二、从遇到的一个异常说起

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与后者相与清理掉其他开关获取真是measure大小
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }
	
    /**
     * <p>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.</p>
     *
     * @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中调用，传入测量过的视图宽度与高度
        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类型中前几位
    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() {
        // 视图的右侧减去左侧的值	
        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;

        ......
		
        // 四个值中任意一个发生改变就行		
        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) {
        ......
        // 当前视图布局时执行，传入当前视图的上下左右边界值		
        boolean changed = setFrame(l, t, r, b);
		
        if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
            ......
            // 上面执行完成后才会触发onLayout
            onLayout(changed, l, t, r, b);
			
            ......
        }
        ......
        mPrivateFlags &= ~FORCE_LAYOUT;
    }
}