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Android Animations and Transitions The Transitions Framework Transitioning Back and Forth

Posted by TERRENCE MAVHUNGA
TERRENCE MAVHUNGA
6,053 Points

Finally, fix the onButtonClicked() method so that the mTransitionManager successfully transitions to the new current sce

im not getting the question

MainActivity.java 
public class MainActivity extends Activity {

    protected TransitionManager mTransitionManager;
    protected Scene mScene1;
    protected Scene mScene2;
    protected Scene mCurrentScene;
    protected TransitionSet mForwardSet;
    protected TransitionSet mBackwardSet;

    // Some code omitted for brevity

    public void onButtonClicked(View view) {

        if (mCurrentScene == mScene2) {
            mCurrentScene = mScene1;
        }
        else {
            mCurrentScene = mScene2;
        }
          mTransitionManager.transitionTo(mCurrentScene);
    }

    protected void setupTransitions() {
        // These methods are setting up scenes and transitions sets like in the video
        initScenes();
        initSets(); 

        // Start here!
         TransitionManager mTransitionManager = new TransitionManager();
      mTransitionManager.setTransition(mScene1,mScene2, mForwardSet);
      mTransitionManager.setTransition(mScene2,mScene1, mBackwardSet);  
     mScene1.enter();


    }
}

2 Answers

Steve Hunter
Steve Hunter
57,712 Points

Hi there,

This is a very easy mistake to make!!

What you have done is redeclared mTransitionManager inside the setupTransitions method. This makes that instance local in scope to that method only. So, outside of that method, the mTransitionManager is not initialized resulting in a Null Pointer Exception.

To fix this remove the preceding TransitionManager as the code already knows that mTransitionManager is of type TransitionManager. Don't redeclare it in the setupTransitions() method, just initialize it; i.e. just assign something to mTransitonManager, as you have done but without the formal data type.

See the comments in here:

public class MainActivity extends Activity {

    protected TransitionManager mTransitionManager;  // declared as 'TransitionManager' here
  .
  .
  .
    public void onButtonClicked(View view) {

        if (mCurrentScene == mScene2) {
            mCurrentScene = mScene1;
        }
        else {
            mCurrentScene = mScene2;
        }
        mTransitionManager.transitionTo(mCurrentScene);
    }

    protected void setupTransitions() {
        initScenes();
        initSets(); 

        // Not this, don't use the data type again; it already is of the right type
        // TransitionManager mTransitionManager = new TransitionManager();

      /* but this - no preceding data type as this 
      is already declared at the top of the class */
      mTransitionManager = new TransitionManager();
      .
      .

Let me know if that's not clear!

Steve.

TERRENCE MAVHUNGA
TERRENCE MAVHUNGA
6,053 Points
TERRENCE MAVHUNGA
TERRENCE MAVHUNGA
6,053 Points

Steve Hunter, hey thts where i was missing it : mTransitionManager = new TransitionManager(); thanx hey

Steve Hunter
Steve Hunter
57,712 Points

No problem! :+1:

Melody Mudehwe
Melody Mudehwe
3,282 Points

Sorry guys I am still getting make sure you transition to correct the scene.

Steve Hunter
Steve Hunter
57,712 Points

Can you post your code, Melody?