Android 布局浅析

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 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);
        }
 
        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 bit
            int 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 position
            invalidate(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 child
                invalidateParentCaches();
            }
 
            // 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