android - 求问Fragment fragment = (Fragment) page.getCls().newInstance();

Question

这段代码是什么意思？里面的newInstance（）的作用是什么？

这个是别人的代码：

 protected void initFromIntent(int pageValue, Intent data) {
        if (data == null) {
            throw new RuntimeException(
                    "you must provide a page info to display");
        }
        SimpleBackPage page = SimpleBackPage.getPageByValue(pageValue);
        if (page == null) {
            throw new IllegalArgumentException("can not find page by value:"
                    + pageValue);
        }

        setActionBarTitle(page.getTitle());

        try {
            /**
             *获得一个Fragment
             */
            Fragment fragment = (Fragment) page.getClz().newInstance();

            Bundle args = data.getBundleExtra(BUNDLE_KEY_ARGS);
            if (args != null) {
                fragment.setArguments(args);
            }

            FragmentTransaction trans = getSupportFragmentManager()
                    .beginTransaction();
            trans.replace(R.id.container, fragment, TAG);
            trans.commitAllowingStateLoss();

            mFragment = new WeakReference<Fragment>(fragment);
        } catch (Exception e) {
            e.printStackTrace();
            throw new IllegalArgumentException(
                    "generate fragment error. by value:" + pageValue);
        }
    }

通过调用getPageByValue()获得page

public enum SimpleBackPage {

    FEEDBACK(1, R.string.setting_about, FeedBackFragment.class),
    ABOUT(2, R.string.setting_about, AboutFrament.class);


    private int values;
    private int title;
    private Class<?> cls;

    private SimpleBackPage(int values, int title, Class<?> cls) {
        this.values = values;
        this.title = title;
        this.cls = cls;
    }

    public int getValues() {
        return values;
    }

    public void setValues(int values) {
        this.values = values;
    }

    public Class<?> getCls() {
        return cls;
    }

    public void setCls(Class<?> cls) {
        this.cls = cls;
    }

    public int getTitle() {
        return title;
    }

    public void setTitle(int title) {
        this.title = title;
    }

    public static SimpleBackPage getPageValue(int val) {
        for (SimpleBackPage p : values()) {
            if (p.getValues() == val) {
                return p;
            }
        }
        return null;
    }
}

Answer 1

newInstance 是 Class 類別的方法，作用是新建類別的實例

---

從程式碼來看，這段程式碼是把多個Fragment的互動做了一個封裝。

傳入的參數是Fragment的Class定義，這樣就能避免不必要的Fragment實例化，而透過Class去實例化Fragment則需要使用newInstance() 這個方法。

Answer 2

依問題追加的程式碼Update

SimpleBackPage是enum類型，意圖是透過數字取得對應的Fragment類別：

SimpleBackPage page = SimpleBackPage.getPageValue(pageValue);
// 如果pageValue=1，getCls返回的就是FeedBackFragment.class
// 如果pageVaule=2，getCls返回的就是AboutFrament.class
page.getCls();

public enum SimpleBackPage {
    FEEDBACK(1, R.string.setting_about, FeedBackFragment.class),
    ABOUT(2, R.string.setting_about, AboutFrament.class);
    
    private SimpleBackPage(int values, int title, Class<?> cls) {
        this.values = values;
        this.title = title;
        this.cls = cls;
    }
    public Class<?> getCls() {
        return cls;
    }    

---

拿到Fragment類別後

Fragment.class.newInstance() 透過呼叫該類別的無參數建構器，建立並傳回該類別的一個實例。
從結果來看等價於：new Fragment();

Returns a new instance of the class represented by this Class, created by invoking the default (that is, zero-argument) constructor. If there is no such constructor, or if the creation fails (eist fails of a such constructor, or if the creation fails of (ei fails of a such constructuseor, or if the creation fails (, aeile fails ofei such constructuseor, or if the creation fails (, aeile fails of aei if lack of available memory or because an exception is thrown by the constructor), an InstantiationException is thrown. If the default constructor exists but is not accessible from the context where this method is invoaled, is not accessible from the context where this method is invoaled, is not accessible from the contextb.

感覺上

不如

清楚明了。而且你還要為其添加try/catch Fragment.class.newInstance().new Fragment()