View框架之layout()流程

　　在上一篇我们分析了view的measure()流程，当我们把view测量出来以后，接着就要算出这个view的在这个屏幕的具体位置，所以这一篇我们接着分析layout()流程，废话不多说，还是先来一张流程图，下面会根据这个流程图进行讲解

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注意：RootView是一个FrameLayout,所以也是一个ViewGroup

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　　layout()过程和measure(）过程一样，起点都是在ViewRootImpl中的performTraverals()方法，在performTraverals()中我们又调用了performLayout()，所以我们直接从performLayout()方法开始分析，先贴上performLayout(）的源码：

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private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
                 ／＊＊省略部分代码

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

                mHandlingLayoutInLayoutRequest = false;

                // Check the valid requests again, this time without checking/clearing the
                // layout flags, since requests happening during the second pass get noop'd
                validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                if (validLayoutRequesters != null) {
                    final ArrayList<View> finalRequesters = validLayoutRequesters;
                    // Post second-pass requests to the next frame
                    getRunQueue().post(new Runnable() {
                        @Override
                        public void run() {
                            int numValidRequests = finalRequesters.size();
                            for (int i = 0; i < numValidRequests; ++i) {
                                final View view = finalRequesters.get(i);
                                Log.w("View", "requestLayout() improperly called by " + view +
                                        " during second layout pass: posting in next frame");
                                view.requestLayout();
                            }
                        }
                    });
                }
            }

        }
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}　　
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　　这里的的host就是我们的根布局DecorView,因为DecorView是一个FrameLayout,而layout()是view中被修饰final的方法，所以我们这里调用的是View的layout的方法，layout的四个参数代表view的四个顶点的位置，默认的DecorView左边距屏幕的距离和上边距屏幕的边距都是0，接着贴下View的layout方法

  @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;
      /**
      *   setFrame()方法执行以后，View的四个顶点的距离我们就确定了
      */
    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;
}
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　　当执行setFrame(）方法后，会直接初始化mLeft , mTop ,mRight,mBottom这几个值，因为我们一般在view中调用 getLeft() getTop(） getRight() getBottom(）获取的就是这几个值，所以我们一般必须得等layout(）结束以后才能获取到这几个值

   protected boolean setFrame(int left, int top, int right, int bottom) {
    boolean changed = false;

  　　/** 省略部分代码
        mLeft = left;
        mTop = top;
        mRight = right;
        mBottom = bottom;
     /** 省略部分代码
}

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　　接着继续执行我们的onLayout(）方法，onLayout()方法是view中的方法，在view中是一个默认空实现，在viewGroup是一个抽象方法（继承viewGroup必须实现，因为要对子view进行布局），所以我们分两种情况来介绍

　　ViewGroup的onLayout()，以RelativeLayout为例

 @Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    //  The layout has actually already been performed and the positions
    //  cached.  Apply the cached values to the children.
    //遍历子view
    final int count = getChildCount();

    for (int i = 0; i < count; i++) {
        View childView = getChildAt(i);
        if (child.getVisibility() != GONE) {
            RelativeLayout.LayoutParams st =
                    (RelativeLayout.LayoutParams) child.getLayoutParams();
                    //调用每个childView的layout()方法，继续走上面的layout方法
            childView.layout(st.mLeft, st.mTop, st.mRight, st.mBottom);
        }
    }

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View的onLayout(),默认是一个空实现，一般情况下我们自定义控件的时候，会去重写它，实现自己的逻辑。

  view中的onLayout(）：

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

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好了，差不多就这么多了，到了这一步基本确定了view的具体位置，接着就是绘制了…..