Android中View的layout mechanism（布局机制）

layout mechanism

　　Android中View的layout mechanism主要分为两个阶段：measure阶段和layout阶段。layout mechanism按照一定的顺序进行，总体上来讲是先measure然后layout。

一、measure阶段

　　以父容器提供的宽高参数作为限制条件，计算本身的大小（宽高）。这个过程需要调用final方法measure(int, int)完成。根据源码，我们可以知道实际的measure过程却是由回调方法onMeasure(int, int)来进行。因此我们常常可以看到在View的子类中，会根据需要，有重写方法onMeasure(int, int)。源码如下所示

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
    }

二、layout阶段

　　为View进行layout(int, int, int, int)：分配大小和指定位置（假设这个View是一个ViewGroup，它的子节点控件也进行layout，这个过程通常是回调重写的onLayout(boolean, int, int, int, int)来执行）。源码如下所示：

public void layout(int l, int t, int r, int b) {
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        boolean changed = 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;
    }