闲谈自定义控件源码-view 测量

measure 调用onMeasure()，因为measure的到的宽高会用MeasureSpec封装，先看MeasureSpec

1. private static final int MODE_SHIFT = 30;
2. private static final int MODE_MASK = 0x3 << MODE_SHIFT;

1. /**
2. * Measure specification mode: The parent has not imposed any constraint
3. * on the child. It can be whatever size it wants.
4. */
5. public static final int UNSPECIFIED = 0 << MODE_SHIFT;
7. /**
8. * Measure specification mode: The parent has determined an exact size
9. * for the child. The child is going to be given those bounds regardless
10. * of how big it wants to be.
11. */
12. public static final int EXACTLY = 1 << MODE_SHIFT;
14. /**
15. * Measure specification mode: The child can be as large as it wants up
16. * to the specified size.
17. */
18. public static final int AT_MOST = 2 << MODE_SHIFT;

MeasureSpec包含三种模式(mode)

   UNSPECIFIED：父控件未约束，也就是子控件大小随意,比如listview，scrollview一般view用不到

   EXACTLY：当宽或高为固定值或match_parent时，子控件约束值是固定的，mode为exactly

   AT_MOST：当宽或高为wrap_content时，我觉得当viewgroup有两个控件时 一个width 60dp，一个width 70dp，那么

1. public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
2. @MeasureSpecMode int mode) {
3. if (sUseBrokenMakeMeasureSpec) {
4. return size + mode;
5. } else {
6. return (size & ~MODE_MASK) | (mode & MODE_MASK);
7. }
8. }

知道1：mode_mask 为11000.... 所以~mode_mask 为001111...

        2：一个二进制数&1的结果不变，一个二进制数&0的结果都是0

        3：一个二进制数 | 1都得1，一个二进制 | 0不变。

110000 | 001100 = 111100 可见 如果将一个二进制数划分为多个部分。处特定含义部分为0 ，| 运算就想当与加法

110000&000000 = 000000 可见&运算起到一个清0的作用

  ( size & ~ MODE_MASK ) | ( mode & MODE_MASK ); 和  size + mode 的作用是一样的只是位运算的算法效率更高

1. public static int getMode(int measureSpec) {
2. //noinspection ResourceType
3. return (measureSpec & MODE_MASK);
4. }
5. ...
6. public static int getSize(int measureSpec) {
7. return (measureSpec & ~MODE_MASK);
8. }

这两个方法通过位的与运算得到Mode和Size

1. /**
2. * Utility to return a default size. Uses the supplied size if the
3. * MeasureSpec imposed no constraints. Will get larger if allowed
4. * by the MeasureSpec.
5. *
6. * @param size Default size for this view
7. * @param measureSpec Constraints imposed by the parent
8. * @return The size this view should be.
9. */
10. public static int getDefaultSize(int size, int measureSpec) {
11. int result = size;
12. int specMode = MeasureSpec.getMode(measureSpec);
13. int specSize = MeasureSpec.getSize(measureSpec);
15. switch (specMode) {
16. case MeasureSpec.UNSPECIFIED:
17. result = size;
18. break;
19. case MeasureSpec.AT_MOST:
20. case MeasureSpec.EXACTLY:
21. result = specSize;
22. break;
23. }
24. return result;
25. }

第一个参数size 代表当 Mode为UNSPECIFIED是取的值

    前2位代表mode，后30位代表size

    进入View的onMeasure()：

1. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
2. setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
3. getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
4. }

先看 setMeasuredDimension () 方法

1. /**
2. *

   This method must be called by {@link #onMeasure(int, int)} to store the

3. * measured width and measured height. Failing to do so will trigger an
4. * exception at measurement time.

5. *
6. * @param measuredWidth The measured width of this view. May be a complex
7. * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
8. * {@link #MEASURED_STATE_TOO_SMALL}.
9. * @param measuredHeight The measured height of this view. May be a complex
10. * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
11. * {@link #MEASURED_STATE_TOO_SMALL}.
12. */
13. protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
14. ...
15. setMeasuredDimensionRaw(measuredWidth, measuredHeight);
16. }

这个方法必须被onMeasure()调用，用来保存测量的宽高，否则在执行测量时会触发异常。measureWidth,measureHeight两个参数是MeasureSpec.getSize()的值。

应用试列：测量viewpager每一页的高度，使viewpager每个页面根据控件高度自适应

    

1. import android.content.Context;
2. import android.support.v4.view.ViewPager;
3. import android.util.AttributeSet;
4. import android.util.Log;
5. import android.view.MotionEvent;
6. import android.view.View;
8. import com.vcredit.vmoney.utils.CommonUtils;
9. import com.vcredit.vmoney.utils.ConstantUtils;
11. /**
12. * Created by qiubangbang on 2016/8/16.
13. */
14. public class WrapContentViewpager extends ViewPager {
15. // private static final String TAG = "qbb_MyButton";
16. private int[] pageHeigeht = {0, 0, 0};
18. public WrapContentViewpager(Context context) {
19. super(context);
20. }
22. public WrapContentViewpager(Context context, AttributeSet attrs) {
23. super(context, attrs);
24. }
26. @Override
27. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
28. CommonUtils.LOG_D(getClass(), "inestmentpages: onMeasure");
29. CommonUtils.LOG_D(getClass(), "inestmentpages: onMeasure_item: " + getCurrentItem());
30. CommonUtils.LOG_D(getClass(), "inestmentpages: getchildCount: " + getChildCount());
31. // for (int i = 0; i < getChildCount(); i++) {
32. // View child = getChildAt(i);
33. // child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
34. // int h = child.getMeasuredHeight();
35. // if (h > height) {
36. // height = h;
37. // }
38. // }
40. View child;
41. if (getChildCount() > 0) {
42. //测量展示的viewpager页的高度
43. //只有pageheight没有值时采取测量
44. if (pageHeigeht[0] == 0 || pageHeigeht[1] == 0 || pageHeigeht[2] == 0) {
45. //点击时出现问题，如果不是按部就班滑动，而是直接点击第三页
46. if (pageHeigeht[1] == 0 && getCurrentItem() == 2) {
47. child = getChildAt(1);
48. } else if (pageHeigeht[2] != 0 && getCurrentItem() == 1) {
49. child = getChildAt(0);
50. } else {
51. if (getChildCount() == 2 && getCurrentItem() == 2) {
52. child = getChildAt(1);
53. } else {
54. child = getChildAt(getCurrentItem());
55. }
56. }
57. child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
58. if (pageHeigeht[getCurrentItem()] == 0) {
59. pageHeigeht[getCurrentItem()] = child.getMeasuredHeight();
60. }
61. }
63. heightMeasureSpec = MeasureSpec.makeMeasureSpec(pageHeigeht[getCurrentItem()], MeasureSpec.EXACTLY);
64. }
66. super.onMeasure(widthMeasureSpec, heightMeasureSpec);
67. }
68. }