法看一下:
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
public final void layout(int l, int t, int r, int b) {
boolean changed = setFrame(l, t, r, b);
if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_LAYOUT);
}
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~LAYOUT_REQUIRED;
}
mPrivateFlags &= ~FORCE_LAYOUT;
}
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
int drawn = mPrivateFlags & DRAWN;
invalidate();
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mPrivateFlags |= HAS_BOUNDS;
int newWidth = right - left;
int newHeight = bottom - top;
if (newWidth != oldWidth || newHeight != oldHeight) {
onSizeChanged(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE) {
mPrivateFlags |= DRAWN;
invalidate();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
}
return changed;
}
给子View布局最终调用的是setFrame方法,四个参数分别代表view在其父视图中的位置。首先判断如果这四个值和之前已经有的值是否相等,如果有一个不相等,就代表要重新布局,此时如果子view现在的高度和宽度和之前的宽高是不相同的,那么就必须要调用onSizeChanged方法,通知程序view的大小发生了变化,最后如果view是VISIBLE的,那么就要执行invalidate操作。
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