Setting layout parameters for an element inside a Gallery

I am using a Gallery to display an horizontal timeline of events. Some of the events get Gravity.TOP and some Gravity.BOTTOM to align them above or below a nice line displaying the years. So far, so good.

I want to change the left margin property of the elements in the top, so there are no huge gaps and the elements look interleaved. For example: setting a negative left margin to every element aligned on top.

Each element of the Gallery consists on a LinearLayout, which can be set up a MarginLayoutParams instance to change the margins programatically. However, I'm getting a ClassCastException when using MarginLayoutParams inside the adapter because the Gallery code does this:

    // Respect layout params that are already in the view. Otherwise
    // make some up...
    Gallery.LayoutParams lp = (Gallery.LayoutParams) child.getLayoutParams();

Any ideas or hints on how to overcome this problem?

Each element of the Gallery consists on a LinearLayout

Just wrap it using another LinearLayout and set the margin in the LinerLayout.LayoutParams for the inner LinearLayout. I have checked it, and it seems to do what you want.

So the layout you inflate for Gallery item should look like this:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/layOuter"
    android:layout_width="wrap_content" android:layout_height="wrap_content">

    <LinearLayout
        android:id="@+id/layInner"
        android:layout_width="wrap_content" android:layout_height="wrap_content"
        android:orientation="vertical">

        <ImageView android:id="@+id/imageView1" android:src="@drawable/icon"
            android:layout_height="wrap_content" android:layout_width="wrap_content"
            android:scaleType="fitXY" />

        <TextView android:text="TextView" android:id="@+id/textView"
            android:layout_width="wrap_content" android:layout_height="wrap_content"
            android:visibility="visible" />
    </LinearLayout>
</LinearLayout>

Then you can access inner LinearLayout in adapter getView method and set margin there depending on your conditions (sample code without convertView reuse optimization):

public View getView(int position, View convertView, ViewGroup parent) {
  Context context = getContext();
  final float density = context.getResources().getDisplayMetrics().density;

  LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
  View layOuter = inflater.inflate(R.layout.row_layout, null);
  View layInner = layOuter.findViewById(R.id.layInner);
  if (...) {  // your condition
    LinearLayout.LayoutParams innerLP = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT);
    innerLP.leftMargin = (int) (50 * density);
    layInner.setLayoutParams(innerLP);
  }
  return layOuter;
}

Please note that you must use LinearLayout.LayoutParams (it extends MarginLayoutParams) for the inner layout, otherwise it will not work.

Gallery.LayoutParams is an entirely different class than android.view.ViewGroup.LayoutParams.

Your child view (built from the adapter) is a LinearLayout that returns a android.view.ViewGroup.LayoutParams, whereas the Gallery returns a Gallery.LayoutParams.

Try using android.view.ViewGroup.LayoutParams instead of Gallery.LayoutParams. If you must use both, then manually set the properties from one to the other where applicable (although I can't really think of a reason why you'd need to use both).