Android布局绘制流程:

从根节点 noodview 递归调用每一级子view 的 Mesure 进行测量

然后

从根节点 noodview 递归调用每一级子view 的 layout 方法,把测量过程得出的子view的位置和size进行传递给子view ,然后子view保存参数

测量是动态化,是需要根view和子view

首先nood view 根节点,在自己的measure 中,根据xml布局中对子view 的 要求规范 和 自己可用空间,得出子view 具体尺寸要求

next:

noodview(父View) 根节点在自己的onMeasure 中 根据xml定义的对子view 的要求,和自己可用空间,得出对子view 的具体尺寸要求

子view在自己的onMeasure 根据noodView 的要求以及特性计算出自己的期望尺寸

如果是viewGroup,还会调用每个子View的measure 进行测量 计算(父View一般会和子View之间宽度相同 或者相加叠加)

如果自定义子view 宽高 和 父View有冲突,则父View会修正子View大小,例如ConstraintLayout 和 LinearLayout 表现出很大差异

父View在子View计算出期望尺寸时后,得出子View的实际尺寸和位置

子View在self的layout中,将父view传来的实际尺寸和位置进行保存

 

   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;boolean changed = isLayoutModeOptical(mParent) ?setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {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 focusclearFocusInternal(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);}notifyAppearedOrDisappearedForContentCaptureIfNeeded(true);}

 

   protected boolean setFrame(int left, int top, int right, int bottom) {boolean changed = false;if (DBG) {Log.d(VIEW_LOG_TAG, this + " View.setFrame(" + left + "," + top + ","+ right + "," + bottom + ")");}if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {changed = true;// Remember our drawn bitint drawn = mPrivateFlags & PFLAG_DRAWN;int oldWidth = mRight - mLeft;int oldHeight = mBottom - mTop;int newWidth = right - left;int newHeight = bottom - top;boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);// Invalidate our old positioninvalidate(sizeChanged);mLeft = left;mTop = top;mRight = right;mBottom = bottom;mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);mPrivateFlags |= PFLAG_HAS_BOUNDS;if (sizeChanged) {sizeChange(newWidth, newHeight, oldWidth, oldHeight);}if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {// If we are visible, force the DRAWN bit to on so that// this invalidate will go through (at least to our parent).// This is because someone may have invalidated this view// before this call to setFrame came in, thereby clearing// the DRAWN bit.mPrivateFlags |= PFLAG_DRAWN;invalidate(sizeChanged);// parent display list may need to be recreated based on a change in the bounds// of any childinvalidateParentCaches();}// Reset drawn bit to original value (invalidate turns it off)mPrivateFlags |= drawn;mBackgroundSizeChanged = true;mDefaultFocusHighlightSizeChanged = true;if (mForegroundInfo != null) {mForegroundInfo.mBoundsChanged = true;}notifySubtreeAccessibilityStateChangedIfNeeded();}return changed;}

 在setFrame时 , left,top,right,bottom 在layout的时候就已经保存下来了,重写layout 可以改变 这些参数

 如果是ViewGroup,在onLayout中调用每个子View 的layout,并把子view的位置传给子view