android(17): View三大属性参数：attrs、defStyleAttr、defStyleRes

标题中的3个字符串，是3个变量。最近使用了ListPopupWindow，需要定义样式，对这3个变量作用进行了深入的了解。首先看下ListPopupWindow的构造函数：

    /**
     * Create a new, empty popup window capable of displaying items from a ListAdapter.
     * Backgrounds should be set using {@link #setBackgroundDrawable(Drawable)}.
     *
     * @param context Context used for contained views.
     * @param attrs Attributes from inflating parent views used to style the popup.
     * @param defStyleAttr Style attribute to read for default styling of popup content.
     * @param defStyleRes Style resource ID to use for default styling of popup content.
     */
    public ListPopupWindow(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        mContext = context;
        mHandler = new Handler(context.getMainLooper());

        final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ListPopupWindow,
                defStyleAttr, defStyleRes);
        mDropDownHorizontalOffset = a.getDimensionPixelOffset(
                R.styleable.ListPopupWindow_android_dropDownHorizontalOffset, 0);
        mDropDownVerticalOffset = a.getDimensionPixelOffset(
                R.styleable.ListPopupWindow_android_dropDownVerticalOffset, 0);
        if (mDropDownVerticalOffset != 0) {
            mDropDownVerticalOffsetSet = true;
        }
        a.recycle();

        mPopup = new AppCompatPopupWindow(context, attrs, defStyleAttr);
        mPopup.setInputMethodMode(PopupWindow.INPUT_METHOD_NEEDED);

        // Set the default layout direction to match the default locale one
        final Locale locale = mContext.getResources().getConfiguration().locale;
        mLayoutDirection = TextUtilsCompat.getLayoutDirectionFromLocale(locale);
    }

看注释，这3个变量都能控制ListPopupWindow的样式，这3个值是什么关系呢？

我们把问题先放一放，我们来看下View的构造函数，发现也有这3个变量。

    /**
     * Perform inflation from XML and apply a class-specific base style from a
     * theme attribute or style resource. This constructor of View allows
     * subclasses to use their own base style when they are inflating.
     * <p>
     * When determining the final value of a particular attribute, there are
     * four inputs that come into play:
     * <ol>
     * <li>Any attribute values in the given AttributeSet.
     * <li>The style resource specified in the AttributeSet (named "style").
     * <li>The default style specified by <var>defStyleAttr</var>.
     * <li>The default style specified by <var>defStyleRes</var>.
     * <li>The base values in this theme.
     * </ol>
     * <p>
     * Each of these inputs is considered in-order, with the first listed taking
     * precedence over the following ones. In other words, if in the
     * AttributeSet you have supplied <code>&lt;Button * textColor="#ff000000"&gt;</code>
     * , then the button's text will <em>always</em> be black, regardless of
     * what is specified in any of the styles.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     * @param attrs The attributes of the XML tag that is inflating the view.
     * @param defStyleAttr An attribute in the current theme that contains a
     *        reference to a style resource that supplies default values for
     *        the view. Can be 0 to not look for defaults.
     * @param defStyleRes A resource identifier of a style resource that
     *        supplies default values for the view, used only if
     *        defStyleAttr is 0 or can not be found in the theme. Can be 0
     *        to not look for defaults.
     * @see #View(Context, AttributeSet, int)
     */
    public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        this(context);

        final TypedArray a = context.obtainStyledAttributes(
                attrs, com.android.internal.R.styleable.View, defStyleAttr, defStyleRes);
        ....

看View构造函数的大段注释，可以大概明白：attrs、defStyleAttr、defStyleRes都是用来确定View的属性（比如宽度、高度、背景等）的值，但使用谁提供的属性值有一定的规则。

先说下这3个的单独用法。
attrs: AttributeSet属性集。
属性：

defStyleAttr: 默认样式属性。Android内置一些默认的样式属性。
主题：

defStyleAttr: 默认样式资源。
样式：

http://www.jianshu.com/p/61b79e7f88fc