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1. View(),沒有暴露出來，等于沒有，過過過

     /**
     * Non-public constructor for use in testing
     */
    View() {
        mResources = null;
        mRenderNode = RenderNode.create(getClass().getName(), this);
    }

2. View(Context context),當(dāng)在代碼中創(chuàng)建view時使用，傳入一個上下文，使該View運(yùn)行在其中，并且可以通過這個上下文獲取主題、資源等。

    /**
     * Simple constructor to use when creating a view from code.
     *
     * @param context The Context the view is running in, through which it can
     *        access the current theme, resources, etc.
     */
    public View(Context context) {...}

3. 從布局文件xml中填充出來的view使用這個2參的構(gòu)造函數(shù)。
   第1個參數(shù)context上文說過了，第2個參數(shù)attr包含了在xml中定義的屬性，初始化的時候需要把這些屬性拿出來用?？梢钥吹竭@個構(gòu)造直接調(diào)用了一個三參構(gòu)造。那么繼續(xù)走

    /**
     * Constructor that is called when inflating a view from XML. This is called
     * when a view is being constructed from an XML file, supplying attributes
     * that were specified in the XML file. This version uses a default style of
     * 0, so the only attribute values applied are those in the Context's Theme
     * and the given AttributeSet.
     *
     * <p>
     * The method onFinishInflate() will be called after all children have been
     * added.
     *
     * @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.
     * @see #View(Context, AttributeSet, int)
     */
    public View(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

4. 三參構(gòu)造

    /**
     * Perform inflation from XML and apply a class-specific base style from a
     * theme attribute. This constructor of View allows subclasses to use their
     * own base style when they are inflating. For example, a Button class's
     * constructor would call this version of the super class constructor and
     * supply <code>R.attr.buttonStyle</code> for <var>defStyleAttr</var>; this
     * allows the theme's button style to modify all of the base view attributes
     * (in particular its background) as well as the Button class's attributes.
     *
     * @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.
     * @see #View(Context, AttributeSet)
     */
    public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        this(context, attrs, defStyleAttr, 0);
    }

2參構(gòu)造會調(diào)用這個3參構(gòu)造，多了一個默認(rèn)樣式defStyleAttr，比如Button的構(gòu)造會傳一個Button的樣式，然后調(diào)用了自己的四參構(gòu)造

public Button(Context context, AttributeSet attrs) {
        this(context, attrs, com.android.internal.R.attr.buttonStyle);
}

而四參構(gòu)造會調(diào)用父類TextView的四參構(gòu)造，TextView會繼續(xù)調(diào)用父類View的四參構(gòu)造。

    public Button(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

所以這個三參構(gòu)造的第三個參數(shù)defStyleAttr是給View的子類傳自己用的屬性配置的。

5. 四參構(gòu)造

    /**
     * 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);
        ...
    }

比三參多了個defStyleRes，子類在覆寫構(gòu)造的時候可以傳入樣式資源到這里。可以看到，四參構(gòu)造也是先調(diào)用了this(context)，然后獲取到TypeArray拿各種屬性,再繼續(xù)對view進(jìn)行初始化操作。

總結(jié)：View(Context)是從代碼中創(chuàng)建view走的構(gòu)造。 View(Context,AttributeSet),是從xml中填充的view走的構(gòu)造,2參會調(diào)用3參，3參調(diào)用4參，4參最后會調(diào)用1參，然后拿到所有屬性再初始化。這樣子類的構(gòu)造可以傳入特定的attr和style來初始化。