闲谈自定义控件源码-view layout draw

    onMeasure

   onMeasure会先测量父view根据父view的measureSpecSize测量出子view，由上向下的传递。onLayout一样先布局父view然后依次布局子view

    看FrameLayout的onLayout方法

    

1. @Override
2. protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
3. layoutChildren(left, top, right, bottom, false /* no force left gravity */);
4. }

调用layoutChildren方法

1. void layoutChildren(int left, int top, int right, int bottom,
2. boolean forceLeftGravity) {
3. final int count = getChildCount();
5. final int parentLeft = getPaddingLeftWithForeground();
6. final int parentRight = right - left - getPaddingRightWithForeground();
8. final int parentTop = getPaddingTopWithForeground();
9. final int parentBottom = bottom - top - getPaddingBottomWithForeground();
11. for (int i = 0; i < count; i++) {
12. final View child = getChildAt(i);
13. if (child.getVisibility() != GONE) {
14. final LayoutParams lp = (LayoutParams) child.getLayoutParams();
16. final int width = child.getMeasuredWidth();
17. final int height = child.getMeasuredHeight();
19. int childLeft;
20. int childTop;
22. ...
23. switch (verticalGravity) {
24. case Gravity.TOP:
25. childTop = parentTop + lp.topMargin;
26. break;
27. case Gravity.CENTER_VERTICAL:
28. childTop = parentTop + (parentBottom - parentTop - height) / 2 +
29. lp.topMargin - lp.bottomMargin;
30. break;
31. case Gravity.BOTTOM:
32. childTop = parentBottom - height - lp.bottomMargin;
33. break;
34. default:
35. childTop = parentTop + lp.topMargin;
36. }
38. child.layout(childLeft, childTop, childLeft + width, childTop + height);
39. }

根据child的layoutparam和gravity计算childTop和childLeft，然后调用child.layout()-->child.onLayout()布局child。一般viewGroup需要重写onLayout和onMeasure，view很少会需要重写。

   ondraw

ondraw算是源码中最简单的方法了，只是传过来一个canvas，我们通过canvas来绘制。但是有算是最复杂的方法，因为在自定义一个view的时候更多的去重写ondraw方法。会重新写关于ondraw方法