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Android-Fragment懒加载

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ViewPager的预加载

在项目中，经常会遇到ViewPager+TabLayout实现对多个fragment的管理。但是由于ViewPager的预加载(默认的预加载为1)，viewpager会调用fragment的onCreateView()进行fragmemt的初始化:

fragment

而在Android中当fragment的onCreateView()会将整个xml文件中的UI控件实例到内存，而且数据的请求初始化基本也是在onCreateView()中进行。导致App启动的时候加载的数据过多。

解决方案

解决这个问题有两种方式，一种是禁止ViewPager的预加载，重写ViewPager,但是该方法会出现左右滑动时会出现卡顿现象，带来不好的用户体验。而另外一种就是我们接下来要讲的通过Fragment的懒加载来实现。当用户切换到某个fragment时再加载。

setUserVisibleHint()方法

懒加载主要使用到fragment中的setUserVisibleHint()，这个方法是当fragment的UI显示状态变化时会被调用。源码如下：

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/**
 * Set a hint to the system about whether this fragment's UI is currently visible
 * to the user. This hint defaults to true and is persistent across fragment instance
 * state save and restore.
 *
 * <p>An app may set this to false to indicate that the fragment's UI is
 * scrolled out of visibility or is otherwise not directly visible to the user.
 * This may be used by the system to prioritize operations such as fragment lifecycle updates
 * or loader ordering behavior.</p>
 *
 * <p><strong>Note:</strong> This method may be called outside of the fragment lifecycle.
 * and thus has no ordering guarantees with regard to fragment lifecycle method calls.</p>
 *
 * @param isVisibleToUser true if this fragment's UI is currently visible to the user (default),
 *                        false if it is not.
 */
public void setUserVisibleHint(boolean isVisibleToUser) {
    if (!mUserVisibleHint && isVisibleToUser && mState < STARTED
            && mFragmentManager != null && isAdded()) {
        mFragmentManager.performPendingDeferredStart(this);
    }
    mUserVisibleHint = isVisibleToUser;
    mDeferStart = mState < STARTED && !isVisibleToUser;
}

实现

  1. 因为fragment的onCreateView()方法会将xml中的控件加载到内存中，所以我们定义一个空的FragmentLayout。

创建一个空的容器作为预加载界面

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<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/other_frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.mz.sanfen.lazyfragment.OtherFragment">
</FrameLayout>

并在onCreateView()中初始化这个布局

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@Nullable
 @Override
 public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
     // Inflate the layout for this fragment
     View inflate = inflater.inflate(R.layout.fragment_other, container, false);
     frameLayout = (FrameLayout) inflate.findViewById(R.id.one_frame);
     Log.e(TAG, "onCreateView: " + frameLayout );
     return inflate;
 }
  1. 将具体的内容的UI定义到一个子xml中，并在setUserVisibleHint()方法中进行初始化控件和数据加载，并将这个xml放入外部的容器中。

重新创建一个子布局

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/other_text"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />
</LinearLayout>

并在setUserVisibleHint()中进行初始化

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    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
//        Log.e(TAG, "setUserVisibleHint: " + isVisibleToUser );
        if (isVisibleToUser) {
            if (contentView == null ) {
                contentView = LayoutInflater.from(getActivity()).inflate(R.layout.content_other, frameLayout, true);
                textView = (TextView) contentView.findViewById(R.id.other_text);
                textView.setText("load other fragment");
            }
        }
    }

注意

ViewPager在预加载fragment的时候是先调用setUserVisibleHint(), 然后再是调用onCreateView()方法。

image

源码传送门