android-View工作原理（四）view的layout过程

本文转载自：http://blog.csdn.net/ff20081528/article/details/17784911

一、android中view的layout过程总概

Layout过程其实就是父视图按照子视图的大小及布局参数将子视图放在窗口的合适的位置上。视图的布局过程是从ViewRoot对象调调用根视图的layout()方法开始，接着layout()方法调用根视图的onLayout()方法，onLayout()方法会对所包含的子视图逐一执行layout操作，如果子视图是ViewGroup子类对象，则继续调用子视图的layout()，重复这一过程。如果子视图是View子类对象，则在子视图重载的onLayout()内部只需要将自己布局到视图中，不需要对子视图进行layout操作，这样一次layout过程结束。过程如下图:

二、layout详细过程

View中的layout()方法源码（ViewGroup类继承了View类，layout过程先从ViewGroup子类开始）:

 /**     * 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().     *     * Derived classes with children should override     * onLayout. In that method, they should     * call layout on each of their their children.     *     * @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     */    public final void layout(int l, int t, int r, int b) {        boolean changed = setFrame(l, t, r, b);        if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {            if (ViewDebug.TRACE_HIERARCHY) {                ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_LAYOUT);            }            onLayout(changed, l, t, r, b);            mPrivateFlags &= ~LAYOUT_REQUIRED;        }        mPrivateFlags &= ~FORCE_LAYOUT;    }

a) 首先我们看这个方法的定义，用了关键字final，说明此方法是不可被重写的，这样也就保证了View的layout过程是不变的。四个参数看注释，左、上、右、下分别相距父视图的距离。

b) 调用setFrame(l,t,r,b)将位置保存起来，这些参数将保存到View内部变量 （mLeft、mTop、mRight、mBottom）中。保存完变量前，会先对比这些参数是否和原来的相同，如果相同，则什么都不做，如果不同则进行重新赋值，并在赋值前给mPrivateFlags中添加DRAWN标识，同时调用invalidate()通知View系统原来占用的位置需要重绘。c) 调用onLayout(),View中定义的onLayout()方法默认什么都不做，View系统提供onLayout()方法的目的是为了使系统包含的子视图的父视图能够在onLayout()方法对子视图进行位置分配，正因为如此，如果是父视图，则必须重写onLayout()，也正因为如此ViewGroup类才会把onLayout重载改成了abstract类型。d）清除mPrivateFlags中的LAYOUT_REQUIRED标识，因为layout操作已经完成。

上面提到的setFrame方法源码如下：

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 + ")");        }        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {            changed = true;            // Remember our drawn bit            int drawn = mPrivateFlags & DRAWN;            // Invalidate our old position            invalidate();            int oldWidth = mRight - mLeft;            int oldHeight = mBottom - mTop;            mLeft = left;            mTop = top;            mRight = right;            mBottom = bottom;            mPrivateFlags |= HAS_BOUNDS;            int newWidth = right - left;            int newHeight = bottom - top;            if (newWidth != oldWidth || newHeight != oldHeight) {                onSizeChanged(newWidth, newHeight, oldWidth, oldHeight);            }            if ((mViewFlags & VISIBILITY_MASK) == VISIBLE) {                // 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, therby clearing                // the DRAWN bit.                mPrivateFlags |= DRAWN;                invalidate();            }            // Reset drawn bit to original value (invalidate turns it off)            mPrivateFlags |= drawn;            mBackgroundSizeChanged = true;        }        return changed;    }

View中的onLayout()方法如下：

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {    }

而ViewGroup中的onLayout()方法如下：

@Override    protected abstract void onLayout(boolean changed,            int l, int t, int r, int b);

因为ViewGroup中的onLayout()方法是一个抽象方法，所以下面我们用他的子类LinearLayout中的onLayout()方法来分析。源码如下：

onlayout()方法：

 @Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        if (mOrientation == VERTICAL) {            layoutVertical();        } else {            layoutHorizontal();        }    }

layoutVertical()方法源码：

void layoutVertical() {        final int paddingLeft = mPaddingLeft;        int childTop = mPaddingTop;        int childLeft;        // Where right end of child should go        final int width = mRight - mLeft;        int childRight = width - mPaddingRight;        // Space available for child        int childSpace = width - paddingLeft - mPaddingRight;        final int count = getVirtualChildCount();        final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;        final int minorGravity = mGravity & Gravity.HORIZONTAL_GRAVITY_MASK;        if (majorGravity != Gravity.TOP) {           switch (majorGravity) {               case Gravity.BOTTOM:                   // mTotalLength contains the padding already, we add the top                   // padding to compensate                   childTop = mBottom - mTop + mPaddingTop - mTotalLength;                   break;               case Gravity.CENTER_VERTICAL:                   childTop += ((mBottom - mTop)  - mTotalLength) / 2;                   break;           }        }        for (int i = 0; i < count; i++) {            final View child = getVirtualChildAt(i);            if (child == null) {                childTop += measureNullChild(i);            } else if (child.getVisibility() != GONE) {                final int childWidth = child.getMeasuredWidth();                final int childHeight = child.getMeasuredHeight();                final LinearLayout.LayoutParams lp =                        (LinearLayout.LayoutParams) child.getLayoutParams();                int gravity = lp.gravity;                if (gravity < 0) {                    gravity = minorGravity;                }                switch (gravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                    case Gravity.LEFT:                        childLeft = paddingLeft + lp.leftMargin;                        break;                    case Gravity.CENTER_HORIZONTAL:                        childLeft = paddingLeft + ((childSpace - childWidth) / 2)                                + lp.leftMargin - lp.rightMargin;                        break;                    case Gravity.RIGHT:                        childLeft = childRight - childWidth - lp.rightMargin;                        break;                    default:                        childLeft = paddingLeft;                        break;                }                childTop += lp.topMargin;                setChildFrame(child, childLeft, childTop + getLocationOffset(child),                        childWidth, childHeight);                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);                i += getChildrenSkipCount(child, i);            }        }    }

a) LinearLayout中的子视图有两种布局方式，一个是纵向的，一个是横向的，这里我们以纵向的分析。b) 获得子视图的宽度。c) 根据父视图中的grarity属性，来判断子视图的起始位置。d) 开始for()循环，为每个子视图分配位置。对于每个子视图首先取出子视图的LayoutParams属性，并且获得gravity的值。根据gravity的值确定水平方向的起始位置，三种值分别为：LEFT,CENTER_HORIZONTAL和RIGHT.接着调用setChildFrame()，该方法内部实际上就是调用child.layout()为子视图设置布局位置。