一，焦點相關Api說明

• View中定義了一個方法，用來判斷View自身是否獲取到焦點或者其內部的子View是否獲取到焦點。

/**
 * Returns true if this view has focus itself, or is the ancestor of the
 * view that has focus.
 *
 * @return True if this view has or contains focus, false otherwise.
 */
@ViewDebug.ExportedProperty(category = "focus")
public boolean
 
 hasFocus() {
    return (mPrivateFlags & PFLAG_FOCUSED) != 0;
}

• View中定義了一個isFocused方法，該方法用於判斷該View自身是否已經獲取到焦點。

/**
 * Returns true if this view has focus
 *
 * @return True if this view has focus, false otherwise.
 */
@ViewDebug.ExportedProperty(category = "focus")
public boolean isFocused() {
    return
 
 (mPrivateFlags & PFLAG_FOCUSED) != 0;
}

• ViewGroup中定義了一個名為setDescendantFocusability的方法，該方法需要指定一個focusability，該值可以為三種類型：

FOCUS_BEFORE_DESCENDANTS：viewgroup會優先其子類控制元件而獲取到焦點
FOCUS_AFTER_DESCENDANTS：viewgroup只有當其子類控制元件不需要獲取焦點時才獲取焦點
FOCUS_BLOCK_DESCENDANTS：viewgroup會覆蓋子類控制元件而直接獲得焦點

二，介面初始化時系統如何標記初始焦點

• ViewRootImpl類中有一個比較重要的方法performTraversals，該方法中有如下一段程式碼：

if (mFirst) {
    // handle first focus request
    if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: mView.hasFocus()="
            + mView.hasFocus());
    if (mView != null) {
        if (!mView.hasFocus()) {
            mView.requestFocus(View.FOCUS_FORWARD);
            if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: requested focused view="
                    + mView.findFocus());
        } else {
            if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: existing focused view="
                    + mView.findFocus());
        }
    }
}

• handle first focus request意味著這裡處理第一次焦點的請求，注意這裡的mView其實就是DecorView，也就是位於view tree的最頂層的View。其本質就是一個FrameLayout，當自身或者其內部子View不包含焦點的時候，就會呼叫requestFocus嘗試標記焦點View。我們再來看下ViewGroup中重寫的requestFocus：

/**
 * {@inheritDoc}
 *
 * Looks for a view to give focus to respecting the setting specified by
 * {@link #getDescendantFocusability()}.
 *
 * Uses {@link #onRequestFocusInDescendants(int, android.graphics.Rect)} to
 * find focus within the children of this group when appropriate.
 *
 * @see #FOCUS_BEFORE_DESCENDANTS
 * @see #FOCUS_AFTER_DESCENDANTS
 * @see #FOCUS_BLOCK_DESCENDANTS
 * @see #onRequestFocusInDescendants(int, android.graphics.Rect)
 */
@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
    if (DBG) {
        System.out.println(this + " ViewGroup.requestFocus direction="
                + direction);
    }
    int descendantFocusability = getDescendantFocusability();

    switch (descendantFocusability) {
        case FOCUS_BLOCK_DESCENDANTS:
            return super.requestFocus(direction, previouslyFocusedRect);
        case FOCUS_BEFORE_DESCENDANTS: {
            final boolean took = super.requestFocus(direction, previouslyFocusedRect);
            return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
        }
        case FOCUS_AFTER_DESCENDANTS: {
            final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
            return took ? took : super.requestFocus(direction, previouslyFocusedRect);
        }
        default:
            throw new IllegalStateException("descendant focusability must be "
                    + "one of FOCUS_BEFORE_DESCENDANTS, FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS "
                    + "but is " + descendantFocusability);
    }
}

• 我們注意到，這裡會根據getDescendantFocusability返回的結果，也就是會先判斷我們有沒有設定focusability，然後會根據型別首先判斷onRequestFocusInDescendants的返回值，如果為真就直接返回了。onRequestFocusInDescendants是個什麼鬼呢，接著看：

@SuppressWarnings({"ConstantConditions"})
protected boolean onRequestFocusInDescendants(int direction,
        Rect previouslyFocusedRect) {
    int index;
    int increment;
    int end;
    int count = mChildrenCount;
    if ((direction & FOCUS_FORWARD) != 0) {
        index = 0;
        increment = 1;
        end = count;
    } else {
        index = count - 1;
        increment = -1;
        end = -1;
    }
    final View[] children = mChildren;
    for (int i = index; i != end; i += increment) {
        View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
            if (child.requestFocus(direction, previouslyFocusedRect)) {
                return true;
            }
        }
    }
    return false;
}

• 我們注意到，其內部遍歷所有子View，當子View為可見時，就嘗試去標記焦點。因為onRequestFocusInDescendants方法為protected型別，所以我們通常可以重寫該方法做一些操作。