正常是通過呼叫碎片的空的構造方法來例項化一個碎片物件，但是這不利於同時管理一批碎片的情況，因為開閉原則的需要，正常的例項方法根本滿足不了。

這裡用的是instantiate

使用方法，通過傳入不同的碎片類名例項化。

Fragment fragment = Fragment.instantiate(mContext,
newNav.getClx().getName(), null);

引數

1.上下文

2.碎片的類名，和碎片的例項一同被維護在某個類中

3.Bundle 你可以理解為arguments 就是你賦給碎片的標識 在導航欄中常見

原始碼

public static Fragment instantiate
 
(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
clazz = context.getClassLoader().loadClass(fname);
sClassMap.put(fname, 
 
clazz);
}
        Fragment f = (Fragment)clazz.newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
f.mArguments = args;
}
        return f;
} catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " 
 
+ fname
                + ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
}
}

首先碎片的例項化有一個快取機制。

private static final SimpleArrayMap<String, Class<?>> sClassMap =
        new SimpleArrayMap<String, Class<?>>();

下面的程式碼就是快取過程，就是以空間換取時間

if (clazz == null) {
    // Class not found in the cache, see if it's real, and try to add it
clazz = context.getClassLoader().loadClass(fname);
sClassMap.put(fname, clazz);
}

clazz = context.getClassLoader().loadClass(fname);

例項化

Fragment f = (Fragment)clazz.newInstance();

最後你的碎片標識

if (args != null) {
    args.setClassLoader(f.getClass().getClassLoader());
f.mArguments = args;
}