图+源码，读懂View的Layout方法

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
                           int desiredWindowHeight) {
    ...
    try {
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());//1
        ...
        if (numViewsRequestingLayout > 0) {
            ...
            if (validLayoutRequesters != null) {
                ...
                host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());//2
                ...
            }
        }
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}
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/**
 * Assign a size and position to a view and all of its
 * descendants
 *
 * <p>This is the second phase of the layout mechanism.
 * (The first is measuring). In this phase, each parent calls
 * layout on all of its children to position them.
 * This is typically done using the child measurements
 * that were stored in the measure pass().</p>
 *
 * <p>Derived classes should not override this method.
 * Derived classes with children should override
 * onLayout. In that method, they should
 * call layout on each of their children.</p>
 *
 * @param l Left position, relative to parent
 * @param t Top position, relative to parent
 * @param r Right position, relative to parent
 * @param b Bottom position, relative to parent
 */
@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;
    boolean changed = isLayoutModeOptical(mParent) ?
        setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);//1
    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        onLayout(changed, l, t, r, b);//2
        ...
    }
    ...
}
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/**
 * Assign a size and position to this view.
 *
 * This is called from layout.
 *
 * @param ...
 * @return true if the new size and position are different than the
 *         previous ones
 * {@hide}
 */
@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
protected boolean setFrame(int left, int top, int right, int bottom) {
    boolean changed = false;
    ...
    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);
        ...
        notifySubtreeAccessibilityStateChangedIfNeeded();
    }
    return changed;
}
/**
 * Called from layout when this view should
 * assign a size and position to each of its children.
 *
 * Derived classes with children should override
 * this method and call layout on each of
 * their children.
 * @param ...
 */
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
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@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);
    }
}
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void layoutVertical(int left, int top, int right, int bottom) {
    ...
    for (int i = 0; i < count; i++) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
            childTop += measureNullChild(i);//1
        } else if (child.getVisibility() != GONE) {
            final int childWidth = child.getMeasuredWidth();
            final int childHeight = child.getMeasuredHeight();
            final LinearLayout.LayoutParams lp =
                (LinearLayout.LayoutParams) child.getLayoutParams();
            ...
            if (hasDividerBeforeChildAt(i)) {
                childTop += mDividerHeight;//2
            }
            childTop += lp.topMargin;
            setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                          childWidth, childHeight);//3
            childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
            i += getChildrenSkipCount(child, i);
        }
    }
}
//注释3的详细代码
private void setChildFrame(View child, int left, int top, int width, int height) {
    child.layout(left, top, left + width, top + height);
}
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public final int getWidth(){
    return mRight - mLeft;
}
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