This tutorial covers the LinearLayout container, which is the simplest layout mechanism available on Android. If you’re familiar with Swing’s BoxLayout then you will have a good idea of what this container has to offer. Linear layouts are really simple… you can add components horizontally or vertically to a ‘bag’ or ‘box’. This is a pretty simple way of getting components assembled on a screen. Here are it’s limitations:

1. Can’t overlay components (one in front of the other)
2. Can’t control layout in a grid fashion (for that you can use TableLayout).

Also, you can’t select items that you’ve inserted into this container (unless the components themselves are selectable). If you want to make every item you add in this container selectable, then you should use ListView.

Android 101

To get started with Android, click here. Instead of reading the SDK docs to get the basics, I recommend that you read this book to get going.

What can I do with LinearLayout?

Here’s a list of things you can do:

1. You can set the orientation of the LinearLayout to LinearLayout.VERTICAL, or LinearLayout.HORIZONTAL. If you select VERTICAL, then all your components added to this container will be stacked on top of each other. If you pick HORIZONTAL, they will all be stacked beside each other.
2. Before adding a component to this container, you have to set its LinearLayout.LayoutParams. This specifies the height and width constraints you want to assign to your component.
3. You can specify the “weight” (as a float which has to be less than or equal to 1.0f) when you create the LayoutParams for a component that you’re adding to the container. This weight determines how much empty space the component should take up in the final layout. A good way to use this feature is if you want an expandable component in a mix of some other components. For example, if you want to layout 3 components, and you want the middle component to take up the space left over by the other 2 then you can specify it’s width to be 1.0f. Alternatively, you can assign non zero weights to many components (as long as they all add up to 1f). Using this approach you can group the spacing of various components.
4. You can specify the “gravity” for each component that’s in the LinearLayout. This determines whether the component is left or right aligned, and top or bottom aligned, or a combination of these.

That’s pretty much it. The LinearLayout is simple to use, and is very useful to quickly get components stacked vertically or horizontally. When creating the LinearLayout itself, don’t forget to setLayoutParams() on it by specifying layout parameter constraints for the view that the LinearLayout itself is going to be added to.

Example with source code

In the following example, I will show you how to create a vertical box that has 2 other boxes in it; the enclosing vertical box itself is added to an Activity. The top component in the vertical box has a weight of 1f which allows it to take up all the screen space (except for what’s taken up by the bottom box). The top box has an image, and a bunch of clocks in it (vertically laid out). The bottom box has a bunch of buttons in it (horizontally laid out).

Here’s the code that creates the enclosing LinearLayout and adds it to an Activity:

@Override
public void onCreate(Bundle icicle) {
  super.onCreate(icicle);
  try {

    // create the UI
    setContentView(PanelBuilder.createPanel1(this));

  }
  catch (Exception e) {
    Log.e(Global.TAG, "ui creation problem", e);
  }

}

The PanelBuilder class is just a static class that assembles the UI in Java code:

public static final ViewGroup createPanel1(final MainActivity ctx) {

  // main "enclosing" linearlayout container - mainPanel
  final LinearLayout mainPanel = new LinearLayout(ctx);
  {
    mainPanel.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,
                                               LayoutParams.FILL_PARENT));
    mainPanel.setOrientation(LinearLayout.VERTICAL);

    mainPanel.setBackground(SCREEN_BG_IMG);

    AnimUtils.setLayoutAnim_slidedownfromtop(mainPanel, ctx);

    Log.i(Global.TAG, "created main panel");
  }

  // top panel
  LinearLayout topPanel = new LinearLayout(ctx);
  {
  ...
  }

  // bottom panel
  LinearLayout bottomPanel = new LinearLayout(ctx);
  {
  ...
  }

  // add the panels
  mainPanel.addView(topPanel);
  mainPanel.addView(bottomPanel);

  Log.i(Global.TAG, "added topPanel and bottomPanel to mainPanel");

  return mainPanel;

}

Here are some notes on this code:

1. This mainPanel is a LinearLayout container that’s displayed in the Activity. I set it’s LayoutParams to fill the parent (ie, full screen on the activity). Alternatively, you can use the LayoutUtils utility class provided in AndroidUtils to save yourself the tedium of setting these verbose LayoutParams (this is shown in the code example for assembling the bottomPanel below).
2. mainPanel.setOrientation() sets this to be a vertical LinearLayout.
3. You can set a background image to fill the background of the LinearLayout with setBackground(). In this case, SCREEN_BG_IMG is simply an image in R.drawable.* (in my res/drawable/ folder).
4. AnimUtils is a utility class that I’ve written (you can get it in AndroidUtils) to make it easier to assign layout animations on Viewgroup objects/containers (LinearLayout, TableLayout, ListView, etc). It can be used to assign different animation sequences to these ViewGroups very easily. With Android’s animation framework, it’s possible to enable layout animation, without you having to write the code. More on the animation framework in this tutorial.
5. There’s some code that creates the top and bottom LinearLayout containers (called topPanel and bottomPanel), which are then added to the mainPanel LinearLayout object.

The following code shows you how the topPanel is assembled:

  // top panel
  LinearLayout topPanel = new LinearLayout(ctx);
  {

    // WEIGHT = 1f, GRAVITY = center
    topPanel.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,
                                              LayoutParams.WRAP_CONTENT,
                                              1));
    topPanel.setOrientation(LinearLayout.VERTICAL);
    topPanel.setGravity(Gravity.CENTER);

    // an imageview with scaling...
    ImageView topIcon = new ImageView(ctx);
    topIcon.setImageResource(R.drawable.stlogo);
    topIcon.setAdjustViewBounds(true);
    topIcon.setMaxHeight(TopIconMaxHeight);
    topIcon.setMaxWidth(TopIconMaxWidth);
    topIcon.setScaleType(ScaleType.FIT_CENTER);
    topIcon.setAlpha(TopIconAlpha);
    topIcon.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,
                                             LayoutParams.WRAP_CONTENT));

    // clocks...
    AnalogClock clock = new AnalogClock(ctx);
    clock.setBackground(topIcon.getDrawable());
    clock.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,
                                           LayoutParams.WRAP_CONTENT));
    DigitalClock clock2 = new DigitalClock(ctx);
    clock2.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,
                                            LayoutParams.WRAP_CONTENT));

    // adding imageview and clocks to topPanel
    topPanel.addView(clock);
    topPanel.addView(clock2);

    Log.i(Global.TAG, "created topIcon & clock, and added it to topPanel");

  }

Here are some notes on this code:

1. The topPanel LinearLayout is created, and LayoutParams are assigned to it. Note that a weight of “1″ is passed in as a parameter to the LayoutParams constructor. This will ensure that the topPanel will fill the screen with whatever space remains after the bottomPanel is drawn.
2. The gravity is set to CENTER. This simply horizontally and vertically centers any components that are placed inside the topPanel.
3. A couple of Views are then added to the container… these are all wrapped to content width and height. So the topPanel will take up as much space as it can, but the components inside of it will not… they will be centered.

The following code shows how the bottomPanel is assembled:

  // bottom panel
  LinearLayout bottomPanel = new LinearLayout(ctx);
  {
    LayoutUtils.Layout.WidthFill_HeightWrap.applyLinearLayoutParams(bottomPanel);
    bottomPanel.setOrientation(LinearLayout.HORIZONTAL);
    bottomPanel.setGravity(Gravity.CENTER_HORIZONTAL);

    bottomPanel.setBackgroundColor(Color.argb(120, 120, 120, 120));

    ImageButton b1 = _newButton(ctx, R.drawable.icon1);
    ImageButton b2 = _newButton(ctx, R.drawable.icon2);
    ImageButton b3 = _newButton(ctx, R.drawable.icon3);

    bottomPanel.addView(b1);
    bottomPanel.addView(b2);
    bottomPanel.addView(b3);

  }

Here are some notes on this code:

1. Instead of creating a LayoutParams object by hand, this code uses the LayoutUtils class (part of AndroidUtils). This is a shorthand version of the code you have seen in earlier snippets.
2. The bottomPanel is horizontal, and it’s gravity is set to center-horizontal, simply meaning that all components will be centered left-right on the bottomPanel.
3. A translucent background color is set on the bottomPanel itself.
4. Finally a bunch of buttons are added to the bottomPanel.

You can download LayoutUtils, AnimUtils and other helpful classes in AndroidUtils.zip.

Feedback and comments

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This entry was posted on Monday, August 4th, 2008 at 11:26 am and is filed under Android. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.