android - 求问Fragment fragment = (Fragment) page.getCls().newInstance();-PHP Chinese Network Q&A

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); } 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; } }

Update code added based on the problem

SimpleBackPage is an enum type, and the intention is toget the corresponding Fragment class through numbers:

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; }

After getting the Fragment class

Fragment.class.newInstance()Creates and returns an instance of this class by calling its parameterless constructor.
The result is equivalent to: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 (either because of a 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 invoked, an IllegalAccessException is thrown.

It feels likeFragment.class.newInstance()is not as clear asnew Fragment(). And you also need to add try/catchexception type IllegalAccessException.

reply all (2)

Ty802017-04-17 14:48:21 2 floor

newInstance is a method of the Class class, which is used to create new instances of the class

From the code point of view, this code encapsulates the interaction of multiple Fragments.

The parameter passed in is the Class definition of Fragment, which can avoid unnecessary instantiation of Fragment. To instantiate Fragment through Class, you need to use the newInstance() method.

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