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[操作系统]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,如果需要重新布局,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。

 

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作者:杰瑞教育
出处:http://www.cnblogs.com/jerehedu/ 
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