編輯:關於Android編程
本篇模擬三個角色:Android 架構師-小福、Android 控件開發工程師-小黑、 Android 開發工程師-小白,下面按照三個角色不同角度分析measure過程。
小福負責分享:
measure的本質 measure代碼流程 onMeasure方法與MeasureSpec 提出問題
小黑負責分享:
布局控件開發中覆寫Measure例子 - ok 從遇到的一個異常說起 什麼時候需要覆寫onMeaure? - ok view.getWidth與view.getMeasureWidth區別 - ok
java.lang.IllegalStateException: onMeasure() did not set the measured dimension by calling setMeasuredDimension()
public class View implements Drawable.Callback, Drawable.Callback2, KeyEvent.Callback, AccessibilityEventSource { /** * Like {@link #getMeasuredWidthAndState()}, but only returns the * raw width component (that is the result is masked by * {@link #MEASURED_SIZE_MASK}). * * @return The raw measured width of this view. */ public final int getMeasuredWidth() { // 直接返回mMeasuredWidth與後者相與清理掉其他開關獲取真是measure大小 return mMeasuredWidth & MEASURED_SIZE_MASK; } /** *
This mehod must be called by {@link #onMeasure(int, int)} to store the * measured width and measured height. Failing to do so will trigger an * exception at measurement time.
* * @param measuredWidth The measured width of this view. May be a complex * bit mask as defined by {@link #MEASURED_SIZE_MASK} and * {@link #MEASURED_STATE_TOO_SMALL}. * @param measuredHeight The measured height of this view. May be a complex * bit mask as defined by {@link #MEASURED_SIZE_MASK} and * {@link #MEASURED_STATE_TOO_SMALL}. */ protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) { // 通常在onMeasure中調用,傳入測量過的視圖寬度與高度 mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; mPrivateFlags |= MEASURED_DIMENSION_SET; } /** * Bits of {@link #getMeasuredWidthAndState()} and * {@link #getMeasuredWidthAndState()} that provide the actual measured size. */ // MeasureSpec中的Mode或占用int類型中前幾位 public static final int MEASURED_SIZE_MASK = 0x00ffffff; }public class View implements Drawable.Callback, Drawable.Callback2, KeyEvent.Callback, AccessibilityEventSource { /** * Return the width of the your view. * * @return The width of your view, in pixels. */ @ViewDebug.ExportedProperty(category = "layout") public final int getWidth() { // 視圖的右側減去左側的值 return mRight - mLeft; } /** * Assign a size and position to this view. * * This is called from layout. * * @param left Left position, relative to parent * @param top Top position, relative to parent * @param right Right position, relative to parent * @param bottom Bottom position, relative to parent * @return true if the new size and position are different than the * previous ones * {@hide} */ protected boolean setFrame(int left, int top, int right, int bottom) { boolean changed = false; ...... // 四個值中任意一個發生改變就行 if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) { changed = true; ...... mLeft = left; mTop = top; mRight = right; mBottom = bottom; ...... } return changed; } public void layout(int l, int t, int r, int b) { ...... // 當前視圖布局時執行,傳入當前視圖的上下左右邊界值 boolean changed = setFrame(l, t, r, b); if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) { ...... // 上面執行完成後才會觸發onLayout onLayout(changed, l, t, r, b); ...... } ...... mPrivateFlags &= ~FORCE_LAYOUT; } }
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