Android View体系（八）从源码解析View的layout和draw流程

前言

上一篇文章我们讲了View的measure的流程，接下来我们讲下View的layout和draw流程，如果你理解了View的measure的流程，那这篇文章自然就不在话下了。

1.View的layout流程

先来看看View的layout()方法：

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;
   }

View Code