Android中View绘制过程（五） performlayout

小灰灰 2022-06-14 11:14 292阅读 0赞

performTraversals()中执行了performmeasure确定了每一个View的width，和height，就会调用performLayout,确定这些View在屏幕的位置。

layout方法也是由decorView自上而下的递归调用，知道调用到叶子节点。就将所有的View位置确定好了，在performlayout()中开始Decor调用layout，代码如下

host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

这里的host就是DecorView根据上一篇的测量，可以知道host.getMeasuredWidth(), host.getMeasuredHeight()分别就是屏幕的宽高。Layout在View和GroupView中差别不大。

在View中代码如下

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);
                    }
                }
            }
    
            mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
            mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
        }

注意到setFrame()这个调用就是当前VIew的left，top，right，bottom赋值，确定View的位置。例如decorView当时就确定是在左上角到右下角屏幕这么大了，之后调用onLayout

这个要到具体实现的ViewGroup中去看，各种layout，这里观察一下FramLayout是怎么做的。

@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();
    
            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);
                }
            }
        }

FrameLayout调用了layoutChildren遍历每一个子View并根据子View测量值来确定子View的位置，之后调用layout子View，这样完成由DecorView到叶子节点的所有View的遍历。