View的绘制流程五、layout

之前文章了解到ViewRootImpl的performTraversals()引导了View的测量、布局、绘制的流程今天我们就从performLayout()方法来分析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());
   ...省略部分代码
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

前文中我们已经知道mView就是DecorView，这里调用 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());就是调用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的代码

@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 记录了View的位置以及是否变动如果变动则保存最新的位置信息如果没有变动则跳过
    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;
}

这里主要牵扯到两个方法一个setFrame（）该方法主要是保存布局走过这一步那么该View的布局已经结束了，而onLayout则是父容器对子View的一个遍历布局作用该方法View是没有实现的主要由ViewGroup实现

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 + ")");
    }
    //判断坐标位置是否变动如果变动重新保存坐标并更新
    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;
}

这步保存了View的顶点位置，也就是从这步开始我们getHeight，getWidth就可以获取到具体的值了。onLayout方法View里面并没有实现所以我们直接看看onLayout中是怎么实现的

@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();
    //获取padding值
    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();
    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();
    
    //遍历布局子元素
    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;
            //根据布局中设置的Gravity属性来设置布局
            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;
            }
            //最后确定坐标点对子元素进行布局
            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

由源码看出，onLayout方法内部直接调用了layoutChildren方法，而layoutChildren则是具体的实现。 FrameLayout会遍历子元素并调用子元素的layout方法来对子View进行遍历布局。到目前为止，View的布局流程就已经全部分析完了。可以看出，布局流程的逻辑相比测量流程来说，简单许多，获取一个View的测量宽高是比较复杂的，而布局流程则是根据已经获得的测量宽高进而确定一个View的四个位置参数。