performLayout

最新推荐文章于 2024-10-16 17:13:23 发布

junjle

最新推荐文章于 2024-10-16 17:13:23 发布

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分类专栏： android 文章标签： performLayout

本文链接：https://blog.youkuaiyun.com/junjle/article/details/62428165

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本文详细剖析了Android中视图的布局流程，包括初始化后的DecorView如何调用performTraversals()，以及performLayout如何通过view的layout方法来确定子view的位置。文章深入探讨了TextView和不同ViewGroup如FrameLayout及LinearLayout在onLayout中的实现细节。

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初始化app后DecorView调用performTraversals()，执行完performMeasure后开始执行
performLayout(lp, mWidth, mHeight);
performLayout里面调用了 view的layout方法
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
先看viewGroup的处理，里面包含两个函数：
1. layout调用了view.layout
  super.layout(l, t, r, b)
2. onLayout
  abstract方法，需要子类实现
所有的方法最终都是调用了view的layout，接下来看view的layout方法
onLayout(changed, l, t, r, b)
调用用了onLayout，而onLayout为空方法。
到这里layout执行完了，没有实现功能，说明layout的计算是在具体的view里面实现的

接下来分别看一下TextView和FrameLayout的onLayout方法

TextView

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    super.onLayout(changed, left, top, right, bottom);
    if (mDeferScroll >= 0) {
        int curs = mDeferScroll;
        mDeferScroll = -1;
        bringPointIntoView(Math.min(curs, mText.length()));
    }
}

很简单，直接交给了super即view进行处理，原因很简单：对于单个view（没有子view）来说，只要按照父view传给的ltrb（left、top、right、bottom layout的四个参数）进行处理就好了，并不需要自己本身去计算

FrameLayout

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);
        }
    }
}

逻辑很清楚，遍历child，获得child的measureWidth和measureHeight和layoutparam，根据gravity计算childLeft和childTop，另外两个参数直接通过width和height计算获得。
最后调用child.layout进行处理
不同的viewgrouop由于自身不同的规则会在onLayout进行处理，这里的fragmeLayout比较简单，没有考虑同一个viewGroup之下的子view之间的互相影响，下面看一下LinearLayout

LinearLayout

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
首先根据orientation进行区分，没毛病
看一下 layoutHorizontal

void layoutHorizontal(int left, int top, int right, int bottom) {
    final boolean isLayoutRtl = isLayoutRtl();
    final int paddingTop = mPaddingTop;

    int childTop;
    int childLeft;
   
    // Where bottom of child should go
    final int height = bottom - top;
    int childBottom = height - mPaddingBottom; 
   
    // Space available for child
    int childSpace = height - paddingTop - mPaddingBottom;

    final int count = getVirtualChildCount();

    final int majorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
    final int minorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;

    final boolean baselineAligned = mBaselineAligned;

    final int[] maxAscent = mMaxAscent;
    final int[] maxDescent = mMaxDescent;

    final int layoutDirection = getLayoutDirection();
    switch (Gravity.getAbsoluteGravity(majorGravity, layoutDirection)) {
        case Gravity.RIGHT:
            // mTotalLength contains the padding already
            childLeft = mPaddingLeft + right - left - mTotalLength;
            break;

        case Gravity.CENTER_HORIZONTAL:
            // mTotalLength contains the padding already
            childLeft = mPaddingLeft + (right - left - mTotalLength) / 2;
            break;

        case Gravity.LEFT:
        default:
            childLeft = mPaddingLeft;
            break;
    }

    int start = 0;
    int dir = 1;
    //In case of RTL, start drawing from the last child.
    if (isLayoutRtl) {
        start = count - 1;
        dir = -1;
    }

    for (int i = 0; i < count; i++) {
        final int childIndex = start + dir * i;
        final View child = getVirtualChildAt(childIndex);
        if (child == null) {
            childLeft += measureNullChild(childIndex);
        } else if (child.getVisibility() != GONE) {
            final int childWidth = child.getMeasuredWidth();
            final int childHeight = child.getMeasuredHeight();
            int childBaseline = -1;

            final LinearLayout.LayoutParams lp =
                    (LinearLayout.LayoutParams) child.getLayoutParams();

            if (baselineAligned && lp.height != LayoutParams.MATCH_PARENT) {
                childBaseline = child.getBaseline();
            }
           
            int gravity = lp.gravity;
            if (gravity < 0) {
                gravity = minorGravity;
            }
           
            switch (gravity & Gravity.VERTICAL_GRAVITY_MASK) {
                case Gravity.TOP:
                    childTop = paddingTop + lp.topMargin;
                    if (childBaseline != -1) {
                        childTop += maxAscent[INDEX_TOP] - childBaseline;
                    }
                    break;

                case Gravity.CENTER_VERTICAL:
                    // Removed support for baseline alignment when layout_gravity or
                    // gravity == center_vertical. See bug #1038483.
                    // Keep the code around if we need to re-enable this feature
                    // if (childBaseline != -1) {
                    //     // Align baselines vertically only if the child is smaller than us
                    //     if (childSpace - childHeight > 0) {
                    //         childTop = paddingTop + (childSpace / 2) - childBaseline;
                    //     } else {
                    //         childTop = paddingTop + (childSpace - childHeight) / 2;
                    //     }
                    // } else {
                    childTop = paddingTop + ((childSpace - childHeight) / 2)
                            + lp.topMargin - lp.bottomMargin;
                    break;

                case Gravity.BOTTOM:
                    childTop = childBottom - childHeight - lp.bottomMargin;
                    if (childBaseline != -1) {
                        int descent = child.getMeasuredHeight() - childBaseline;
                        childTop -= (maxDescent[INDEX_BOTTOM] - descent);
                    }
                    break;
                default:
                    childTop = paddingTop;
                    break;
            }

            if (hasDividerBeforeChildAt(childIndex)) {
                childLeft += mDividerWidth;
            }

            childLeft += lp.leftMargin;
            setChildFrame(child, childLeft + getLocationOffset(child), childTop,
                    childWidth, childHeight);
            childLeft += childWidth + lp.rightMargin +
                    getNextLocationOffset(child);

            i += getChildrenSkipCount(child, childIndex);
        }
    }
}

代码比较长，逻辑挺简单，拆开看
1. 获取count（子view个数），childLeft（开始显示子view的left位置）
2. 遍历child
  1. 和上面一样获取child的measureHeight和measureWidth，说明layout是在measure的基础上的
  2. 累加childLeft，水平的linearlayout子view从左到右依次显示
  3. 最终调用child.layout

综上layout过程分析完毕，总结一下
1. 相对于measure，layout过程相对简单，view不需要特殊处理，viewgroup要特殊处理
2. viewGroup中必须实现onLayout方法，里面根据父view的layoutGravity计算子view的显示区域和开始的显示位置
3. 遍历view，根据子view的layoutHeigth和layoutWidth以及gravity计算子view位置
4. 最后说一下getWidth、getHeigth和getMeasureHeight、getMeasureWidth方法
  1. public final int getWidth() {
    return mRight - mLeft;
    }
  2. public final int getHeight() {
    return mBottom - mTop;
    }
  3. public final int getMeasuredWidth() {
    return mMeasuredWidth & MEASURED_SIZE_MASK;
    }
  4. public final int getMeasuredHeight() {
    return mMeasuredHeight & MEASURED_SIZE_MASK;
    }
  5. 由上可知getWidth、getHeigth是layout后计算出来的实际值，而getMeasureHeight、getMeasureWidth则是measure计算后获取的理论值