Android 打开片段时启动 animation

Question

I am trying to start an animation when the fragment opens. I don't want it to start with an onClick, but by itself. Tried it in onCreate, but it just crashes the app. I also wanted to try the method: onWindowFocusChanged() but its complicated to use in fragments.

Fragment JAVA class:

    package com.example.appsplashscreen;

import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.interpolator.view.animation.FastOutSlowInInterpolator;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.Animation;
import android.view.animation.AnimationUtils;
import android.view.animation.Interpolator;
import android.view.animation.LinearInterpolator;
import android.view.animation.RotateAnimation;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.TextView;


/**
 * A simple {@link Fragment} subclass.
 */
public class SecondFragment extends Fragment implements View.OnClickListener {

    // Attributes
    private ImageView image;
    private ImageButton button;
    private Animation moveAnimation;

    public SecondFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View v = inflater.inflate(R.layout.fragment_second, container, false);

        button = v.findViewById(R.id.button);
        button.setOnClickListener(this);

        image = (ImageView) getView().findViewById(R.id.imageView4);
        moveAnimation = AnimationUtils.loadAnimation(getActivity(), R.anim.move_animation);
        image.startAnimation(moveAnimation);

        return v;
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.button :
                // your button click
                button.setVisibility(View.GONE);
                break;
        }
    }
}

Errormessage: This is the Errormessage when the app crashes.:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.appsplashscreen, PID: 32704
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
    at com.example.appsplashscreen.SecondFragment.onCreateView(SecondFragment.java:49)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
    at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
    at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
    at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2079)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1869)
    at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
    at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
    at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

Answer 1

This post will add some color: getView returning null when fragment has been created from an activity

But essentially getView() returns the view returned by onCreateView, so it will be null until after this method.

You should use the view resolved by the inflator for resolving your components