Android開發學習之View測量的內置常用方法

 

measureChildWithMargins

代碼如下

1. protected void measureChildWithMargins(View child,

2. int parentWidthMeasureSpec, int widthUsed,

3. int parentHeightMeasureSpec, int heightUsed) {

4. final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

5.  

6. final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,

7. mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin

8. + widthUsed, lp.width);

9. final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,

10. mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin

11. + heightUsed, lp.height);

12.  

13. child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

14. }

主要調用了getChildMeasureSpec()方法

 

getChildMeasureSpec

代碼如下

1. public static int getChildMeasureSpec(int spec, int padding, int childDimension) {

2. // 以寬度爲例，傳入的參數分別是：父view的widthMeasureSpec、已用寬度(子view左右內外間距+已用的間距)、子view要求的寬度

3.  

4. int specMode = MeasureSpec.getMode(spec);

5. int specSize = MeasureSpec.getSize(spec);

6.  

7. int size = Math.max(0, specSize - padding); // 父寬度 - 子view的內外間距

8. // 也就是子view的最大寬度

9.  

10. int resultSize = 0;

11. int resultMode = 0;

12.  

13. switch (specMode) {

14. // Parent has imposed an exact size on us

15. case MeasureSpec.EXACTLY:

16. if (childDimension >= 0) {

17. resultSize = childDimension;

18. resultMode = MeasureSpec.EXACTLY;

19. } else if (childDimension == LayoutParams.MATCH_PARENT) {

20. // Child wants to be our size. So be it.

21. resultSize = size;

22. resultMode = MeasureSpec.EXACTLY;

23. } else if (childDimension == LayoutParams.WRAP_CONTENT) {

24. // Child wants to determine its own size. It can't be

25. // bigger than us.

26. resultSize = size;

27. resultMode = MeasureSpec.AT_MOST;

28. }

29. break;

30.  

31. // Parent has imposed a maximum size on us

32. case MeasureSpec.AT_MOST:

33. if (childDimension >= 0) {

34. // Child wants a specific size... so be it

35. resultSize = childDimension;

36. resultMode = MeasureSpec.EXACTLY;

37. } else if (childDimension == LayoutParams.MATCH_PARENT) {

38. // Child wants to be our size, but our size is not fixed.

39. // Constrain child to not be bigger than us.

40. resultSize = size;

41. resultMode = MeasureSpec.AT_MOST;

42. } else if (childDimension == LayoutParams.WRAP_CONTENT) {

43. // Child wants to determine its own size. It can't be

44. // bigger than us.

45. resultSize = size;

46. resultMode = MeasureSpec.AT_MOST;

47. }

48. break;

49.  

50. // Parent asked to see how big we want to be

51. case MeasureSpec.UNSPECIFIED:

52. if (childDimension >= 0) {

53. // Child wants a specific size... let him have it

54. resultSize = childDimension;

55. resultMode = MeasureSpec.EXACTLY;

56. } else if (childDimension == LayoutParams.MATCH_PARENT) {

57. // Child wants to be our size... find out how big it should

58. // be. sdk < 23時，sUseZeroUnspecifiedMeasureSpec爲true

59. resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;

60. resultMode = MeasureSpec.UNSPECIFIED;

61. } else if (childDimension == LayoutParams.WRAP_CONTENT) {

62. // Child wants to determine its own size.... find out how

63. // big it should be.

64. resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;

65. resultMode = MeasureSpec.UNSPECIFIED;

66. }

67. break;

68. }

69. //noinspection ResourceType

70. return MeasureSpec.makeMeasureSpec(resultSize, resultMode);

71. }

 

combineMeasuredState

1. public static int combineMeasuredStates(int curState, int newState) {

2. return curState | newState;

3. }

沒啥好說的，合併兩個state

 

getMeasuredState

1. public final int getMeasuredState() {

2. return (mMeasuredWidth&MEASURED_STATE_MASK)

3. | ((mMeasuredHeight>>MEASURED_HEIGHT_STATE_SHIFT)

4. & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));

5. }

通過位運算合併高度和寬度的state，結果就是第1個字節是寬度的state，第3個字節是高度的state

 

resolveSizeAndState

1. public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { // size是自己需要的尺寸，measureSpec是父view指定的尺寸

2. final int specMode = MeasureSpec.getMode(measureSpec);

3. final int specSize = MeasureSpec.getSize(measureSpec); // 父view指定的尺寸

4. final int result;

5. switch (specMode) {

6. case MeasureSpec.AT_MOST:

7. if (specSize < size) {

8. result = specSize | MEASURED_STATE_TOO_SMALL; // 設定標誌位，表示父view得分配的再大一些

9. } else {

10. result = size;

11. }

12. break;

13. case MeasureSpec.EXACTLY:

14. result = specSize;

15. break;

16. case MeasureSpec.UNSPECIFIED:

17. default:

18. result = size;

19. }

20. return result | (childMeasuredState & MEASURED_STATE_MASK);

21. }

最後保存狀態時，由於childMeasureState是第一個字節是寬度的狀態，第三個字節是高度的狀態，所以再測量寬度高度時調用此方法，要對childState進行位運算

 

setMeasuredDimension

這個是用來保存當前view尺寸的，當我們使用自定義view並且覆寫了onMeasure()方法時，不在最後調用這個方法，直接就報錯了

1. protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {

2. boolean optical = isLayoutModeOptical(this);

3. if (optical != isLayoutModeOptical(mParent)) { // layout_mode是LAYOUT_MODE_OPTICAL的情況很少出現，所以測量過程中涉及optical的，我們一般都可以直接忽略不計

4. Insets insets = getOpticalInsets();

5. int opticalWidth = insets.left + insets.right;

6. int opticalHeight = insets.top + insets.bottom;

7.  

8. measuredWidth += optical ? opticalWidth : -opticalWidth;

9. measuredHeight += optical ? opticalHeight : -opticalHeight;

10. }

11. setMeasuredDimensionRaw(measuredWidth, measuredHeight);

12. }

直接調用了setMeasuredDimensionRaw()方法

1. private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {

2. mMeasuredWidth = measuredWidth;

3. mMeasuredHeight = measuredHeight;

4.  

5. mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;

6. }

嗯，就是這一步，保存了measuredHeight和measuredWidth.不過，這裏要說明一下，measuredHeight/Width高八位是狀態，低二十四位纔是真正的尺寸，這也就是View.getMeasuredWidth/Height和View.getMeasuredWidth/HeightAndState()方法的區別所在

 

getDefaultSize

1. public static int getDefaultSize(int size, int measureSpec) {

2. int result = size; // size是view的默認尺寸

3. int specMode = MeasureSpec.getMode(measureSpec);

4. int specSize = MeasureSpec.getSize(measureSpec);

5.  

6. switch (specMode) {

7. case MeasureSpec.UNSPECIFIED:

8. result = size;

9. break;

10. case MeasureSpec.AT_MOST:

11. case MeasureSpec.EXACTLY:

12. result = specSize;

13. break;

14. }

15. return result;

16. }

 

getSuggestMinimunHeight/Width

以高度爲例，代碼如下

1. protected int getSuggestedMinimumHeight() {

2. return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

3. }

涉及view的背景，mBackground是Drawable類的，它的getMinimumHeight()由不同的子類分別實現，反正就是返回最小的高度，然後再跟view本身的最小高度取最大值(view本身的最小高度不手動設置的話，就是0)