之前文章了解到ViewRootImpl的performTraversals()引導(dǎo)了View的測量苏章、布局寂嘉、繪制的流程今天我們就從performLayout()方法來分析View的布局流程

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    mLayoutRequested = false;
    mScrollMayChange = true;
    mInLayout = true;
    final View host = mView;
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
    try {
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
   ...省略部分代碼
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

前文中我們已經(jīng)知道m(xù)View就是DecorView奏瞬，這里調(diào)用 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());就是調(diào)用decorView的layout方法

@Override
public final void layout(int l, int t, int r, int b) {
    if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
        if (mTransition != null) {
            mTransition.layoutChange(this);
        }
        super.layout(l, t, r, b);
    } else {
        // record the fact that we noop'd it; request layout when transition finishes
        mLayoutCalledWhileSuppressed = true;
    }
}

再看看super.layout的代碼

@SuppressWarnings({"unchecked"})
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;
    //setFrame 記錄了View的位置以及是否變動如果變動則保存最新的位置信息如果沒有變動則跳過
    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);
            }
        }
    }
    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

這里主要牽扯到兩個方法一個setFrame（）該方法主要是保存布局走過這一步那么該View的布局已經(jīng)結(jié)束了，而onLayout則是父容器對子View的一個遍歷布局作用該方法View是沒有實現(xiàn)的主要由ViewGroup實現(xiàn)

protected boolean setFrame(int left, int top, int right, int bottom) {
    boolean changed = false;
    if (DBG) {
        Log.d("View", this + " View.setFrame(" + left + "," + top + ","
                + right + "," + bottom + ")");
    }
    //判斷坐標(biāo)位置是否變動如果變動重新保存坐標(biāo)并更新
    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;
        if (mForegroundInfo != null) {
            mForegroundInfo.mBoundsChanged = true;
        }
        notifySubtreeAccessibilityStateChangedIfNeeded();
    }
    return changed;
}

這步保存了View的頂點位置泉孩，也就是從這步開始我們getHeight硼端，getWidth就可以獲取到具體的值了。onLayout方法View里面并沒有實現(xiàn)所以我們直接看看onLayout中是怎么實現(xiàn)的

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}

void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
    final int count = getChildCount();
    //獲取padding值
    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();
    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();
    
    //遍歷布局子元素
    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;
            //根據(jù)布局中設(shè)置的Gravity屬性來設(shè)置布局
            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;
            }
            //最后確定坐標(biāo)點對子元素進行布局
            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

由源碼看出寓搬，onLayout方法內(nèi)部直接調(diào)用了layoutChildren方法珍昨，而layoutChildren則是具體的實現(xiàn)。 FrameLayout會遍歷子元素并調(diào)用子元素的layout方法來對子View進行遍歷布局句喷。到目前為止镣典，View的布局流程就已經(jīng)全部分析完了⊥偾恚可以看出兄春，布局流程的邏輯相比測量流程來說，簡單許多锡溯，獲取一個View的測量寬高是比較復(fù)雜的赶舆，而布局流程則是根據(jù)已經(jīng)獲得的測量寬高進而確定一個View的四個位置參數(shù)。