闪烁的背景

Question

我有LinearLayout个Buttons和TextViews。我希望我的背景以定时间隔闪烁，比如说从红色到白色再到红色等等。现在，我正在尝试这个代码，但它给了我一个空指针异常。

LinearLayout ll = (LinearLayout) findViewById(R.layout.activity_main);
Animation anim = new AlphaAnimation(0.0f, 1.0f);
anim.setDuration(50); 
anim.setStartOffset(20);
anim.setRepeatMode(Animation.REVERSE);
anim.setRepeatCount(Animation.INFINITE);
ll.startAnimation(anim); // shows null pointer exception at this line

请帮帮我哪里出错了？

Answer 1

您在View指定了错误的findViewById(R.layout.activity_main) ID。它应该是这样的：

findViewById(R.id.your_view_id);

另外，请务必在setContentView(R.layout.activity_main)

之后立即致电super.onCreate

修改：

以下代码允许您仅使用您想要的任何颜色更改背景颜色。它看起来像AnimationDrawable.start() doesn't work if called from Activity.onCreate，所以我们必须在这里使用Handler.postDelayed。

final LinearLayout layout = (LinearLayout) findViewById(R.id.layout);
final AnimationDrawable drawable = new AnimationDrawable();
final Handler handler = new Handler();

drawable.addFrame(new ColorDrawable(Color.RED), 400);
drawable.addFrame(new ColorDrawable(Color.GREEN), 400);
drawable.setOneShot(false);

layout.setBackgroundDrawable(drawable);
handler.postDelayed(new Runnable() {
    @Override
    public void run() {
        drawable.start();
    }
}, 100);

Answer 2

试试这个

LinearLayout ll = (LinearLayout) findViewById(R.id.activity_main);
Animation anim = new AlphaAnimation(0.0f, 1.0f);
anim.setDuration(50); 
anim.setStartOffset(20);
anim.setRepeatMode(Animation.REVERSE);
anim.setRepeatCount(Animation.INFINITE);
ll.startAnimation(anim);

如果activity_main是您的XML文件名，那么

setContentView(R.layout.activity_main);

并在此处使用您的布局ID

LinearLayout ll = (LinearLayout) findViewById(R.id.linear_layout_id);