眾所周知功舀，UI繪制三部曲是measure、layout、draw
本篇我們分析View#performLayout
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
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;
}
方法的第一個(gè)參數(shù)lp是頂層布局容器的布局屬性胖喳，后面兩個(gè)參數(shù)mWidth，mHeight是頂層布局容器的寬高贮竟，將頂層布局容器mView賦值給host丽焊，mView為DecorView，后面接著調(diào)用host.layout()方法咕别，并傳入自己的測(cè)量寬高技健，繼續(xù)跟進(jìn)View#layout
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
// 注釋1
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 注釋2
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
final boolean wasLayoutValid = isLayoutValid();
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if (!wasLayoutValid && isFocused()) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
if (canTakeFocus()) {
// We have a robust focus, so parents should no longer be wanting focus.
clearParentsWantFocus();
} else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) {
// This is a weird case. Most-likely the user, rather than ViewRootImpl, called
// layout. In this case, there's no guarantee that parent layouts will be evaluated
// and thus the safest action is to clear focus here.
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
clearParentsWantFocus();
} else if (!hasParentWantsFocus()) {
// original requestFocus was likely on this view directly, so just clear focus
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
// otherwise, we let parents handle re-assigning focus during their layout passes.
} else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
View focused = findFocus();
if (focused != null) {
// Try to restore focus as close as possible to our starting focus.
if (!restoreDefaultFocus() && !hasParentWantsFocus()) {
// Give up and clear focus once we've reached the top-most parent which wants
// focus.
focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
}
}
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}
看到注釋1處，調(diào)用了setFrame(l, t, r, b)來(lái)確定自身的左上右下的位置惰拱，看到注釋2處雌贱，接著調(diào)用了onLayout方法，繼續(xù)跟進(jìn)View#onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
發(fā)現(xiàn)是一個(gè)空方法偿短，供子類實(shí)現(xiàn)欣孤，如果當(dāng)前View是一個(gè)容器，則需要重寫onLayout方法來(lái)確定子View的位置昔逗，如果是子View降传，則不需要重寫onLayout方法，我們以FrameLayout為例來(lái)看看它的onLayout方法纤子，F(xiàn)rameLayout#onLayout
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
繼續(xù)跟進(jìn)FrameLayout#layoutChildren
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);
}
}
}
可以看到這方法就是通過(guò)for循環(huán)去遍歷子View搬瑰，然后調(diào)用child.layout來(lái)遞歸對(duì)子View進(jìn)行位置擺放
我們來(lái)總結(jié)下，View的布局是調(diào)用view.layout確定自身的位置控硼，即mLeft,mTop,mRight,mBottom的值泽论，如果是ViewGroup類型，則需要調(diào)用onLayout確定子View的位置
