Android GUI之View布局

在清楚了view绘制机制中的第一步测量之后，我们继续来了解分析View绘制的第二个过程，那就是布局定位。继续跟踪分析源码，根据之前的流程分析我们知道View的绘制是从RootViewImpl的performTraversals方法开始的，在此方法中依次调用了performMeasure、performLayout、performDraw等方法进行测量、布局、绘制，那么下面我们就看看则方performLayout中都做了哪些事情，该方法的关键源码如下：

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
        mLayoutRequested = false;
        mScrollMayChange = true;
        mInLayout = true;
        final View host = mView;
          ……
        try {
            host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
……
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        mInLayout = false;
    }

从中看出，最关键的代码就是调用了host.layout方法，那么大家还记不记得host是个什么东东呢？对了，正是我们之前说的根视图DecorView。那么我们就回到DecorView看看它在layout方法中到底做了什么事情。令人失望的是，我们在DecorView中并没有发现该方法，不要急，根据该类的继承体系，我们最终追踪到layout方法在View中。

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);
            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;
    }

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

该方法中的4个参数代表了当前的View与父View之间4个方向上的距离，同时从说明中可以看出，此方法不应该被子类重写，假如需要重新布局，可以在子类中重写的方法是onLayout，此方法在View中是个空方法，什么都没有写。可实际上layout的方法在View中并没有被标识为final，这就意味是可以被重写的。

继续查看ViewGoup中的相关代码，果然layout被重写了并添加了final标识，同时onLayout被标识为抽象方法，所以继承了ViewGroup的类是，是不能重写layout方法的，并且要实现onLayout方法。从代码可以看出，虽然ViewGroup重写了layout，实际本质上还是调用了View的layout，然后通过调用onLayout方法最终完成布局定位的。

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

在DecorView中，并没有发现onLayout方法，所以它使用的肯定是其父类FrameLayout中的，找到FrameLayout的源码，可以查看到onLayout方法，具体如下：

  @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();

        mForegroundBoundsChanged = true;

        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);
            }
        }
    }

从方法中，我们可以看出在Framelayout中最终调用了layoutChildren方法，在该方法中根据测量结果和一些布局属性对容器中每一个View都调用了layout方法进行了布局。根据以上的代码分析，我们可以得出View布局定位的流程图如下。